UNIT 1

INTRODUCTION TO GEOLOGY
WHAT IS GEOLOGY

 Geology is the study of Earth, including the materials that it is

made of, the physical and chemical changes that take place on its
surface and in its interior and the history of the planet and its life
form.

 Geology is broadly divided into Physical Geology and Historical

Geology.

• Physical geology is concerned with the materials and processes

which compose and operate on the surface of, and within the
Earth.

• Historical geology is concerned with the origin and evolution of

Earth's continents, oceans, atmosphere, and life.
BRANCHES OF GEOLOGY
 GEOLOGY IN OUR LIVES
 Geology is relevant to everyone’s
day-to-day life.

 Metals and energy sources such

as coal and petroleum are
geologic products that build and
power modern society.

 Water, a precious natural

resource is used in industry,
agriculture and domestic purpose
is also a geologic product.

 Natural hazards like tsunamis,

landslides, earthquakes and
volcanic eruptions are also
related to geology,
WHAT DOES A GEOLOGIST DO
 Geologists work to understand the history of our planet. The better they can understand Earth’s
history the better they can foresee how events and processes of the past might influence the future.

 Geologists study earth processes:

 Many processes such as landslides, earthquakes, floods and volcanic eruptions can be hazardous to people.
 Geologists work to understand these processes well enough to avoid building important structures where they
might be damaged.
 If geologists can prepare maps of areas that have flooded in the past they can prepare maps of areas that
might be flooded in the future.
 These maps can be used to guide the development of communities and determine where flood protection or
flood insurance is needed.

 Geologists study earth materials:

 People use earth materials every day. They use oil that is produced from wells, metals that are produced from
mines, and water that has been drawn from streams or from underground.
 Geologists conduct studies that locate rocks that contain important metals, plan the mines that produce them
and the methods used to remove the metals from the rocks. They do similar work to locate and produce oil,
natural gas and ground water.

 Geologists study earth history:

 Today we are concerned about climate change. Many geologists are working to learn about the past climates of
earth and how they have changed across time.
 This historical geology news information is valuable to understand how our current climate is changing and what
the results might be.
WHERE DO GEOLOGISTS WORK
 Jobs in geology are found in government
agencies, private companies, and non-
profit and academic institutions.

 Government agencies hire geologists to

investigate, plan and evaluate
excavations, construction sites, natural
disaster preparedness, and natural
resources.

 Private companies hire geologists to help

locate natural resources (minerals, oil
and natural gas), evaluate environmental
impact and comply with government
regulations, among many other tasks.

 Geologists who prefer an academic

career usually work, either as educators,
researchers or both, in middle or high
schools, colleges, universities and
museums.
PROCESSES ACTING ON THE EARTH

 The Earth is a Dynamic system that is it

undergoes constant changes with time both
internally and on its surface.

Internal Processes

 Processes that originate deep within the Earth

are termed as internal processes. These are
the driving forces that raise mountains, cause
earthquakes and produce volcanic eruptions.

Surface Processes

 Surface processes are all those processes

which take place on the earth’s surface and
result in sculpting the earth’s surface. Most of
the surface processes are driven by water,
though wind, ice and gravity also play an
important role.
GEO-SPHERES
 The area near the surface of the earth can be divided
into four inter-connected "geo-spheres:" the
atmosphere, hydrosphere, lithosphere and the
biosphere.
ATMOSPHERE
 The atmosphere is the body of air which
surrounds our planet.

 Most of our atmosphere is located close to

the earth's surface where it is most dense.

 The atmosphere not only provides the air

that we breathe but also acts to protect us
from the Sun’s intense heat and
dangerous ultraviolet radiation.

 The energy exchanges that continually

occur between the atmosphere and the
surface and between the atmosphere and
space produce the effects we call weather
and climate.

 The air of our planet is 79% nitrogen and

just under 21% oxygen; the small amount
remaining is composed of carbon dioxide
and other gasses.
HYDROSPHERE
 The hydrosphere is composed
of all of the water on or near
the earth.

 This includes the oceans,

rivers, lakes, and even the
moisture in the air.

 Ninety-seven percent of the

earth's water is in the oceans.

 The remaining three percent is

fresh water; three-quarters of
the fresh water is solid and
exists in ice sheets.
LITHOSPHERE
 Beneath the atmosphere and the oceans is the solid Earth, or lithosphere.

 The lithosphere is the solid, rocky crust covering entire planet.

 This crust is inorganic and is composed of minerals.

BIOSPHERE
 The biosphere is composed of all living organisms.

 Plants, animals, and one-celled organisms are all part of the biosphere.

 Most of the planet's life is found from three meters below the ground to thirty
meters above it and in the top 200 meters of the oceans and seas.
 UNIFORMITARIANISM AND CATASTROPHISM

 James Hutton a Scotish gentleman in 18th century gave the

concept of Uniformitarianism.

 According to the Principle of Uniformitarianism, geological

changes take place over a long period of time.

 Hutton summarized that geological processes operating

today also operated in the past therefore scientists can
explain events that occurred in the past by observing
changes that are occurring today.

 Sometimes this whole idea is also known as “The present is

the key to the past.”
UNIFORMITARIANISM

The formation of river valleys due to the action of running water or the
movement of the continents are examples of very slow and gradual changes
 UNIFORMITARIANISM AND CATASTROPHISM

 William Whewell, another early geologist, agreed that

the Earth is very old, but he argued that geologic change
was sometimes rapid.

 He wrote that the geologic past may have “consisted of

epochs catastrophic action, interposed between periods
of comparative tranquility.”

 This phenomenon where earth was subjected to rapid

geological change as a result of certain catastrophe
came to be known as Catastrophism
CATASTROPHISM
UNIFORMITARIANISM AND CATASTROPHISM

 Today, geologists know that both Hutton’s

uniformitarianism and Whewell’s catastrophism
are correct.

 Thus, over the great expanses of geologic time,

slow, uniform processes are significant, but
improbable, catastrophic events radically
modify the path of slow change.
ROCK CYCLE
 Rock is the most common and abundant material on Earth.

 A rock consists of smaller crystals or grains called minerals. Minerals are chemical
compounds (or sometimes single elements), each with its own composition and
physical properties. The grains or crystals may be microscopically small or easily
seen with the unaided eye.

 The nature and appearance of a rock is strongly influenced by the minerals that
compose it.

 A rock’s texture—the size, shape, and/or arrangement of its constituent minerals—

also has a significant effect on its appearance.

 A rock’s mineral composition and texture, in turn, are a reflection of the geologic
processes that created it

 Geologists divide rocks into three major groups: igneous, sedimentary, and
metamorphic.
 Rock Cycle is the fundamental concept of Geology that describes the dynamic
transition through geologic time among the three rock types Each type of rock
is altered or destroyed when it is forced out of equilibrium condition.
IGNEOUS ROCKS
 Magma is the molten material which is
formed when the pressure and
temperature conditions are high enough
to melt the rocks.

 The Magma is formed in the interior of

the earth and then gradually migrates
upwards to the earth’s crust.

 When it reaches the surface its cools

and solidifies by the process of
crystallization.

 The rocks formed as a result are known

as Igneous Rocks.
SEDIMENTARY ROCKS
 These igneous rocks when exposed
to the atmosphere undergo
weathering where they disintegrate
into smaller particles.

 These particles known as sediments

are transported by the agents of
erosion such as water, wind and ice.

 Finally these sediments are

deposited.

 These sediments are then converted

to rocks by the process of
lithification.

 The resulting rocks are known as

Sedimentary Rocks.
METAMORPHIC ROCKS
 If the resulting sedimentary rock is
buried deep within Earth and involved in
the dynamics of mountain building or
intruded by a mass of magma, it will be
subjected to great pressures and/or
intense heat.

 The sedimentary rock will react to the

changing environment and turn into the
third rock type, metamorphic rock.

 When metamorphic rock is subjected to

additional pressure changes or to still
higher temperatures, it will melt,
creating magma, which will eventually
crystallize into igneous rock, starting the
cycle all over again.
FORMATION OF THE SOLAR SYSTEM
 Earth was not around at the beginning—
the universe began without us some 10
billion years earlier than Earth.

 The universe started out with only two

elements, hydrogen and helium gas.

 They formed stars that burned these

elements in nuclear fusion reactions.

 Generations of stars were born in gas

clouds and died in explosive novas.

 Long, long ago (some 5 billion years

ago) a supernova exploded, pushing a
lot of its heavy-element wreckage into a
nearby cloud of hydrogen gas and
interstellar dust.
FORMATION OF THE SOLAR SYSTEM
 The mixture grew hot and
compressed under its own
gravity, and at its center a
new star began to form.

 Around it swirled a disk of

the same material, which
grew white-hot from the
great compressive forces.

 That new star became our

Sun, and the glowing disk
gave rise to Earth and its
sister planets.
FORMATION OF THE SOLAR SYSTEM
FORMATION OF THE SOLAR SYSTEM
 The planets of the Solar system can be divided into groups
depending on their proximity to the sun and their density.

 The terrestrial planets are the four closest to the sun and are
all similar to the Earth in density. They include Mercury,
Venus, Earth and Mars. All four terrestrial planets are small,
rocky and dense (3 g/cm3 or more).
FORMATION OF THE SOLAR SYSTEM
 The Jovian planets are those farther from the sun than Mars
 They include Jupiter, Saturn, Uranus and Neptune. They are much
larger than the Earth but their densities are very low.
 They are made up of light elements most Hydrogen and Helium
and hence their densities are low,
FORMATION OF THE EARTH
 The Earth is approximately 4.6 billion years old and is believed that it was
formed by accretion of small particles.

 The Earth has a layered structure. The center is a dense, hot core composed
mainly of iron and nickel .

 A thick mantle, composed mainly of solid rock, surrounds the core and contains
80 percent of the Earth’s volume.

 The crust is a thin surface also composed of rock.

STRUCTURE OF THE EARTH
GEOLOGIC TIME
 The earth is estimated to be 4.6 Billion Years old.
 Geologic time differs from the human perspective of time.
 Earth goes through cycles of much longer duration than the human
perspective of time.
 The geologic time scale is the calendar that geologists use to date past
events in Earth’s history.
 The Geological time scale is divided into Eons, Eras, Periods, and Epochs
and is identified primarily by the types of life that existed at the various
times.
 The two earliest eons, the Hadean and Archean, cover the first 2.5 billion
years of Earth history.
 Life originated during Archean time and with the passage of time the life
form evolved.
 Evolution was very gradual until the last 5 million years where many new
species evolved which were more complex than their ancestors.
The Geological Time
Scale
UNIT 2

MATTER AND MINERALS

ROCKS AND MINERALS
 A rock is any solid mass of mineral, or
mineral-like, matter that occurs
naturally as part of our planet.

 Most rocks, like the common rock

granite, occur as aggregates of several
different minerals.

 The term aggregate implies that the

minerals are joined in such a way that
their individual properties are retained.

 However, some rocks are composed

almost entirely of one mineral.

 A common example is the sedimentary

rock limestone, which consists of
impure masses of the mineral calcite.
WHAT IS A MINERAL

 A mineral is a naturally
occurring, inorganic
solid with a
characteristic chemical
composition and a
crystalline structure.

 Chemical composition
and crystalline structure
are the two most
important properties of
a mineral: They
distinguish any mineral
from all others.
WHAT ARE MINERALS
 Naturally occurring

 Minerals form by natural, geologic processes.

 Synthetic materials, meaning those produced in a laboratory or by human

intervention, are not considered minerals.

 Solid substance

 Only solid crystalline substances are considered minerals.

 Ice (frozen water) fits this criterion and is considered a mineral, whereas liquid
water and water vapor do not.

 The exception is mercury, which is found in its liquid form in nature.

WHAT ARE MINERALS
 Generally inorganic

 Minerals do not contain compounds of organic carbon.

 Organic carbon which is found in all living organisms bonds with hydrogen to form compounds.

 Inorganic carbon is formed when carbon combines with elements other than hydrogen.

 Thus coal is not a mineral because it contains organic carbon derived from plant remains.

 Orderly crystalline structure

 Minerals are crystalline substances, which means their atoms

are arranged in an orderly, repetitive manner

 This orderly packing of atoms is reflected in the regularly

shaped objects called crystals.

 To have a crystalline structure, a substance must be solid at at

Earth’s surface temperature and not in the liquid or gaseous
phase.

 Some naturally occurring solids, such as volcanic glass

(obsidian), lack a repetitive atomic structure and are not
considered minerals.
NaCl (Sodium Chloride)
HALITE
WHAT ARE MINERALS
 Can be represented by a chemical formula

 Most minerals are chemical compounds having compositions that can be expressed by a
chemical formula.

 For example, the common mineral quartz has the formula SiO2, which indicates that quartz
consists of silicon (Si) and oxygen (O) atoms in a ratio of one-to-two.

 This proportion of silicon to oxygen is true for any sample of pure quartz, regardless of its origin.

 However, the compositions of some minerals vary within specific, well-defined limits.

 This occurs because certain elements can substitute for others of similar size without changing
the mineral’s internal structure.

 An example is the mineral olivine in which either the element magnesium (Mg) or the element
iron (Fe) may occupy the same site in the crystal structure.

 Therefore, olivine’s formula, (Mg, Fe)2SiO4, expresses variability in the relative amounts of
magnesium and iron. However, the ratio of magnesium plus iron to silicon (Si) and oxygen (O)
remains fixed at 2:1:4.
ELEMENTS & ATOM

 An element cannot be broken into simpler

particles by ordinary chemical processes.

 Most common minerals consist of a small

number—usually two to five of different
chemical elements

ROCKS – MINERALS – ELEMNTS - ATOMS

COMMON ELEMENTS

 A total of 91 elements occur naturally in the Earth’s crust.

However, eight elements make up more than 98 percent of
the earth’s crust. These elements are

 Oxygen,
 Silicon,
 Aluminum,
 Iron,
 Calcium,
 Magnesium,
 Potassium and
 Sodium
THE PERIODIC TABLE

As of June 2011, the periodic table includes 118 chemical elements whose
discoveries have been confirmed. Of these, 91 are regularly occurring primordial or
recurrently produced elements found naturally on the Earth,
 STRUCTURE OF AN ATOM
 An atom is the basic unit of an element.

 An atom is tiny; the diameter of the average

atom is about 10ˉ10 meters.

 An atom consists of a small, dense, positively

charged center called a Nucleus .

 The Nucleus contains dense particles with

positive electric charge known as Protons .

 and equally dense particles with neutral electric

charges know as Neutrons.

 The nucleus is surrounded by negatively

charged Electrons.

 An electron is a fundamental particle; it is not

made up of smaller components. An electron
orbits the nucleus, but not in a clearly defined
path.
ATOMIC MASS AND NUMBER
 In its normal state an atom is electrically
neutral as the number of protons(+
charge) is always equal to the number of
electrons (- charge)

 Atomic weight/mass of an atom is equal to

the total number of Neutrons + Protons

 Atomic Number of an atom is equal to its

number of Proton or Electron.
IONS
 Electrons concentrate in spherical layers
or shells around the nucleus.

 Each shell can hold a certain number of

electrons.

 An atom is completely stable when its

outermost shell is completely fill with
electrons.

 However in their neutral states most

atoms do not have a filled outer shell.

 The atoms can loose or gain an electron to

make its outer shell complete.

 When an atom loses one or more

electrons, its protons outnumber its
electrons and it develops a positive
charge.

 If an atom gains one or more extra

electrons, it becomes negatively charged.

 A charged atom is called an Ion.

IONS

 A positively charged ion is a Cation.

(Na+ ).

 Atom with a negative charge is

called Anion, (Cl- ).

 Atoms and ions rarely exist

independently. Instead, they unite
to form Compounds. (NaCl).

 The forces that hold atoms and

ions together to form compounds
are called chemical bonds.
ELECTRON PATTERN FOR THE FIRST 20
ELEMENTS
Atomic Number of Electrons in Each shell
Element Symbol
Number First (2 is stable) Second (8 is stable) Third (8 is stable) Fourth (8 is stable)
Hydrogen H 1 1
Helium He 2 2
Lithium Li 3 2 1
Beryllium Be 4 2 2
Boron B 5 2 3
Carbon C 6 2 4
Nitrogen N 7 2 5
Oxygen O 8 2 6
Fluorine F 9 2 7
Neon Ne 10 2 8
Sodium Na 11 2 8 1
Magnesium Mg 12 2 8 2
Aluminum Al 13 2 8 3
Silicon Si 14 2 8 4
Phosphorus P 15 2 8 5
Sulfur S 16 2 8 6
Chlorine Cl 17 2 8 7
Argon Ar 18 2 8 8
Potassium K 19 2 8 8 1
Calcium Ca 20 2 8 8 2
CHEMICAL BONDS

 Four types of chemical bonds are found in

minerals:

1. Ionic,

2. Covalent

3. Metallic

4. Van der Waals forces

IONIC BONDS
 Cations and anions are attracted
by their opposite electronic
charges and thus bond together.

 This union is called an ionic bond.

 An ionic compound (made up of

two or more ions) is neutral
because the positive and negative
charges balance each other.

 For example, when sodium and

chlorine form an ionic bond, the
sodium atom loses one electron to
become a cation and chlorine
gains one to become an anion.

 When they combine, the +1

charge balances the -1 charge
COVALENT BONDS
 Not all atoms combine by
transferring electrons to form
ions.

 Other atoms share electrons.

 A covalent bond develops

when two or more atoms share
their electrons to produce the
effect of filled outer electron
shells.

 For example the gaseous

elements Oxygen,O2;
Hydrogen,H2 and Chlorine, Cl2
exist as stable molecules
consisting of two atoms
bonded together by sharing
their valence electrons
IONIC AND COVALENT BONDS
METALLIC BONDS

 In a metallic bond, the outer electrons

are loose; that is, they are not
associated with particular atoms.

 This arrangement allows the nuclei to

pack together as closely as possible,
resulting in the characteristic high
density of metals and metallic minerals,
such as pyrite.

 Because the electrons are free to move

through the entire crystal, metallic
minerals are excellent conductors of
electricity and heat.
VAN DER WAALS FORCES
 Weak electrical forces called van
der Waals forces also bond
molecules together.

 These weak bonds result from an

uneven distribution of electrons
around individual molecules, so
that one portion of a molecule
may have a greater density of
negative charge while another
portion has a partial positive
charge.

 Because van der Waals forces are

weak, minerals in which these
bonds are important, such as talc
GRAPHITE
and graphite, tend to be soft and
cleave easily along planes of van
der Waals bonds.
MATTER AND MINERALS

 Most minerals are compounds.

 When ions bond together to form a mineral,

they do so in proportions so that the total
number of negative charges exactly balances
the total number of positive charges.

 Thus, minerals are always electrically neutral.

MATTER AND MINERALS

 The composition of a mineral can be

expressed as a chemical formula, which is
written by combining the symbols of the
individual elements.

Quartz (SiO2) Calcite (CaCO3)

MATTER AND MINERALS

 A few minerals, such as gold and silver, consist of only a

single element. Their chemical formulas, respectively, are Au
(the symbol for gold) and Ag (the symbol for silver).

 Most minerals, however, are made up of two to five essential

elements.

 For example, the formula of quartz is SiO2: It consists of one

atom of silicon (Si) for every two of oxygen (O). The chemical
composition of Quartz is uniform throughout the universe
and this is one of the main criteria for a substance to be
called as a mineral.
MATTER AND MINERALS

The 91 elements that occur naturally in


the Earth’s crust can combine in many  They are
ways to form many different minerals.

1. Olivine,
 However, the eight abundant elements 2. Pyroxene,
commonly combine in only a few ways.
3. Amphibole,
As a result, only nine rock forming

minerals (or mineral “groups”) make up 4. Mica,
most rocks of the Earth’s crust.
5. Clays,
6. Quartz,
7. Feldspar,
8. Calcite, and
9. Dolomite.
CRYSTALS

 A crystal is any substance

whose atoms are arranged
in a regular, periodically
repeated pattern.

 All minerals are crystalline.

 The mineral halite (common

table salt) has the
composition NaCl: one
sodium ion (Na+) for every
chlorine ion (Cl-).
UNIT CELL

 The sodium and chlorine ions

alternate in orderly rows and
columns intersecting at right
angles.

 This arrangement is the

crystalline structure of halite.

 In every crystal, a small group

of atoms, like a single brick in
a wall, repeats itself over and
over. This small group of
atoms is called a unit cell.
CRYSTAL FACE
 A crystal face is a planar
surface that develops if a
crystal grows freely in an
uncrowded environment.

 In nature, the growth of

crystals is often impeded
by adjacent minerals that
are growing
simultaneously or that
have formed previously.

 For this reason, minerals

rarely show perfect
development of crystal
faces.
PHYSICAL PROPERTIES OF MINERALS

1. Crystal habit
2. Cleavage
3. Fracture
4. Hardness
5. Specific gravity
6. Color
7. Streak
8. Luster
CRYSTAL HABIT
 Crystal habit is the characteristic
shape of a mineral and the
manner in which aggregates of
crystals grow.

 If a crystal grows freely, it

develops a characteristic shape
controlled by the arrangement of
its atoms, as in the cubes of
halite. Some minerals such as
Quartz occur in more than 1
different crystal habits.

 When crystal growth is

obstructed by other crystals, a
mineral cannot develop its
characteristic habit. They form
interlocking texture because
some crystals grew around
others as the magma solidified.
CLEAVAGE

 Cleavage is the tendency of some

minerals to break along flat surfaces.

 The surfaces are planes of weak

bonds in the crystal. Some minerals,
such as mica and graphite, have one
set of parallel cleavage planes.
Others have two, three, or even four
different sets.

 Many minerals have no cleavage at

all because they have no planes of
weak bonds.

 A flat surface created by a cleavage

and a crystal face can appear
identical as both are smooth and
flat, however a cleavage plane is
duplicated when the sample is
broken whereas the crystal face is
not.
FRACTURE
 Fracture is the pattern in
which a mineral breaks other
than along planes of cleavage. Conchoidal Fracture

 Many minerals fracture into

characteristic shapes.

 Conchoidal fracture creates

smooth, curved surfaces. It is Splintery Fracture
characteristic of quartz and
olivine.

 Some minerals break into

splintery or fibrous fragments.

 Most fracture into irregular Jagged Fracture

shapes.
SPECIFIC GRAVITY

 Specific gravity is the weight of a substance in air divided by

the weight of an equal volume of water.

 If a mineral weighs 2.5 times as much as an equal volume of

water, its specific gravity is 2.5.

 Most common minerals have specific gravities of about 2.7.

 Metals have much greater specific gravities; for example,

gold has the highest specific gravity of all minerals, 19.3.
Lead is 11.3, silver is 10.5, and copper is 8.9.
HARDNESS

 Hardness is the resistance of a mineral to scratching.

 It is easily measured and is a fundamental property of

each mineral because it is controlled by bond strength
between the atoms in the mineral.

 Geologists commonly use a knife or other object of

known hardness.

 If the blade scratches the mineral, the mineral is softer

than the knife. If the knife cannot scratch the mineral,
the mineral is harder.
HARDNESS
 To measure hardness more
accurately, geologists use a
scale based on ten
minerals, numbered 1
through 10.

 Each mineral is harder than

those with lower numbers
on the scale, so 10
(diamond) is the hardest
and 1 (talc) is the softest.

 The scale is known as the

Mohs hardness scale after
F. Mohs, the Austrian
mineralogist who
developed it in the early
nineteenth century.
 COLOR

 Color is the most obvious

property of a mineral, but it is
commonly unreliable for
identification.

 Color would be a reliable

identification tool if all minerals
were pure and had perfect
crystal structures.

 However, both small amounts of

chemical impurities and
imperfections in crystal
structure can dramatically alter
color.
STREAK

 Streak is the color of a fine

powder of a mineral.

 It is observed by rubbing the

mineral across a piece of
unglazed porcelain known as a
streak plate.

 Many minerals leave a streak of

powder with a diagnostic color
on the plate.

 Streak is commonly more

reliable than the color of the
mineral itself for identification.
LUSTER

 Luster is the manner in which

a mineral reflects light.

 A mineral with a metallic look,

irrespective of color, has a
metallic luster.

 The luster of nonmetallic

minerals is usually described
by self-explanatory words such
as glassy, pearly, earthy, and
resinous.
TYPES OF MINERALS

 Although about 3500 minerals are known to exist in the Earth’s

crust, only a small number — between 50 and 100 are important
because they are common or valuable. These minerals can be
grouped into 5 categories. They include

 Rock Forming Minerals

 Accessory minerals

 Gem

 Ore minerals

 Industrial Minerals
 ROCK FORMING MINERALS
 Rock Forming Minerals
make up the bulk of most
rocks in the Earth’s crust.
Olivine Pyroxene
 They are important to
geologists simply because
they are the most common
minerals. They include Mica
Amphibole

1. olivine,
2. pyroxene,
3. amphibole, Quartz Clay
4. mica, minerals
5. the clay minerals,
6. feldspar,
7. quartz,
8. Calcite and Feldspar
Dolomite Calcite
9. dolomite
ACCESSORY MINERALS

 Accessory minerals
are minerals that are
common but usually Garnet
are found only in
small amounts.

 Chlorite, garnet,
hematite, limonite,
magnetite, and pyrite
are common Chlorite
accessory minerals.
GEMS
 A gem is a mineral that is
prized primarily for its beauty,
although some gems, like Diamond
diamonds, are also used
industrially.

 Depending on its value, a gem Ruby

can be either precious or
semiprecious.

 Precious gems include Sapphire

diamond, emerald, ruby, and
sapphire. Several varieties of
quartz, including amethyst,
agate, jasper, and tiger’s eye,
are semiprecious gems. Emerald
ORE MINERALS

 Ore minerals are minerals Gold

from which metals or other
elements can be profitably
recovered. A few, such as
native gold and native silver, Silver
are composed of a single
element.
Galena
 However, most metals are
chemically bonded to anions.
Pyrite
 Copper, lead, and zinc are
commonly bonded to sulfur to
form the important ore Chalcopyrite
minerals chalcopyrite, galena.
ORE MINERALS

 Argentite: Ag2S for production of  Columbite-Tantalite or Coltan: (Fe,

silver Mn)(Nb, Ta)2O6
 Barite: BaSO4  Galena: PbS
 Bauxite Al2O3 for production of  Gold: Au, typically associated with
aluminium quartz or as placer deposits
 Beryl: Be3Al2(SiO3)6  Hematite: Fe2O3
 Bornite: Cu5FeS4  Ilmenite: FeTiO3
 Cassiterite: SnO2  Magnetite: Fe3O4
 Chalcocite: Cu2S for production of  Molybdenite: MoS2
copper  Pentlandite:(Fe, Ni)9S8
 Chalcopyrite: CuFeS2  Pyrolusite:MnO2
 Chromite: (Fe, Mg)Cr2O4 for  Scheelite: CaWO4
production of chromium  Sphalerite: ZnS
 Cinnabar: HgS for production of  Uraninite (pitchblende): UO2 for
mercury production of metallic uranium
 Cobaltite: (Co, Fe)AsS  Wolframite: (Fe, Mn)WO4
INDUSTRIAL MINERALS
 Industrial Minerals are industrially
important, although they are not
considered ore because they are Apatite
mined for purposes other than the
extraction of metals.

 Halite is mined for table salt, and

gypsum is mined as the raw
material for plaster and
sheetrock.
Halite
 Apatite and other phosphorus
minerals are sources of the
phosphate fertilizers crucial to
modern agriculture.

 Many limestones are made up of

nearly pure calcite and are mined Limestone
as the raw material of cement.
MINERAL CLASSIFICATION

 Minerals are classified according to their anions i.e. the negatively

charged ions.

 Anions can be either simple or complex. A simple anion is a single

negatively charged ion, such as O2-.

 Alternatively, two or more atoms can bond firmly together and acquire
a negative charge to form a complex anion. Two common examples
are the silicate, (SiO4)4-, and carbonate, (CO3)2-.

 Each mineral group (except the native elements) is named for its
anion. For example, the oxides all contain O2-, the silicates contain
(SiO4)4-, and the carbonates contain (CO3)2-.
MAJOR MINERAL GROUPS
NATIVE ELEMENTS
 About 20 elements occur naturally in Diamond
their native states as minerals.

 Fewer than ten, however, are common

enough to be of economic importance.
Sulfur
 Gold, silver, platinum, and copper are all
mined in their pure forms.

 Pure carbon occurs as both graphite and

diamond. Gold
 The minerals have identical compositions
but different crystalline structures and
are called polymorphs.
Copper
 Graphite is one of the softest minerals
and is opaque and an electrical
conductor.

 Diamond, the hardest mineral known, is Platinum

transparent and an electrical insulator.
OXIDES

 The oxides are a large group of

minerals in which oxygen is Magnetite
combined with one or more metals.

 Oxide minerals are the most

important ores of iron, manganese,
tin, chromium, uranium, titanium,
and several other industrial metals. Haematite

 Hematite (iron oxide, Fe2O3) occurs

widely in many types of rocks and is
the most abundant ore of iron.

 Magnetite (Fe3O4), a naturally

magnetic iron oxide, is another ore Rutile
of iron.
SULFIDES
 Sulfide minerals consist of sulfur
combined with one or more
metals. Pyrite

 Many sulfides are extremely

important ore minerals.

 They are the world’s major sources Chalcopyrite

of copper, lead, zinc, molybdenum,
silver, cobalt, mercury, nickel, and
several other metals.

 The most common sulfides are Galena

pyrite (FeS2), chalcopyrite
(CuFeS2), galena (PbS), and
sphalerite (ZnS).
Sphalerite
SULFATES

 The sulfate minerals contain

the sulfate complex anion
(SO4)2-.

 Gypsum (CaSO4. 2H2O) and

anhydrite (CaSO4) are two Gypsum
important industrial sulfates
used to manufacture plaster
and sheetrock.

 Both form by evaporation of

seawater or salty lake water.
Anhydrite
PHOSPHATES

 Phosphate minerals contain the complex anion (PO4)3-

.
 Apatite, Ca5 (F,Cl,OH)(PO4)3-, is the substance that
makes up both teeth and bones. Phosphate is an
essential fertilizer in modern agriculture.

Apatite
CARBONATES
 The complex carbonate
anion (CO3)2- is the basis
Limestone
of two common rock-
forming minerals, calcite
(CaCO3) and dolomite
[CaMg(CO3)2].
Dolomite
 Limestone is mined as a
raw ingredient of cement.

 Aragonite is a polymorph
of calcite that makes up Aragonite
the shells of many marine
animals.
SILICATES

 The silicate minerals contain the

(SiO4)4- complex anion. Silicates
make up about 95 percent of the
Earth’s crust.

 They are abundant for two reasons.

 First, silicon and oxygen are the two
most abundant elements in the crust.
 Second, silicon and oxygen combine
readily.
 SILICATE MINERALS

 Every silicon atom surrounds itself with four

oxygens. The bonds between each silicon and its
four oxygens are very strong.

 The silicon atom and its four oxygens form a

pyramid-shaped structure called the silicate
tetrahedron with silicon in the center and oxygens
at the four corners.

 The silicate tetrahedron has a 4- charge and forms

the (SiO4)4- complex anion. The silicate tetrahedron
is the fundamental building block of all silicate
minerals.

 To make silicate minerals electrically neutral, other

cations must combine with the silicate tetrahedra
to balance their negative charges.

 Silicate tetrahedra commonly link together by

sharing oxygens. Thus, two tetrahedra may share a
single oxygen, bonding the tetrahedra together.
ROCK FORMING SILICATE MINERALS

 The rock-forming
silicates (and most
other silicate minerals)
fall into five classes,
based on five ways in
which tetrahedra
share oxygens.

 Each class contains at

least one of the rock-
forming mineral
groups.
 SILICATE MINERALS
 In independent tetrahedra silicates, adjacent tetrahedral
do not share oxygens.

 Olivine is an independent tetrahedra mineral that occurs in

small quantities in basalt of both continental and oceanic
crust .

 Mantle is composed mainly of Olivine and Pyroxenes

 In the single-chain silicates, each tetrahedron links to two

others by sharing oxygens, forming a continuous chain of
tetrahedral.

 The pyroxenes are a group of similar minerals with single

chain structures.

 Pyroxenes are a major component of both oceanic crust

and the mantle and are also abundant in some continental
rocks.
SILICATE STRUCTURE

 The double-chain silicates

consist of two single chains
crosslinked by the sharing of
additional oxygens between
them.

 The amphiboles are a group

of double-chain silicates
with similar properties. They
occur commonly in many
continental rocks.
SILICATE MINERALS
 In the sheet silicates, each tetrahedron shares
oxygens with three others in the same plane,
forming a continuous sheet.

 All of the atoms within each sheet are strongly

bonded, but each sheet is only weakly bonded to
those above and below. Therefore, sheet silicates
have excellent cleavage.

 The micas are sheet silicates and typically grow as

plate-shaped crystals, with flat surfaces.

 Mica is common in continental rocks.

 The clay minerals are similar to mica in structure,

composition, and platy habit.

 Clay minerals are abundant near the Earth’s

surface and are an important component of soil
and of sedimentary rocks.
 SILICATE MINERALS
 In the framework silicates, each
tetrahedron shares all four of its oxygens
with adjacent tetrahedral.

 Because tetrahedra share oxygens in all

directions, minerals with the framework
structure tend to grow blocky crystals that
have similar dimensions in all directions.

 Feldspar and Quartz have framework

structures.

 Feldspars make up more than 50 percent

of the earth’s crust.

 Quartz is the only common silicate mineral

that contains no cations other than silicon;
it is pure SiO2
UNIT 3

IGNEOUS ROCKS
ROCK TYPES
 The Earth is almost entirely
rock to a depth of 2900
kilometers, where the solid
mantle gives way to the liquid
outer core.

 Geologists group rocks into

three categories on the basis
of how they form:

 Igneous rocks,
 Sedimentary rocks, and
 Metamorphic rocks
ORIGIN OF THE MAGMA

 In the asthenosphere (between depths of about 100 to 350 kilometers), the

temperature is so high that rocks melt in certain environments to form magma
FORMATION OF MAGMA
 Under certain conditions, rocks of
the upper mantle and lower crust
melt, forming a hot liquid called
magma.

 An igneous rock forms when magma

solidifies.

 About 95 percent of the Earth’s

crust consists of igneous rock and
metamorphosed igneous rock.

 Granite and basalt are two common

and familiar igneous rocks.
FORMATION OF MAGMA
 Three different processes melt the
asthenosphere:

1. rising temperature,

2. decreasing pressure and

3. addition of water
FORMATION OF MAGMA
 Rising Temperature

 A solid melts when it becomes hot enough. Therefore an increase in

temperature melts a hot rock.

 Most magma originates when essentially solid rock, located in the crust and
upper mantle, melts. The most obvious way to generate magma from solid
rock is to raise the temperature above the rock’s melting point.

 The temperatures gets higher as we go deeper inside the earth. Although

the rate of temperature change varies considerably from place to place, it
averages about 25 °C per kilometer in the upper crust.

 This increase in temperature with depth, known as the geothermal gradient,

is somewhat higher beneath the oceans than beneath the continents.

 Tectonic processes exist that can increase the geothermal gradient

sufficiently to trigger melting.
FORMATION OF MAGMA
 Decreasing Pressure

 If temperature were the only factor that determined whether or not rock
melts, our planet would be a molten ball covered with a thin, solid outer
shell.

 This, of course, is not the case. The reason is that pressure also increases
with depth.

 Melting, which is accompanied by an increase in volume, occurs at higher

temperatures at depth because of greater confining pressure.

 Consequently, an increase in confining pressure causes an increase in the

rock’s melting temperature. Conversely, reducing confining pressure lowers
a rock’s melting temperature. When confining pressure drops sufficiently,
decompression melting is triggered.
FORMATION OF MAGMA
 Addition of Water

 Another important factor affecting the melting temperature of rock is

its water content.

 Volatiles cause rock to melt at lower temperatures. Further, the effect

of volatiles is magnified by increased pressure.

 Deeply buried “wet” rock has a much lower melting temperature than
“dry” rock of the same composition.

 Therefore, in addition to a rock’s composition, its temperature, depth

(confining pressure), and water content determine whether it exists
as a solid or liquid.
 NATURE OF MAGMA
 Magma is the completely or
partially molten material,
which on cooling solidifies to
form an igneous rock.

 Once a magma is formed it

rises towards the surface as its
is less dense than the
surrounding rocks.

 Sometimes these magma may

reach the earth’s surface in
the form of volcanoes. Magma
that reaches the earth’s
surface is known as Lava.
COMPONENTS OF THE MAGMA
 Most magma consists of 3 distinct parts: a liquid component, a solid
component and a gaseous phase.

 The melt is mostly made up of the ions of Silicon and Oxygen which readily
combines to form Silica (SiO2). Less amounts of Aluminum, Potassium, Calcium,
Sodium, Iron and Magnesium are also found.

 The solid component if any are silicate minerals that have already crystallized
from the melt. As the magma cools, the size and the number of crystals
increase. During the last stage of cooling, the magma body is mostly a
crystalline solid with only minor amounts of melt.

 Water (H2O), carbon dioxide (CO2) and Sulfur Dioxide (SO2) are the most
common gases found in the magma. Theses gaseous components also known
as the volatiles are dissolved within the melt.

 Volatiles remain a part of the magma until the magma body crystallizes or it
moves near the surface (low pressure), at which time any remaining volatile
freely moves away.
CHARACTERISTICS OF THE MAGMA
 Temperature: The temperature of magma varies from about
600ºC to 1400ºC, depending on its chemical composition
and the depth at which it forms.

 Generally, basaltic magma forms at great depth and has a

temperature near the high end of this scale.

 Granitic magmas, which form at shallower depths, tend to

lie near the cooler end of the scale.

Composition Source Material Viscosity Temperature

Basaltic Magma Upper Mantle Low 1200 °C
Andesitic Magma Oceanic Crust Inetermediate 800-1000 °C
Granitic Magma Continental Crust High 750-850 °C
CHARACTERISTICS OF THE MAGMA
 Chemical Composition: Because oxygen and silicon are the two most
abundant elements in the crust and mantle, nearly all magmas are
silicate magmas.

 In addition to oxygen and silicon, they also contain lesser amounts of

the six other common elements of the Earth’s crust: aluminum, iron,
magnesium, calcium, potassium, and sodium.

 The main variations among different types of magmas are

differences in the relative proportions of these eight elements.

 For example, basaltic magma contains more iron and magnesium

than granitic magma, but granitic magma is richer in silicon,
potassium, and sodium. A few rare magmas are of carbonate
composition.
EVOLUTION OF MAGMA
 A large variety of Igneous rocks are found on the earth but a single magma can lead to the formation of different
types of igneous rocks.

 This idea was first investigated by geologist Norman L. Bowen (1887–1956).

 The order of crystallization of the different minerals from the magma came to be known as the Bowen Reaction series
which allows geologists to predict chemical composition and texture based upon the temperature of a cooling
magma.
EVOLUTION OF MAGMA
 Bowen's reaction series is usually diagramed as a "Y" with horizontal lines drawn across the "Y

 The horizontal temperature lines divide the "Y" into four compositional sections.

 Mineral formation is not possible above 1800°C.

 Between 1100°C and 1800°C, rocks are ultramafic in composition.

 Between 900°C and 1100°C, rocks are mafic in composition.

 Between 600°C and 900°C, rocks are intermediate in composition. Below 600°C, felsic rocks form.

 The upper arms of the "Y" represent two different series. By convention, the left upper arm represents
the discontinuous series. The upper right arm represents the continuous series.

 The continuous series describes the evolution of the plagioclase feldspars as they evolve from being
calcium-rich to more sodium-rich.

 The discontinuous series describes the formation of the mafic minerals olivine, pyroxene, amphibole,
and biotite mica. These minerals are associated with the mafic and intermediate types of rocks.

 At lower temperatures, the branches merge and we obtain the minerals common to the felsic rocks -
orthoclase feldspar, muscovite mica, and quartz.
BOWEN REACTION SERIES
 EXTRUSIVE AND INTRUSIVE IGNEOUS ROCKS
 Igneous rocks which are formed when the molten magma solidifies at the earth’s
surface are known as Extrusive Igneous Rocks or Volcanic Rocks.

 Sometimes the magma loses its mobility before reaching the surface and
crystallizes at depths. Igneous rocks which are formed by the crystallization of the
molten magma beneath the earth’s surface or at depths are known as Intrusive
Igneous Rocks or Plutonic Rocks.
TEXTURE OF IGNEOUS ROCKS
 The texture of a rock refers to the size, shape, and
arrangement of its mineral grains, or crystals.

 Some igneous rocks consist of mineral grains that

are too small to be seen with the naked eye;
others are made up of thumb-size or even larger
crystals.

 Volcanic /extrusive igneous rocks are usually fine

grained, whereas plutonic/intrusive igneous rocks
are medium or coarse grained.
FACTORS AFFECTING CRYSTAL SIZE

 A magma at depths, slowly loses its heat to the

surrounding. The cooling of the magma may take tens or
even thousands of years as fewer but large crystals are
formed.

 On the other hand if the magma cools rapidly for example

on the earth’s surface, the crystals do not get time to
increase in size and the result is a rock with small
intergrown crystals.

 Sometimes the magma cools so rapidly that the ions do

not get time to arrange themselves and the magma
solidifies to form glass. There is no internal arrangement
of ions in Glass.
FACTORS AFFECTING CRYSTAL SIZE
 Three factors influence the
textures of igneous rocks:

 (1) the rate at which molten

rock cools;

 (2) the amount of silica

present and

 (3) the amount of dissolved

gases in the magma.

 Among these, the rate of

cooling tends to be the
dominant factor.
APHANITIC TEXTURE
 When the magma reaches the
earth’s surface, it undergoes
rapid cooling as a result the
crystals do not get a chance
to grow in size.

 The resulting igneous rocks

have a fine grained texture
which is known as Aphanitic
texture.

 The mineral grains in rocks

having Aphanitic texture are
so small that they can be
seen only with the aid of a
microscope
PHANERITIC TEXTURE
 Phaneritic textured rocks are
comprised of large crystals that
are clearly visible to the eye
with or without a hand lens or
binocular microscope.

 The entire rock is made up of

large crystals, which are
generally 1/2 mm to several
centimeters in size; no fine
matrix material is present.

 This texture forms by slow

cooling of magma deep
underground in the plutonic
environment.
PORPHYRITIC TEXTURE
 If magma rises slowly through
the crust before erupting,
some crystals may grow while
most of the magma remains
molten.

 If this mixture of magma and

crystals then erupts onto the
surface, it solidifies quickly,
forming porphyry, a rock with
the large crystals, called
phenocrysts, embedded in a
fine-grained matrix/ground
mass. Such a texture is known
as porphyritic texture
GLASSY TEXTURE
 Glassy textured igneous rocks
are non-crystalline meaning the
rock contains no mineral grains.

 Glass results from cooling that

is so fast that minerals do not
have a chance to crystallize.

 This may happen when magma

or lava comes into quick contact
with much cooler materials near
the Earth's surface.

 Pure volcanic glass is known as

obsidian.
VESICULAR TEXTURE
 Vesicular texture refers to
vesicles (holes, pores, or
cavities) within the igneous
rock.

 Vesicles are the result of gas

expansion (bubbles), which
often occurs during volcanic
eruptions.

 Pumice and scoria are

common types of vesicular
rocks.

 The image to the right shows

a basalt with vesicles, hence
the name "vesicular basalt".
PYROCLASTIC TEXTURE
 Some igneous rocks are
formed from the
consolidation of individual
rock fragments that are
ejected during a volcanic
eruption.

 These particles may consist

of fine ash, large angular
blocks or molten blobs.

 The rocks composed of

these rock fragments are
said to have a pyroclastic
texture.
PEGMATITIC TEXTURE
 Under special conditions
certain course grained
igneous rocks called
pegmatites are formed.

 These rocks which are

composed up of
interlocking crystals
which are generally
more than a centimeter
in diameter are said to
have a pegmatitic
texture.
TEXTURE OF IGNEOUS ROCKS
IGNEOUS ROCKS COMPOSITION

 Igneous rocks are composed up of silicate minerals. Chemical analysis shows that
Silica (Si) and Oxygen (O) is the most abundant constituent of igneous rocks.

 As the magma cools it solidifies to form the two major group of silicate minerals.

 The dark or (ferromagnesian) silicates are rich in iron, magnesium and are low in
silica.

 Olivine, pyroxene, amphibole and biotite mica is common dark silicate minerals
found on the earth.

 The light (nonferromagnesian) silicates contain greater amounts of potassium,

sodium and calcium.

 These minerals are also rich in silica content. The light silicate minerals include
feldspars, muscovite mica and quartz.
FERROMAGNESIAN AND NON-FERROMAGNESIAN
MINERALS
NAMING IGNEOUS ROCKS
 Geologists use both the minerals and texture to classify
and name igneous rocks.

 The various igneous textures result mainly from the

different cooling histories, whereas the mineral
composition of an igneous rock is the result of the
chemical makeup of the parent magma.

 Two igneous rocks having the same mineral composition

but different textures will have a different name.
CLASSIFICATION OF IGNEOUS ROCKS
MINERALOGY OF COMMON IGNEOUS ROCKS
FELSIC (GRANITIC) IGNEOUS ROCKS
 Granite is the best known of all
igneous rocks.

 Granite (and metamorphosed

granitic rocks) are the most
common rocks in continental
crust.

 Granite has a phaneritic texture

and is composed up of 25
percent Quartz and about 65
percent feldspar mostly the
potassium and sodium rich
varieties.

 The remaining 10 percent is

made up of muscovite, biotite
and some amphibole.
FELSIC (GRANITIC) IGNEOUS ROCKS
 Rhyolite is the extrusive
equivalent of granite
and like granite is
composed essentially of
light colored silicate
minerals.

 Rhyolite has an
aphanitic texture and
frequently contains
glass fragments and
voids indicating rapid
cooling in the surface
environment.
FELSIC (GRANITIC) IGNEOUS ROCKS
 Obsidian is a dark colored
glass rock that usually
forms when silica rich lava
is quenched quickly.

 This means that there is

no crystals formation in
obsidian and it consists of
unordered ions.

 Though obsidian appears

dark in color but its
chemical composition is
similar to that of granites.
FELSIC (GRANITIC) IGNEOUS ROCKS
 Pumice is a volcanic
igneous rock that like
obsidian has a glassy
texture but is formed
when large amounts of
gas escape through
lava.

 Because of the large

percentage of voids,
many samples of
pumice will float when
placed in water.
INTERMEDIATE (ANDESITIC) IGNEOUS ROCKS

 Andesite is a volcanic rock

intermediate in composition between
basalt and granite.

 It is commonly gray or green and

consists of plagioclase and dark
minerals (usually biotite, amphibole,
or pyroxene).

 Andesite is a volcanic rock and is

typically very fine grained.

 Andesite and Rhyolite sometimes

appear similar but microscopic
examination shows that Rhyolite is
composed up of about 25 percent
quartz whereas Andesite contains
only minor amount of Quartz.
INTERMEDIATE (ANDESITIC) IGNEOUS ROCKS
 Diorite is the plutonic equivalent
of andesite.

 It is a coarse grained intrusive

igneous rock that forms from the
same magma as andesite.

 It can be distinguished from

granite by the absence of visible
Quartz crystals and because it
contains a higher percentage of
dark silicate minerals.

 The mineral makeup of diorite is

primarily sodium rich plagioclase
feldspar and amphibole, with
lesser amounts of biotite.
MAFIC (BASALTIC) IGNEOUS ROCKS

 Basalt is a very dark green to

black fine-grained volcanic
rock.

 It is composed primarily of
pyroxene and calcium-rich
plagioclase feldspar, with
lesser amounts of olivine and
amphibole present.

 Basalt is the most common

extrusive igneous rock.
MAFIC (BASALTIC) IGNEOUS ROCKS
 Gabbro is the intrusive
equivalent of basalt.

 Like basalt, it is very dark

green to black in color and
composed primarily of
pyroxenes and calcium
rich plagioclase feldspars.

 Gabbro is uncommon at
the Earth’s surface,
although it is abundant in
deeper parts of oceanic
crust, where basaltic
magma crystallizes slowly.
ULTRAMAFIC IGNEOUS ROCKS

 Peridotite is an
ultramafic igneous rock
that makes up most of
the upper mantle but is
rare in the Earth’s crust.

 It is coarse grained and

composed of olivine,
and it usually contains
pyroxene, amphibole, or
mica but no feldspar.
NAMING IGNEOUS ROCKS
IGNEOUS ROCK CLASSIFICATION
UNIT 4

SEDIMENTARY ROCKS
WHAT ARE SEDIMENTS
 Sediments are loose Earth materials (unconsolidated materials) such
as sand which are transported by the action of water, wind, glacial ice
and gravity.

 These materials are accumulate on the land surface, (such as in river

and lake beds), and / or on the ocean floor and form sedimentary
rocks
FORMATION OF SEDIMENTARY ROCKS
 Weathering begins the process. It involves the physical disintegration and chemical
decomposition of preexisting igneous, metamorphic, and sedimentary rocks.

 Then they are eroded from the site of weathering and moved downslope by gravity, a
process termed mass wasting and are transported by wind, water, ice, and mass
wasting.

 Transportation moves these materials from the sites where they originated to
locations where they accumulate.

 Finally sediment settles out and accumulates after transport : This process is known
as deposition.

 As deposition continues, older sediments are buried beneath younger layers and are
gradually converted to sedimentary rock by compaction and cementation. This and
other changes are referred to as diagenesis (Changes that take place in texture,
composition, and other physical properties after sediments are deposited).
 FORMATION OF SEDIMENTARY ROCKS

 WEATHERING

 TRANSPORTATION

 DEPOSITION

 DIAGENESIS

 COMPACTION
 CEMENTATION
WHAT ARE SEDIMENTARY ROCKS
 The loose sediments after
their deposition become
compact and hard to form
sedimentary rock.

 Sedimentary rocks make

up only about 5 percent of
the Earth’s crust but since
they are formed on the
Earth’s surface, they cover
about 75 percent of
continents.
DIAGENESIS
 Diagenesis refers to all of the physical,
chemical, and biological changes that occur
after sediments are deposited and during and
after the time they are turned into
sedimentary rock.

 Diagenesis includes lithification.

 Burial promotes diagenesis because as

sediments are buried, they are subjected to
increasingly higher temperature and pressure.

 Diagenesis occurs within the upper few

kilometers of earth’s crust as temperatures
that are generally less than 150 degree C to
200 degree C.

 Lithification refers to processes that convert

loose sediment to hard rock

 Two of the most important processes involved

in lithification are compaction and
cementation.
TYPES OF SEDIMENTARY ROCKS
 Because there are a variety of ways that the products of weathering are transported,
deposited, and transformed into solid rock, three categories of sedimentary rocks
are recognized.

 As the overview reminded us, sediment has two principal sources. First, it may be an
accumulation of material that originates and is transported as solid particles derived
from both mechanical and chemical weathering. Deposits of this type are termed
detrital, and the sedimentary rocks that they form are called detrital sedimentary
rocks.

 The second major source of sediment is soluble material produced largely by

chemical weathering. When these ions in solution are precipitated by either
inorganic or biologic processes, the material is known as chemical sediment, and
the rocks formed from it are called chemical sedimentary rocks.

 The third category is organic sedimentary rocks. The primary example is coal. This
black combustible rock consists of organic carbon from the remains of plants that
died and accumulated on the floor of a swamp.
CLASTIC/DETRITAL SEDIMENTARY ROCKS
 Detrital sedimentary rocks consists of grains and particles that were eroded
from weathered rocks and then were transported and deposited in loose,
unconsolidated layers at the Earth’s surface.

 Detrital sediments are named according to particle size.

 Gravel includes all rounded particles larger than 2 millimeters in diameter.

 Sand ranges from 1/16 to 2 millimeters in diameter.

 Silt varies from 1/256 to 1/16 millimeter.

 Clay is less than 1/256 millimeter in diameter. Mud is wet silt and clay.
 PARTICLE SIZE
CLASSIFICATION OF DETRITAL
ROCKS
COMMON DETRITAL SEDIMENTARY ROCKS
 Conglomerate consists largely
of gravels.

 The particle size in

conglomerate varies from 2
mm to more than 256 mm.

 In a conglomerate the
particles are rounded.

 Conglomerates are poorly

sorted and the openings
between the particles are
filled with sand or mud.
COMMON DETRITAL SEDIMENTARY ROCKS

 If the large particles are angular rather than

rounded, the rock is called breccia.
COMMON DETRITAL SEDIMENTARY ROCKS
 Sandstone is the name given to
any rock in which sand size
particles are dominant.

 Most sandstones are quartz

sandstone and contain more
than 90 percent quartz.

 Arkose is a sandstone
comprising 25 percent or more
feldspar grains, with most of
the remaining grains being
quartz.

 Graywacke is poorly sorted

sandstone with considerable
quantities of silt and clay in its
pores.
 COMMON DETRITAL SEDIMENTARY ROCKS

 Shale is a sedimentary rock

consisting of silt and clay sized
particle. They consist of clay
minerals and small amount of
quartz.

 The tiny particles in shale indicate

that deposition has taken place in
water in a very quiet and non
turbulent environment.

 Shale has a finely layered structure

called fissility, along which the rock
splits easily.

 The layered structure in shale is

also sometimes known as laminae.
COMMON DETRITAL SEDIMENTARY ROCKS

 Mudstone also has the same particle size as

that of shale but is a non-fissile and breaks
as chunks or blocks.
COMMON DETRITAL SEDIMENTARY ROCKS
 Siltstone is lithified silt.

 The main component of

most siltstones is
quartz, although clays
are also commonly
present.

 Siltstones often show

layering but lack the fine
fissility of shales
because of their lower
clay content.
PARTICLE SIZE AND SHAPE OF DETRITAL SEDIMENTARY ROCKS

 Particle size of a detrital sedimentary rock

indicates the depositional environment and the
strength of the transporting water current.

 Shallow environment and strong currents will

result in the formation of coarse grained
sedimentary rocks.

 Calm and deep depositional environment will help

in the formation of fine grained sedimentary rocks.
PARTICLE SIZE AND SHAPE OF DETRITAL SEDIMENTARY ROCKS

 Particle shape in detrital sedimentary rocks determines how far the particles travelled before getting
deposited to form sedimentary rocks.

 Round particles will indicate long transportation where as angular particles indicate a short
transportation history.
CHEMICAL SEDIMENTARY ROCKS
 In contrast to detrital sedimentary rocks which form from the solid
product of weathering, chemical sedimentary rocks are formed from
materials that is carried in solution to lakes and seas.

 Chemical sediments form from water which are saturated with

dissolved cations and anions. Crystallization occurs when these ions
develop covalent or ionic bonds and thus create chemical
compounds, producing minerals such as calcite and salt.

 The precipitation of the material can take place in two ways.

1. Organic processes such as activities of water dwelling organisms.

2. Inorganic processes such as evaporation and chemical activities.

CHEMICAL SEDIMENTARY ROCKS
 Limestone represents
about 10 percent of the
total volume of all
sedimentary rocks and is
the most abundant in
chemical sedimentary
rocks.

 It is composed mainly of
the mineral calcite
(CaCO3) and can be
formed either by inorganic
means or as a result of
biochemical processes.
CHEMICAL SEDIMENTARY ROCKS
 Dolostone is closely related to
limestone and consists mainly
of the mineral dolomite which
is calcium magnesium
carbonate.

 Although dolostone can form

by direct precipitation from
seawater, it is thought that
most of them are formed
when magnesium in the sea
water replaces some of the
calcium in limestones.
CHEMICAL SEDIMENTARY ROCKS
 Coquina is bioclastic limestone
consisting wholly of coarse
shell fragments cemented
together.

 Chalk is a very fine-grained,

soft, white bioclastic limestone
made of the shells and
skeletons of microorganisms
that float near the surface of
the oceans.

 When they die, their remains

sink to the bottom and
accumulate to form chalk.
CHEMICAL SEDIMENTARY ROCKS
 Chert is a name used for a
number of very compact
and hard rock made up of
microcrystalline silica.

 Microscopic examination
of bedded chert often
shows that it is made up
of the remains of tiny
marine organisms that
make their skeletons of
silica rather than calcium
carbonate.
CHEMICAL SEDIMENTARY ROCKS

 Evaporites form when

evaporation concentrates
dissolved ions to the point at
which they precipitate from
solution.

 The most common minerals

found in evaporite deposits
are gypsum (CaSO4.2H2O)
and halite (NaCl).
CHEMICAL SEDIMENTARY ROCKS

 Corals are an example

of organisms that are
capable of creating
large quantities of
marine limestones.

 Corals are capable of

forming massive
structures which are
known as coral reefs.
ORGANIC SEDIMENTARY ROCKS
 When plants die, their remains
usually decompose by reaction
with oxygen.

 However, in warm swamps and in

other environments where plant
growth is rapid, dead plants
accumulate so rapidly that the
oxygen is used up long before the
decay process is complete.

 The undecayed or partially

decayed plant remains form
peat.

 As peat is buried and compacted

by overlying sediments, it
converts to coal, a hard, black,
combustible rock.
SEDIMENTARY STRUCTURES

 Nearly all sedimentary rocks contain

sedimentary structures, features that
developed during or shortly after deposition of
the sediment.

 These structures help us understand how the

sediment was transported and deposited.
BEDDING OR STRATIFICATION
 Sedimentary rocks form as layer
upon layer of sediment
accumulates in various
depositional environments.

 These layers, called strata or beds,

are probably the single most
common and characteristic feature
of sedimentary rocks.

 Each stratum/layer is unique. The

variations in texture, composition,
and thickness reflect the different
conditions under which each layer
was deposited.
CROSS BEDDING AND GRADED BEDDING
 Sediments usually accumulate as particles that
settle from a fluid, most strata are originally
deposited as horizontal layers.

 There are circumstances, however, when sediments

do not accumulate in horizontal beds and are
inclined to the horizontal.

 When this occurs, it is called cross-bedding and is

most characteristic of sand dunes, river deltas.

 Graded beds represent another special type of

bedding. In this case the particles within a single
sedimentary layer gradually change from coarse at
the bottom to fine at the top.

 Graded beds are most characteristic of rapid

deposition from water containing sediment of
varying sizes.
RIPPLE MARKS
 Ripple marks are small, nearly parallel
sand ridges and troughs that are also
formed by moving water or wind.

 They are like dunes and sand waves,

but smaller.

 If the water or wind flows in a single

direction, the ripple marks become
asymmetrical, like miniature dunes.

 In other cases, waves move back and

forth in shallow water, forming
symmetrical ripple marks in bottom
sand.

 Ripple marks are often preserved in

sandy sedimentary rocks.
MUD CRACKS

 Mud cracks are

polygonal cracks that
form when mud
shrinks as it dries.

 They indicate that the

mud accumulated in
shallow water that
periodically dried up.
FOSSILS
 Fossils are another feature of the
sedimentary rocks that are formed
during the time of deposition of
these rocks.

 Fossils are the remains or

impressions of plants or animals that
were persevered in the crust of the
earth due to natural causes.

 They are important tools for

interpreting the geologic past.

 Knowing the nature of the life forms

that existed at a particular time
helps researchers decipher past
environmental conditions.

 Further, fossils are important time

indicators and play a key role in
correlating rocks that are of similar
ages but are from different places.
UNIT - 5

METAMORPHIC ROCKS
WHAT IS METAMORPHISM
 Metamorphism (from the Greek words for “changing form”) is the process by
which rising temperature and changes in other environmental conditions
transform rocks and minerals.

 In other word metamorphism is the transformation of one rock type into

another.

 Metamorphism takes place where the pre-existing rocks are subjected to

temperature and pressure unlike those in which they were formed.

 In response to these new conditions the rock gradually changes until it reaches
a state of equilibrium with the new environment.

 Metamorphism is always gradual and it ranges from low grade metamorphism

to high grade metamorphism.

 However rock should always be in the solid state during metamorphism.

 If the rocks melts at a certain point, then we enter the zone of igneous activity.
ENVIRONMENT OF METAMORPHISM
 Most metamorphism occurs in one of the three settings:

 Contact or thermal metamorphism occurs where hot magma

intrudes cooler country rock. The country rock may be of any
type—sedimentary, metamorphic, or igneous.

 The highest-grade metamorphic rocks form at the contact,

closest to the magma. Lower-grade rocks develop farther out.

 The change is driven by the rise in temperature within the host

rock surrounding an igneous intrusion.

 Most contact metamorphic rocks are fine-grained, dense,tough

rocks of various chemical compositions.

 Because directional pressure is not a major factor, these rocks

are not generally foliated.

 Hydrothermal metamorphism (also called hydrothermal

alteration and metasomatism) occurs when hot water and ions
dissolved in the hot water react with a rock to change its
chemical composition and minerals.

 Regional metamorphism occurs during the process of mountain

building, great quantities of rocks are subjected to directed
pressure and high temperatures associated with large scale
deformation.
AGENTS OF METAMORPHISM
 Heat contributes to the process in two ways. First, atoms may
combine differently at different temperatures.

 This means that a mineral stable at one temperature might become

unstable at a higher (or lower) temperature and be converted to a
different mineral with a more stable atomic structure.

 This may or may not involve changing the exact elemental

composition.

 Second, heat makes practically all chemical reactions go faster,

meaning that mineral transformations are much easier at higher
temperature.
AGENTS OF METAMORPHISM
 Pressure also has two effects. As with heat, it can also control which minerals or
forms of minerals are stable.

 Some minerals may be converted to minerals with similar composition but different
atomic packing simply because pressure is increased.

 The exact nature of the pressure is not important in this case. Only the amount is
important. Thus the confining or lithostatic pressure created by deep burial of rocks
under sediment may have this effect as well as the directed pressure during
mountain building processes.

 The second effect of pressure is to reorient minerals with linear or platy structure or
to create a preferred orientation of them as they form.

 Thus elongate minerals such as amphiboles, or platy minerals such as clays or

micas tend to align themselves parallel to each other when under pressure.

 This only happens when there is directed pressure; confining pressure does not
accomplish it.
AGENTS OF METAMORPHISM

Confining Pressure and Directed Pressure

AGENTS OF METAMORPHISM

 The diagram illustrates the effect. A texture of

this sort in a metamorphic rock is called
foliation and the rocks are said to be foliated.
AGENTS OF METAMORPHISM

 Fluids serve only to speed up other metamorphic processes, or

perhaps even allow them to happen at all.

 Chemical reactions require water, and most proceed much faster

as the amount of water goes up.

 Dissolved ions in the fluid also make those mineral

transformations that require chemical changes in the minerals to
occur, whether by supplying needed ions or flushing away excess
ones.
GRADE OF METAMORPHISM
 Different locations in the crust experience different levels of heat and
pressure as result the rocks may experience different grades of
metamorphism.

 The changes that occur during metamorphism are recorded in the form of
texture and mineral assemblages.

 High grade metamorphic rocks are greatly altered from its original form and
often have a completely different mineralogy than the parent rock.
INDEX MINERALS
 Through the study of metamorphic rocks it has been found that some minerals are good indicators of
the metamorphic environment in which they formed. These minerals are known as index minerals.

 Using these index minerals, geologists distinguish among different zones of regional metamorphism.

 For example, the mineral chlorite begins to form when temperatures are relatively low, less than 200
°C Thus, rocks that contain chlorite are referred to as low-grade.

 By contrast, the mineral sillimanite only forms in extreme environments where temperatures exceed
500 °C, and rocks containing it are considered high-grade.

 Quartz and Feldspar also appear

in metamorphic products but
since they are found in both low
and high grade metamorphic
rocks, they are not considered
as index minerals.
METAMORPHIC ROCK TEXTURES
 Metamorphic rocks exhibit a variety of textures. They can
be either foliated or granular.

 Foliation refers to any planar arrangement of mineral

grains or structural features within a rock.

 Most metamorphic rocks form in the influence of a

directed stress field. Because of this they develop
conspicuous directional textures.

 As metamorphism proceeds, the sheet structure silicates

(flat minerals with basal cleavage) such as mica (biotite
and muscovite) and chlorite start to grow.

 The sheets orient themselves perpendicular to the

direction of maximum stress.

 The new parallel mineral flakes produce a planar texture

called foliation. (from the Latin folium - leaf).

 Foliation can be subtle or pronounced depending on the

degree of metamorphism.
 METAMORPHIC ROCK TEXTURES
 The foliated textures develop in the sequence listed
below as temperature and pressure increases. Here
we just define the textures. Below are descriptions
and illustrations of how each texture develops.

 Slaty cleavage is formed as a result of parallel

foliation (layering) of fine-grained platy minerals
(chlorite) in a direction perpendicular to the
direction of maximum stress. Exmaples of such
rocks are Slate and Phyllite.

 Schistosity is formed as a result of the layering in a

coarse grained, crystalline rock due to the parallel
arrangement of platy mineral grains such as
muscovite and biotite.

 Other minerals present are typically quartz and

feldspar, plus a variety of other minerals such as
garnet, staurolite, kyanite, sillimanite.
METAMORPHIC ROCK TEXTURES

 Mineral Banding (Gneiss)

is the layering in a rock in
which bands or lenses of
granular minerals (quartz
and feldspar) alternate
with bands or lenses in
which platy (mica) or
elongate (amphibole)
minerals predominate.
METAMORPHIC ROCK TEXTURES
 Non foliated textures are formed around igneous
intrusions where the temperatures are high but
the pressures are relatively low and equal in all
directions (confining pressure).

 The original minerals within the rock recrystallize

into larger sizes and the atoms become more
tightly packed together, increasing the density of
the rock.

 Examples of such rock types are Quartzite and

Marble.
METAMORPHIC ROCK TEXTURES

 When the parent rock is composed only of a

single mineral, metamorphism changes the
rocks into one composed of the same mineral
but with a coarser texture.
 Example Limestone changes to Marble and
Quartz Sandstone changes to Quartzite
METAMORPHIC ROCK TEXTURES
 In contrast, metamorphism of a parent rock containing
several minerals usually forms a rock with new and
different minerals and a new texture.

 For example, a typical shale contains large amounts of

clay, as well as quartz and feldspar.

 When heated, some of those minerals decompose, and

their atoms recombine to form new minerals such as
mica, garnet, and a different kind of feldspar to form a
rock called hornfels which has a different texture and as
well as minerals than shale.
COMMON METAMORPHIC ROCKS

 Metamorphic Rocks
are divided into two
basic divisions

 1. Foliated/Banded

 2.Non-Foliated (also,
granular or
equidimensional)
FOLIATED METAMORPHIC ROCKS
 Slate is a fine grained (less than 0.5 mm)
foliated rock composed of mica flakes.

 Slate is dull colored and closely resembles

shale.

 The most important characteristic of shale is

its tendency to break into flat slabs.

 Slate is generally formed by low grade

metamorphism of shale, mudstone or
siltstone.

 Color of slate depends upon its mineral

composition.

 Black slate contains organic material. Red

slate contains Iron Oxide.

 Green slate contains chlorite.

FOLIATED METAMORPHIC ROCKS
 Phyllite: It represents a degree of
metamorphism in between slate and
schist.

 Its constituent platy minerals are larger

than those in slate but still not large
enough to be identified with the naked
eye.

 Phyllite appears similar to slate but

can be distinguished from slate by its
glossy sheen and wavy surface.

 Phyllite also breaks as a flat surface

and is composed of fine crystals of
muscovite or chlorite or both.
FOLIATED METAMORPHIC ROCKS
 Schists are medium to coarse grained metamorphic
rocks in which platy minerals predominate.

 These minerals include muscovite and biotite.

 These platy minerals are arranged in a planar fashion

that gives the rock its foliated texture.

 In addition to mica, schist also contains other

minerals such as quartz and feldspars.

 Like slate, the parent rock for many schists are also
shale which has undergone medium to high grade
metamorphism during the process of mountain
building.

 Schist is the name given to the texture of the

metamorphic rock.

 To indicate the composition minerals names are used

such as mica scist, talc schist, chlorite schist.
FOLIATED METAMORPHIC ROCKS
 Gneiss is the term applied to medium or
coarse grained banded metamorphic rocks
in which granular and elongated minerals
predominate.

 The most common minerals in gneiss are

Quartz. Potassium feldspar, Na feldspar.

 Gneiss also contains smaller amounts of

biotite, muscovite and amphibole that
develop a preferred orientation.

 During the high grade metamorphism the

dark and light colored minerals segregate
giving the gneisses their typical banded or
layered appearance.

 Most gneisses have a felsic composition,

however some of them are also formed by
the high grade metamorphism of shale.
FOLIATED METAMORPHIC ROCKS
NON-FOLIATED METAMORPHIC ROCKS
 Marble: It is a coarse grained crystalline rock whose parent rock
was limestone or dolostone.

 Pure marble is white and is composed entirely of the mineral

calcite.

 The parent rocks from which marbles are formed often contain
some impurities and this imparts color to marble.

 Marble can be pink, gray or even black in color.

 Quartzite: It is a very hard metamorphic rock formed from quartz

sandstone.

 They are formed under moderate to high grade metamorphism.

 The crystals of quartz fuse together when they undergo

metamorphism and as result the crystal size for quartzite is
much bigger than its parent rock.
UNIT 6

PLATE TECTONICS
 CONTINENTAL DRIFT
 Alfred Wegner proposed the theory that the
crustal plates are moving over the mantle.

 He argued that today’s continents once

formed a single landmass, called Pangaea
(Greek for "all land").

 It broke into pieces due to the weaknesses in

the earth's crust as they were made up of
less dense materials, which drifted
centimeter by centimeter over millions of
years until they arrived at where they are
now.

 This was supported by fossil and rock type

evidence; also matching of coastline shapes.
CONTINENTAL DRIFT
EVIDENCES OF CONTINENTAL DRIFT

The Continental Jigsaw Puzzle

Fossils Match across the Seas

Rock Types and Geologic Features

Ancient Climates
DRAWBACKS OF CONTINENTAL DRIFT
HYPOTHESIS
 One of the main objections to Wegener’s hypothesis was his inability to identify a
credible mechanism for continental drift.

 Wegener proposed that gravitational forces of the Moon and Sun that produce
Earth’s tides were also capable of gradually moving the continents across the globe.

 However, the prominent physicist Harold Jeffreys correctly countered that tidal forces
of the magnitude needed to displace the continents would bring Earth’s rotation to a
halt in a matter of a few years.

 Wegener also incorrectly suggested that the larger and sturdier continents broke
through thinner oceanic crust, much like ice breakers cut through ice.

 However, no evidence existed to suggest that the ocean floor was weak enough to
permit passage of the continents without the continents being appreciably deformed
in the process.
THE PLATE TECTONIC THEORY
 By 1968, a far more encompassing theory than continental drift, known as plate tectonics.

 According to the plate tectonics model, the uppermost mantle and the overlying crust behave as a
strong, rigid layer, known as the lithosphere, which is broken into segments commonly referred to as
plates.

 The lithosphere is thinnest in the oceans where it varies from as little as a few kilometers along the
axis of the oceanic ridge system to about 100 kilometers in the deep-ocean basins.

 By contrast, continental lithosphere is generally thicker than 100 kilometers and may extend to a
depth of 200 to 300 kilometers beneath stable continental cratons.

 The lithosphere, in turn, overlies a weak region in the mantle known as the asthenosphere.

 The temperatures and pressures in the upper asthenosphere (100 to 200 kilometers in depth) are
such that the rocks there are very near their melting temperatures and, hence, respond to stress by
flowing.

 As a result, Earth’s rigid outer shell is effectively detached from the layers below, which permits it to
move independently.
THE EARTH LAYERS
THE EARTH LAYERS
 Crust: The crust is the outermost and thinnest layer. Because the crust is relatively cool, it consists of
hard, strong rock. Crust beneath the oceans differs from that of continents.

 Oceanic crust is 5 to 10 kilometers thick and is composed mostly of a dark, dense rock called basalt.

 In contrast, the average thickness of continental crust is about 20 to 40 kilometers, although under
mountain ranges it can be as much as 70 kilometers thick.

 Continents are composed primarily of a light-colored, less dense rock called granite.
 THE EARTH LAYERS
 Mantle: The mantle lies directly
below the crust.

 It is almost 2900 kilometers thick

and makes up 80 percent of the
Earth’s volume.

 Although the chemical

composition may be similar
throughout the mantle, Earth
temperature and pressure
increase with depth.

 These changes cause the

strength of mantle rock to vary
with depth, and thus they create
layering within the mantle.

 The upper part of the mantle

consists of two layers.
 THE EARTH LAYERS
 The Lithosphere: The uppermost mantle
is relatively cool and consequently is
hard, strong rock.

 The outer part of the Earth, including

both the uppermost mantle and the
crust, make up the lithosphere (Greek
for “rock layer”).

 The lithosphere can be as thin as 10

kilometers where tectonic plates
separate.

 However, in most regions, the

lithosphere varies from about 75
kilometers thick beneath ocean basins
to about 125 kilometers under the
continents.

 A tectonic (or lithospheric) plate is a

segment of the lithosphere.
THE EARTH LAYERS
 The Asthenosphere: At
a depth varying from
about 75 to 125
kilometers, the strong,
hard rock of the
lithosphere gives way
to the weak, plastic
asthenosphere.

 The asthenosphere
extends from the base
of the lithosphere to a
depth of about 350
kilometers.
THE EARTH LAYERS
 The Core: The core is the innermost
of the Earth’s layers.

 It is a sphere with a radius of about

3470 kilometers and is composed
largely of iron and nickel.

 The outer core is molten because

of the high temperature in that
region.

 Near its center, the core’s

temperature is about 6000ºC, as
hot as the Sun’s surface.

 The pressure is greater than 1

million times that of the Earth’s
atmosphere at sea level.

 The extreme pressure overwhelms

the temperature effect and
compresses the inner core to a
solid.
THE EARTH LAYERS
PLATES AND PLATE TECTONICS

 In most places, the lithosphere is less dense than the asthenosphere.

Consequently, it floats on the asthenosphere much as ice floats on water.

 The lithosphere is broken into seven large tectonic plates and several
smaller ones.

 The plates move slowly, at rates ranging from less than 1 to about 16
centimeters per year.

 The great forces generated at a plate boundary build mountain ranges and
cause volcanic eruptions and earthquakes.

 These processes and events are called tectonic activity.

TECTONIC PLATES
TYPES OF PLATE BOUNDARIES
 Neighboring plates can move relative to
one another in three different ways.

 Divergent boundaries (Constructive)

occur where two plates slide apart from
each other. (A)

 Convergent boundaries (Destructive) (or

active margins) occur where two plates
slide towards each other commonly
forming either a subduction zone (if one
plate moves underneath the other) or a
continental collision (if the two plates
contain continental crust). (B)

 Transform boundaries (Conservative)

occur where plates slide or, perhaps
more accurately, grind past each other
along transform faults. (C)
TYPE OF PLATE BOUNDARIES
TYPE OF PLATE BOUNDARIES
TYPE OF PLATE BOUNDARIES
DIVERGENT PLATE BOUNDARIES

 At a divergent plate boundary, also

called a spreading center and a rift
zone, two lithospheric plates spread
apart.

 The underlying asthenosphere then

rises upward to fill the gap between
the separating plates.

 As the asthenosphere rises between

separating plates, some of it melts to
form molten rock called magma.

 Most of this activity occurs beneath

the seas because most divergent
plate boundaries lie in the ocean
basins.
THE MID-OCEANIC RIDGE: RIFTING IN THE
OCEANS
 A spreading center lies
directly above the hot, rising
asthenosphere.

 The newly formed

lithosphere at an oceanic
spreading center is hot and
therefore of low density.

 As a results, the sea floor at

a spreading center floats to
a high elevation, forming an
undersea mountain chain
called the mid-oceanic ridge.
SPLITTING CONTINENTS: RIFTING IN
CONTINENTAL CRUST

 A divergent plate boundary can rip

a continent in half in a process
called continental rifting.

 A rift valley develops in a

continental rift zone because
continental crust stretches,
fractures, and sinks as it is pulled
apart.

 Continental rifting is now taking

place along a zone called the East
African rift
CONVERGENT PLATE BOUNDARIES
 At a convergent plate boundary, two lithospheric
plates move toward each other. Convergence
can occur in three different ways

 (1) between a plate carrying oceanic crust and

another carrying continental crust,

 (2) between two plates carrying oceanic crust,

and

 (3) between two plates carrying continental

crust.

 Differences in density determine what happens

where two plates converge. When two plates
converge, the denser plate dives beneath the
lighter one and sinks into the mantle. This
process is called subduction.

 A subduction zone is a long, narrow belt where a

lithospheric plate is sinking into the mantle.
CONVERGENCE OF TWO PLATES CARRYING
OCEANIC CRUST
 Newly formed oceanic lithosphere
is hot, thin, and light.

 As it spreads away from the mid-

oceanic ridge, it becomes older,
cooler, thicker, and denser.

 Thus the density of oceanic

lithosphere increases with its age.
When two oceanic plates
converge, the denser one sinks
into the mantle.

 Convergence of two oceanic

plates creates and island arc and
trench. e.g. Japan
CONVERGENCE OF OCEANIC CRUST WITH
CONTINENTAL CRUST
 When an oceanic plate
converges with a continental
plate, the denser oceanic plate
sinks into the mantle beneath
the edge of the continent.

 As a result, many subduction

zones are located at continental
margins.

 Convergent boundary of an
oceanic and continental plates
forms a volcanic mountain range
and trenches. e.g. Andes
Mountains
CONVERGENCE OF TWO PLATES CARRYING
CONTINENTS
 If two converging plates carry
continents, neither can sink into
the mantle because of their low
densities.

 In this case, the two continents

collide and crumple against each
other, forming a huge mountain
chain.

 The Himalayas, the Alps, and the

Appalachians all formed as
results of continental collisions.
TRANSFORM PLATE BOUNDARIES
 A transform plate boundary
forms where two plates slide
horizontally past one another
as they move in opposite
directions.

 California’s San Andreas fault

is the transform boundary
between the North American
plate and the Pacific plate.

 This type of boundary can

occur in both oceans and
continents.
CHARACTERISTICS OF LITHOSPHERIC PLATES
 A plate is a segment of the lithosphere; thus, it includes the uppermost mantle and
all of the overlying crust.

 A single plate can carry both oceanic and continental crust.

 A plate is composed of hard, mechanically strong rock.

 A plate floats on the underlying hot, plastic asthenosphere and glides horizontally
over it.

 A plate behaves like a large slab of ice floating on a pond. In general, however, each
plate moves as a large, intact sheet of rock.

 A plate margin is tectonically active. Earthquakes and volcanoes are common at

plate boundaries. In contrast, the interior of a lithospheric plate is normally
tectonically stable.

 Tectonic plates move at rates that vary from less than 1 to 16 centimeters per year.
CONSEQUENCES OF MOVING PLATES

 Volcanoes

 A volcanic eruption occurs where hot magma rises to the Earth’s surface.
 Volcanic eruptions are common at both divergent and convergent plate boundaries.

 Earthquakes

 Earthquakes are common at all three types of plate boundaries, but less common within the
interior of a tectonic plate.
 Quakes concentrate at plate boundaries simply because those boundaries are zones of deep
fractures in the lithosphere where one plate slips past another.

 Mountain Building

 Mountains can be formed both at divergent as well as convergent plate margins.

 Several processes combine to build a mountain chain at a subduction zone.
CONSEQUENCES OF MOVING PLATES
 Oceanic Trenches

 An oceanic trench is a long, narrow trough in the sea floor that develops where a
subducting plate sinks into the mantle.
 A trench can form wherever subduction occurs—where oceanic crust sinks beneath the
edge of a continent, or where it sinks beneath another oceanic plate.
 Trenches are the deepest parts of the ocean basins.
 The deepest point on Earth is in the Mariana trench in the southwestern Pacific Ocean,
where the sea floor is as much as 10.9 kilometers below sea level (compared with the
average sea-floor depth of about 5 kilometers).

 Migrating Continents and Oceans

 Continents migrate over the Earth’s surface because they are integral parts of the moving
lithospheric plates.
 Measurements of these movements show that North America is now moving away from
Europe at about 2.5 centimeters per year, as the mid-Atlantic ridge continues to separate.
 South America is drawing away from Africa at a rate of about 3.5 centimeters per year.
 As the Atlantic Ocean widens, the Pacific is shrinking at the same rate.
 Thus, as continents move, ocean basins open and close over geologic time.
FORCE BEHIND PLATE TECTONICS
 Mantle convection may cause plate
movement.

 Alternatively, a plate may move

because it slides downhill from a
spreading center, as its cold leading
edge sinks into the mantle and
drags the rest of the plate along.

 The concept that the lithosphere

floats on the asthenosphere is
called isostasy.

 When weight such as a glacier is

added to or removed from the
Earth’s surface, the lithosphere
sinks or rises.

 This vertical movement in response

to changing burdens is called
isostatic adjustment.
UNIT - 7

EARTHQUAKES
WHAT IS AN EARTHQUAKE
 An earthquake is a sudden motion or trembling of the Earth
caused by the abrupt release of energy that is stored in
rocks.

 Modern geologists know that most earthquakes occur along

plate boundaries, where huge tectonic plates separate,
converge, or slip past one another.

 Earthquakes also occur when rock slips along previously

established faults.

 Tectonic plate boundaries are huge faults that have moved

many times in the past and will move again in the future.
HOW EARTHQUAKE OCCURS
 Every rock has a limit beyond which it cannot
deform elastically.

 Under certain conditions, when its elastic limit

is exceeded, a rock continues to deform.

 This behavior is called plastic deformation.

 Earthquakes do not occur when rocks deform

plastically.

 Under other conditions, an elastically stressed

rock may rupture by brittle fracture.

 The fracture releases the elastic energy, and

the surrounding rock springs back to its
original shape.

 This rapid motion creates vibrations that travel

through the Earth and are felt as an
earthquake.
THE ELASTIC REBOUND THEORY
 The Elastic Rebound Theory explains how energy is stored in rocks

 Rocks bend until the strength of the rock is exceeded

 Rupture occurs and the rocks quickly rebound to an undeformed shape
 Energy is released in waves that radiate outward from the fault
THE FOCUS AND EPICENTER OF AN
EARTHQUAKE
• The point within Earth where faulting begins is the
focus, or hypocenter.

• The point directly above the focus on the surface is the

epicenter
WHAT ARE SEISMIC WAVES
 Waves that travel through rock are called seismic waves.

 Earthquakes and explosions produce seismic waves.

 Seismology is the study of earthquakes and the nature of the Earth’s interior based
on evidence from seismic waves.

 An earthquake produces several different types of seismic waves.

 Two types of waves are produced during an Earthquake

 Body waves
 P and S

 Surface waves
 R and L
BODY WAVES: P WAVES
 Two main types of body waves travel
through the Earth’s interior.

 P wave (also called a compressional

wave) is an elastic wave that causes
alternate compression and
expansion (rarefaction) of the rock.

 P waves travel through air, liquid, and

solid material.

 P waves travel at speeds between 4

and 7 kilometers per second in the
Earth’s crust and at about 8
kilometers per second in the
uppermost mantle.

 P waves are also known as Primary

waves.
BODY WAVES: S WAVES
 A second type of body wave, called an
S wave, is a shear wave.

 They are transverse waves meaning

that wave particles travel perpendicular
to the direction of the propagation of
the wave.

 S waves are slower than P waves and

travel at speeds between 3 and 4
kilometers per second in the crust.

 As a result, S waves arrive after P

waves.

 The S waves are also known as

Secondary waves.

 Unlike P waves, S waves move only

through solids.
SURFACE WAVES
 Surface waves travel more slowly than body waves. Two types of surface
waves occur simultaneously in the Earth.

 1. Rayleigh wave moves with an up-and down rolling motion like an

ocean wave.
 2. Love waves produce a side-to-side vibration.
MEASUREMENT OF SEISMIC WAVES
 A seismograph is the device that scientists use to measure earthquakes.

 The goal of a seismograph is to accurately record the motion of the ground during a
quake.

 A seismograph has a pen that is hanging in the air. The pen touches a roll of paper called
a drum. When an earthquake happens, the roll of paper shakes. The pen does not. A
weight holds the pen still.

 The marks on the paper show the size of the earthquake. A small motor rolls the drum of
paper. This lets the seismograph record what happens as time passes.

 A seismogram is a graph output by a seismograph. It is a record of the ground motion at a

measuring station as a function of time
MEASUREMENT OF EARTHQUAKE STRENGTH
 Earthquake intensity is a measure of the effects of an earthquake in a particular place.

 Modified Mercalli Scale is use for measuring the intensity of earthquakes, adapted from the
original Mercalli scale.

 The Mercalli scale was devised in 1902 by Italian seismologist Giuseppe Mercalli.

 American seismologists Harry O. Wood and Frank Neumann created the Modified Mercalli scale
in 1931 to measure the intensity of earthquakes that occur in California.

 The Modified Mercalli scale has 12 levels of intensity.

 Each level is defined by a group of observable earthquake effects, such as shaking of the
ground and damage to structures such as buildings, roads, and bridges.

 The levels are designated by the Roman numerals I to XII.

 Levels I through VI are used to describe what people see and feel during a small to moderate
earthquake.

 Levels VII through XII are used to describe damage to structures during a moderate to
catastrophic earthquake.
MEASUREMENT OF EARTHQUAKE STRENGTH
MEASUREMENT OF EARTHQUAKE STRENGTH
 Earthquake Magnitude is a measure of the strength of an earthquake, or the amount of strain that
rocks in Earth’s crust release when an earthquake occurs.

 The Richter scale and the moment magnitude scale are used to measure the magnitude of
earthquakes.

 Richter Scale, method of ranking the strength or size of an earthquake.

 The Richter scale, also known as the local magnitude scale, was devised in 1935 by the American
seismologist Charles F. Richter to rank earthquakes occurring in California.

 Richter and his associates later modified it to apply to earthquakes anywhere in the world.

 The Richter scale ranks earthquakes based on how much the ground shakes 100 km (60 mi) from
the earthquake’s epicenter, the site on the earth’s surface directly above the earthquake’s origin.

 The Richter scale is a logarithmic scale—each increase of 1 on the Richter scale represents a
tenfold increase in movement.

 Thus, an earthquake registering 7 on the scale is 10 times as strong as an earthquake registering 6,

and the earth moves 10 times as far.
MEASUREMENT OF EARTHQUAKE STRENGTH
DAMAGE FROM EARTHQUAKES
 Large earthquakes can displace rock and alter the Earth’s
surface. Most earthquake fatalities and injuries occur
when falling structures crush people. Structural damage,
injury, and death depend on the magnitude of the quake,
its proximity to population centers, rock and soil types,
topography, and the quality of construction in the region.

 CONSTRUCTION DESIGN AND EARTHQUAKE DAMAGE

 A magnitude 6.4 earthquake struck central India in 1993,

killing 30,000 people. In contrast, the 1994 magnitude
6.6 quake in Northridge (near Los Angeles) killed only 55.
The tremendous mortality in India occurred because
buildings were not engineered to withstand earthquakes.

 Some common framing materials used in buildings, such

as wood and steel, bend and sway during an earthquake
but resist failure. However, brick, stone, concrete, adobe
(dried mud), and other masonry products are brittle and
likely to fail during an earthquake.
DAMAGE FROM EARTHQUAKES
 FIRE

Earthquakes commonly rupture

buried gas pipes and electrical
wires, leading to fire, explosions, and
electrocutions. Water pipes may also
break, so fire fighters cannot fight
the blazes effectively.

 LANDSLIDES

Landslides are common when the

Earth trembles. They occur mostly
when earthquake occurs in a hilly
region.
DAMAGE FROM EARTHQUAKES
TSUNAMIS

 When an earthquake occurs beneath the sea, part of the

sea floor rises or falls and water is displaced in response
to the rock movement, forming a wave.

 Sea waves produced by an earthquake are often called

tidal waves, but they have nothing to do with tides.

 Therefore, geologists call them by their Japanese name,

tsunami.

 In the open sea, a tsunami is so flat that it is barely

detectable. Typically, the crest may be only 1 to 3 meters
high, and successive crests may be more than 100 to
150 kilometers apart.

 However, a tsunami may travel at 750 kilometers per

hour.

 When the wave approaches the shallow water near shore,

the base of the wave drags against the bottom and the
water stacks up, increasing the height of the wave. The
rising wall of water then flows inland.
FORMATION OF A TSUNAMIS
 DEPTH OF EARTHQUAKES
 Earthquakes can occur anywhere between
the Earth's surface and about 700
kilometers below the surface. For scientific
purposes, this earthquake depth range of 0 -
700 km is divided into three zones: shallow,
intermediate, and deep.

 Shallow earthquakes are between 0 and 70

km deep;

 Intermediate earthquakes: 70 - 300 km

deep; and

 Deep earthquakes:300 - 700 km deep.

 In general, the term "deep-focus

earthquakes" is applied to earthquakes
deeper than 70 km.

 All earthquakes deeper than 70 km are

localized within great slabs of shallow
lithosphere that are sinking into the Earth's
mantle.
EARTHQUAKE AND TECTONIC PLATE
BOUNDARIES
 Although many faults are located within tectonic plates, the largest
and most active faults are the boundaries between tectonic plates.

 Therefore, earthquakes occur most frequently along plate

boundaries.

 Earthquake occurs at all the three type of plate boundaries that is

the divergent plate boundaries, convergent plate boundaries and
transform plate boundaries.

 However only shallow earthquakes occur along the mid-oceanic ridge

(divergent plate boundary)because here the asthenosphere rises to
within 20 to 30 kilometers of the Earth’s surface and is too hot and
plastic to fracture.
EARTHQUAKE AND TECTONIC PLATE
BOUNDARIES
WHERE DO EARTHQUAKES OCCUR AND HOW
OFTEN

80% of all earthquakes occur in the circum-Pacific belt. Most of these result from convergent margin activity~15%
occur in the Mediterranean-Asiatic belt.

Remaining 5% occur in the interiors of plates and on spreading ridge centers. More than 150,000 quakes strong
enough to be felt are recorded each year
EARTHQUAKE PREDICTION

 Long term prediction: Earthquakes occur over and over in the same
places because it is easier for rocks to move along an old fracture
than for a new fault to form in solid rock.

 Many of these faults lie along tectonic plate boundaries.

 Therefore, long-term earthquake prediction recognizes that

earthquakes have recurred many times in a specific place and will
probably occur there again.

 Short-term prediction is based on occurrences of foreshocks, release

of radon gas, changes in the land surface, the water table, electrical
conductivity, and erratic animal behavior.
LOCATING THE SOURCE OF AN EARTHQUAKE
 P waves travel faster then the S waves and the surface waves are the slowest.

 If a seismograph is located close to an earthquake epicenter, the different

waves will arrive in rapid succession.

 On the other hand, if a seismograph is located far from the epicenter, the S
waves arrive at correspondingly later times after the P waves arrive, and the
surface waves are even farther behind.
LOCATING THE SOURCE OF AN EARTHQUAKE
 Geologists use a time-travel curve to
calculate the distance between an
earthquake epicenter and a
seismograph.

 To make a time-travel curve, a

number of seismic stations at
different locations record the times
of arrival of seismic waves from an
earthquake with a known epicenter
and occurrence time. Then a graph is
drawn.

 The figure on the right shows us that

if the first P wave arrives 5 minutes
before the first S wave, the recording
station is about 3400 kilometers
from the epicenter.
LOCATING THE SOURCE OF AN EARTHQUAKE
 But this distance does not indicate
whether the earthquake originated to the
north, south, east, or west.

 To pinpoint the location of an earthquake,

geologists compare data from three or
more recording stations.

 If a seismic station in Darwin records an

earthquake with an epicenter 4900
kilometers away, geologists know that the
epicenter lies somewhere on a circle
4900 kilometers from Darwin.

 The same epicenter is reported to be

8200 kilometers from a seismic station in
Paris and 3400 kilometers from one in
Nagpur, India.

 If one circle is drawn for each recording

station, the arcs intersect at the epicenter
of the quake.
UNIT - 8

VOLCANOES
WHAT IS A MAGMA
 Magma is a mixture of
molten rock, volatiles and
solids that is found
beneath the surface of
the Earth.

 In some instances, it
solidifies within the crust
to form plutonic rocks. In
others, it erupts onto the
Earth’s surface to form
volcanic rocks
MAGMA BEHAVIOUR
 The magma cools as it
enters shallower and
cooler levels of the Earth.

 Second, pressure drops

because the weight of
overlying rock decreases.

 Cooling tends to solidify

the magma, but
decreasing pressure
tends to keep it liquid.
TYPES OF MAGMA
Solidified Chemical
Magma Type Temperature Viscosity Gas Content
Rock Composition
45-55 SiO2 %,
high in Fe, Mg,
Basaltic Basalt 1000 - 1200oC Low Low
Ca,
low in K, Na

55-65 SiO2 %,
Andesitic Andesite intermediate in 800 - 1000oC Intermediate Intermediate
Fe, Mg, Ca, Na, K

65-75 Sio 2 %,
Rhyolitic/ low in Fe, Mg,
Rhyolite 650 - 800oC High High
Granitic Ca,
high in K, Na.

•Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content
magmas (viscosity increases with increasing SiO2 concentration in the magma).
•Lower temperature magmas have higher viscosity than higher temperature
magmas (viscosity decreases with increasing temperature of the magma).
PLUTONS

 In most cases, granitic magma solidifies within

the Earth’s crust to form a pluton.

 A batholith is a pluton exposed over more than

100 square kilometers of the Earth’s surface.

 A stock is similar to a batholith but is exposed

over less than 100 square kilometers.
PLUTONS

BATHOLITH AND STOCK

PLUTONS

 A dike is a tabular, or sheet like, intrusive rock that

forms when magma oozes into a fracture Dikes cut
across sedimentary layers or other features in
country rock and range from less than a
centimeter to more than a kilometer thick.

 Magma that oozes between layers of country rock

forms a sheet like rock parallel to the layering,
called a sill.
PLUTONS
DIKE

SILL
VOLCANIC ROCKS AND VOLCANOES
 The material erupted from volcanoes creates a
wide variety of rocks and landforms, including lava
plateaus and several types of volcanoes.

 Lava is fluid magma that flows onto the Earth’s

surface. Lava generally comes on to the earth’s
surface through volcanoes.

 A volcano is an opening, or rupture, in a planet's

surface or crust, which allows hot magma, volcanic
ash and gases to escape from below the surface
VOLCANIC ROCKS AND VOLCANOES

LAVA
VOLCANIC ROCKS AND VOLCANOES

VOLCANO
TYPES OF LAVA

 A’a: Pronounced “ah-ah”, this is a basaltic lava

that doesn’t flow very quickly. These types of
lava erupt at temperatures above 1000 to
1100 degrees C
TYPES OF LAVA
 Pahoehoe: Pronounced “pa-ho-ho”, this type of
lava is much thinner and less viscous than a’a. It
can flow down the slopes of a volcano in vast
rivers. Pahoehoe erupts at temperatures of 1100
to 1200 degree C
TYPES OF LAVA

 Pillow Lava: Pillow lava is typically found

erupting from underwater volcano vents.
STRUCTURES IN VOLCANIC ROCKS
 When lava cools, escaping gases such as water and
carbon dioxide form bubbles in the lava.

 If the lava solidifies before the gas escapes, the bubbles

are preserved as holes called vesicles
STRUCTURES IN VOLCANIC ROCKS
 Hot lava shrinks as it cools and solidifies. The
shrinkage pulls the rock apart, forming cracks
that grow as the rock continues to cool. Such
cracks, called columnar joints
PYROCLASTIC ROCKS
 If a volcano erupts explosively, it may eject both liquid
magma and solid rock fragments. A rock formed from
particles of magma that were hurled into the air from a
volcano is called a pyroclastic rock

 The smallest particles is called volcanic ash

PYROCLASTIC ROCKS

 Cinders vary in size from 2 to 64 millimeters

PYROCLASTIC ROCKS

 Particles greater than 64 mm in diameter are

called volcanic bombs
FISSURE ERUPTIONS AND LAVA PLATEAUS
 The gentlest type of volcanic eruption occurs when
magma comes out from the cracks in the land surface
called fissures and flows over the land like water.

 Basaltic magma commonly erupts in this manner

because of its low viscosity
FISSURE ERUPTIONS AND LAVA PLATEAUS
 Some times fissures extend for tens or hundreds of
kilometers and pour thousands of cubic kilometers of
lava onto the Earth’s surface.

 A fissure eruption of this type creates a flood basalt,

which covers the landscape like a flood
FISSURE ERUPTIONS AND LAVA PLATEAUS

 Many such eruptions may occur in rapid

succession and to create a lava plateau
covering thousands of square kilometers
VOLCANOES

 If lava is too viscous to spread out as a flood, it

builds a hill or mountain called a volcano.
VOLCANOES
 Volcanoes differ widely in shape, structure, and size.

 Lava and rock fragments commonly erupt from an opening called a

vent.

 The vent joins the crater which is a bowl shaped depression present
at the top of the volcano.
VOLCANO TYPES BASED ON ACTIVITY
 An active volcano is one that is erupting or is
expected to erupt

 A dormant volcano is one that is not now

erupting but has erupted in the past and will
probably do so again

 An extinct volcano is one that is expected never

to erupt again
TYPES OF VOLCANOES
TYPES OF VOLCANOES

BASLAT PLATEAU

SHIELD VOLCANO
TYPES OF VOLCANOES

COMPOSITE VOLCANO

CINDER CONE CALDERA

DISTRIBUTION OF WORLD VOLCANOES
PLATE TECTONICS AND VOLCANIC ACTIVITY
 Most active volcanoes are associated with plate boundaries.

 Active areas of volcanism are found along mid-ocean ridges

where seafloor spreading is occurring (divergent plate
boundaries),

 in the vicinity of ocean trenches where one plate is being

subducted beneath another (convergent plate boundaries),
and

 In the interiors of plates themselves (intraplate volcanism).

Rising plumes of hot mantle rock are the source of most
intraplate volcanism.
OCCURRENCE AND PREDICTION OF VOLCANOES
 Volcanic eruptions are common near a subduction zone,
near a spreading center, and at a hot spot over a mantle
plume but are rare in other tectonic environments.

 Eruptions on a continent are often violent, whereas those

in oceanic crust are gentle. Such observations form the
basis of regional predictions of volcanic hazards.

 Short-term predictions are made on the basis of

earthquakes caused by magma movements, swelling of a
volcano, increased emissions of gas and ash from a vent,
and other signs that magma is approaching the surface
UNIT - 9

GEOLOGIC TIME
GEOLOGIC TIME

 Normally we think of time in terms of days or

years but geologists commonly refer to events
that happened millions or billions of years ago

 For example earth is approximately 4.6 billion

years old
GEOLOGIC TIME

 Geologists measure geologic time in two

different ways

 Relative Age and Absolute Age

GEOLOGIC TIME
RELATIVE AGE
 Determination of relative age is based on a simple
principle:

 In order for an event to affect a rock, the rock must exist

first. Thus, the rock must be older than the event.

FOLDED ROCKS
GEOLOGIC TIME
ABSOLUTE AGE
 Absolute age is age in years

 Dinosaurs became extinct 65 million years ago

RELATIVE GEOLOGIC TIME
The principle of original horizontality

 It is based on the fact that sediment usually accumulates in

horizontal layers.

 If sedimentary rocks lie at an angle, we can infer that tectonic

forces tilted them after they formed
RELATIVE GEOLOGIC TIME
The principle of superposition

 It states that sedimentary rocks become younger from bottom to

top (as long as tectonic forces have not turned them upside
down).

 This is because younger layers of sediment always accumulate

on top of older layers. In the figure below the sedimentary layers
become progressively younger in the order E, D, C, B, and A.
RELATIVE GEOLOGIC TIME
The principle of cross-cutting relationships

 It states that a rock must first exist before anything can happen to it.

 The figure below shows sedimentary rocks intruded by three granite

dikes.

 Dike B cuts dike C, and dike A cuts dike B, so dike C is older than B, and
dike A is the youngest. The sedimentary rocks must be older than all of
the dikes.
RELATIVE GEOLOGIC TIME
The principle of unconformities

 Layers of sedimentary rocks are conformable if they were deposited

without interruption. An unconformity represents an interruption in
deposition, usually of long duration.

 During the interval when no sediment was deposited, some rock

layers may have been eroded

 Thus, an unconformity represents a long time interval for which no

geologic record exists in that place. The lost record may involve
hundreds of millions of years

 There are several types of unconformities

UNCONFORMITIES
Disconformity

 In this case the sedimentary layers above and below the

unconformity are parallel.

 Geologists identify disconformities by determining the ages of

rocks using methods based on fossils and absolute dating
UNCONFORMITIES
Angular unconformity

 In this case tectonic activity tilted older sedimentary

rock layers before younger sediment accumulated
UNCONFORMITIES
Nonconformity
 In this case sedimentary rocks lie on igneous
or metamorphic rocks
RELATIVE GEOLOGIC TIME

The principle of faunal succession

It states that fossil
organisms succeeded one
another through time in a
definite and recognizable
order and that the relative
ages of rocks can therefore
be recognized from their
fossils
RELATIVE GEOLOGIC TIME

 Paleontologists study fossils, the remains and other traces of

prehistoric life, to understand the history of life and evolution.

 Fossils also provide information about the ages of sedimentary

rocks and their depositional environments
FOSSILS AND FAUNAL SUCCESSION
 The theory of evolution states that life forms have changed
throughout geologic time.

 Fossils are useful in determining relative ages of rocks because

different animals and plants lived at different times in the Earth’s
history.

 For example, trilobites lived from 535 million to 245 million years
ago, and the first dinosaurs appeared about 220 million years
ago.
CORRELATION
 To assemble a complete and continuous a
record, geologists combine evidence from many
localities. To do this, rocks of the same age
from different localities must be matched in a
process called correlation

 There are two kinds of correlation

 Time correlation and
 Lithologic correlation
CORRELATION

 Time correlation: matching of rocks deposited

at the same time (e.g. Mesozoic sedimentary
rocks in the U.S. with Mesozoic sedimentary
rocks in Mexico)

 Time correlation requires the use of index

fossils to demonstrate rocks were deposited at
the same time
CORRELATION

 Index fossils are fossils used to define and

identify geologic periods.

 They work on the premise that, although

different sediments may look different
depending on the conditions under which they
were laid down, they may include the remains
of the same species of fossil.
CORRELATION
 To be useful, an index fossil is produced by an organism
that
 is abundantly preserved in rocks,
 was geographically widespread,
 existed as a species or genus for only a relatively short
time, and
 is easily identified in the field.
EXAMPLES OF INDEX FOSSILS
CORRELATION
 Lithologic correlation: matching rocks of the same
character from one place to another. Usually it is not as
accurate as time correlation, but easier

 This doesn't require index fossils, but lithologic

correlation may not correlate rocks deposited at the
same time.

 Lithologic correlation requires the use of key

beds/marker beds
CORRELATION

 A key bed/marker bed is a thin, widespread

sedimentary layer that was deposited rapidly
and synchronously over a wide area and is
easily recognized

 Examples are the ash deposits from volcanic

eruptions
CORRELATION

The K-T boundary layer which is marker bed found almost all over the
world.The layer shows high concentration of the element iridium.
iridium does not occur naturally on Earth in high concentrations, but it
does occur in higher concentrations in certain types of meteorites. It
points to a metorite impact 65 million years ago which was responsible
for the extiction of the dinosaurs
ABSOLUTE GEOLOGIC TIME

 Natural Radioactivity of the elements present

in rocks provides a way for measuring the
absolute geologic time
•Elements having the same atomic
number but different atomic mass
are known as Isotopes

•The difference in mass is due to

the difference in the number of
neutrons
ABSOLUTE GEOLOGIC TIME

 Many isotopes are stable and do not change

with time. For example potassium-39 remains
unchanged even after 10 billion years
•Other isotopes are unstable or
radioactive. Given time, their
nuclei spontaneously break apart

•Potassium-40 decomposes
naturally to form two other
isotopes, argon-40 and calcium-
40
ABSOLUTE GEOLOGIC TIME

 A radioactive isotope such as potassium-40 is

known as a parent isotope.

 An isotope created by radioactivity, such as argon-

40 or calcium-40, is called a daughter isotope.
ABSOLUTE GEOLOGIC TIME

 The half-life is the time it takes for half of the

atoms in a sample to decompose.
•The half-life of potassium- 40 is
1.3 billion years. Therefore, if 1
gram of potassium-40 were
placed in a container, 0.5 gram
would remain after 1.3 billion
years, 0.25 gram after 2.6 billion
years, and so on.

Each radioactive isotope has its

own half-life; some half-lives are
fractions of a second and others
are measured in billions of years.
ABSOLUTE GEOLOGIC TIME
 Two aspects of radioactivity are essential to the calendars in
rocks

 First, the half-life of a radioactive isotope is constant. It is easily

measured in the laboratory and is unaffected by geologic
processes. So radioactive decay occurs at a known, constant
rate

 Secondly as a parent isotope decays, its daughter accumulates

in the rock. The longer the rock exists, the more daughter
isotope accumulates. The accumulation of a daughter isotope
is similar to marking off days on a calendar
ABSOLUTE GEOLOGIC TIME
ABSOLUTE GEOLOGIC TIME
 Radiometric dating is the process of determining the ages of
rocks, minerals, and fossils by measuring their parent and
daughter isotopes

•At the end of one half-life, 50

percent of the parent atoms have
decayed to daughter.

•At the end of two half-lives, the

mixture is 25 percent parent and 75
percent daughter.

•To determine the age of a rock, a

geologist measures the proportions
of parent and daughter isotopes in a
sample and compares the ratio.
 THE GEOLOGICAL COLUMN
AND TIME SCALE
•The largest time units are eons, which
are divided into eras.

•Eras are subdivided, in turn, into

periods, which are further subdivided
into epochs

•The Phanerozoic Eon is finely and

accurately subdivided because
sedimentary rocks deposited at this time
are often well preserved and they
contain abundant well-preserved fossils

•In contrast, Precambrian rocks and time

are only coarsely subdivided because
fossils are scarce and poorly preserved
and the rocks are often altered.
UNIT 10

MOUNTAIN BUILDING
AND
EVOLUTION OF CONTINENTS
 ROCK DEFORMATION
STRESS
 Tectonic forces exert different
types of stress on rocks in
different geologic
environments.

 The first, called confining stress

or confining pressure, occurs
when rock or sediment is
buried.

 Confining pressure merely

compresses rocks but does not
distort them, because the
compressive force acts equally
in all directions
 ROCK DEFORMATION

 STRESS
 In contrast directed stress or directed pressure, acts only in one
direction.
 Tectonic processes create three types of directed stress.
 ROCK DEFORMATION
DIRECTED PRESSURE

 Compressive stress is common in

convergent plate boundaries,where two
plates converge and the rock.

 Extensional stress (often called tensional

stress) pulls rock apart and is the
opposite of tectonic compression Rocks
at a divergent plate boundary stretch and
pull apart because they are subject to
extensional stress.

 Shear stress acts in parallel but opposite

directions. Shearing deforms rock by
causing one part of a rock mass to slide
past the other part, as in a transform fault
or a transform plate boundary.
ROCK DEFORMATION
STRAIN

 Strain is the deformation produced by stress.

 Deformation can be of two types

1. Elastic deformation: An elastically deformed rock springs back to its

original size and shape when the stress is removed.

2. Plastic deformation :During plastic deformation, a rock deforms like putty

and retains its new shape.

 Once the substance/rock has reached the limit of plastic deformation, it

breaks or ruptures. This is known as the brittle deformation.
GEOLOGICAL STRUCTURES
 A geologic structure is any feature produced by rock deformation.

 Tectonic forces create three types of geologic structures: folds, faults, and
joint.

 A fold is a bend in rock. Some folded rocks display little or no fracturing,

indicating that the rocks deformed in a plastic manner. In other cases,
folding occurs by a combination of plastic deformation and brittle fracture.
Folds formed in this manner exhibit many tiny fractures
GEOLOGICAL STRUCTURES
FOLDS
 Folding usually results
from compressive
stress. For example,
tightly folded rocks in
the Himalayas indicate
that the region was
subjected to
compressive stress.

 Folding always
shortens the horizontal
distances in rock.
GEOLOGICAL STRUCTURES
PARTS OF A FOLD
 The sides of a fold are called
the limbs.

 A line dividing the two limbs

of a fold and running along
the crest of an anticline or the
trough of a syncline is the fold
axis.

 The axial plane is an

imaginary plane that runs
through the axis and divides a
fold as symmetrically as
possible into two halves.
GEOLOGICAL STRUCTURES
TYPES OF FOLDS
 An anticline is a convex up fold in which the
limbs of the fold dip away from each other.
The oldest rocks lie in the center of the fold
GEOLOGICAL STRUCTURES
TYPES OF FOLDS
 In a syncline the limbs of the fold dip
towards each other. The youngest beds are
in the center of the fold
GEOLOGICAL STRUCTURES
TYPES OF FOLDS

 A special type of fold with only one limb is a

monocline.
 GEOLOGICAL STRUCTURES
TYPES OF FOLDS

 A symmetrical fold is one

in which the axial plane is
vertical.

 An asymmetrical fold is
one in which the axial
plane is inclined.

 In an overturned fold, the

beds dip in the same
direction on both sides of
the axial plane.
GEOLOGICAL STRUCTURES
TYPES OF FOLDS

 In a recumbent fold the axial plane is

horizontal and the limbs of the fold are
parallel to each other.
GEOLOGICAL STRUCTURES
 A circular or elliptical anticlinal structure is called a
dome. The layer dips away from the center of a dome in
all directions.

 A circular or elliptical synclinal structure is called a

basin. The layer dips towards the center of the basin in
all directions.
GEOLOGICAL STRUCTURES
FAULTS

 A fault is a fracture along

which rock on one side has
moved relative to rock on the
other side.

 Slip is the distance that

rocks on opposite sides of a
fault have moved.

 Some faults are a single

fracture in rock; others
consist of numerous closely
spaced fractures called a
fault zone.
GEOLOGICAL STRUCTURES
 The two sides of a non-vertical fault are known as
the hanging wall and footwall.

 By definition, the hanging wall occurs above the fault

and the footwall occurs below the fault.

 Fault Plane is the plane along which the rock or crustal

material has fractured.
GEOLOGICAL STRUCTURES

Normal Fault
 Hanging wall moves down relative to footwall.

 Caused by horizontal tension stress.

 Results in extension.
GEOLOGICAL STRUCTURES
Reverse Fault
 Hanging wall moves up relative to footwall.

 Caused by compressive stress.

 Results in shortening.

 Fault plane is oriented between 30 and 90 degrees (measured from

horizontal).
GEOLOGICAL STRUCTURES
Strike-Slip Faults
 A strike–slip fault is one in which the fracture is
vertical, or nearly so, and rocks on opposite sides
of the fracture move horizontally past each other.

 A transform plate boundary is a strike–slip fault

GEOLOGICAL STRUCTURES

Thrust Fault
 A thrust fault is a special type of reverse fault
that is nearly horizontal
 Fault plane is at less than 30 degrees
GEOLOGICAL STRUCTURES
Horsts and Grabens

 Horsts are up thrown blocks bounded on either side by non-parallel

normal faults.

 Grabens are downthrown blocks bounded on either side by non-

parallel normal faults.
GEOLOGICAL STRUCTURES
JOINTS
 A joint is a fracture in rock and is therefore similar
to a fault except that in a joint rocks on either side
of the fracture have not moved