ROCKS

Neeladri Mishra,

NEEL ACADEMICS
INTRODUCTION –
Rocks form an integral part of our lives. We find them in our lawns,
parks, museums and gardens. But they are also found in our homes.
The tiles and kitchen platform are, of
course made of stone. The bricks of a
building are also made of stone. Stone is
also used for decoration. Statues are also
carved from stone. Limestone and Marble
statues are very popular amongst people.
The lead of your pencil is also made from
a type of stone, Graphite. The pictures of
gods are also sometimes made with glazing stone. Stones also were
an important contributor in the growth and development of
Humanity. Yes, this is because our
ancestors made their tools with
stone. These helped in protecting
them from Apex Predators of that
time. Another interesting but
easily noticeable thing is that, the
caves that they lived in were also
made up of stone!

TYPES OF ROCKS –
Based on their sources and formations, the stones are divided into
types –

1. When volcanic lava cools down and hardens, it creates a type of

stone, Igneous Rocks. These rocks are usually very hard but

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 some are soft, like Graphite. Some gems are also made by
volcanos like obsidian gems, amethyst geodes and the better
known, Sapphires. There are two types of Igneous rocks –
Extrusive and Intrusive. Intrusive rocks are those which
hardened far below the Earth’s surface. And extrusive is the
opposite, meaning that it cooled just on top of Earth’s Crust.

2. The next one is Sedimentary Rocks. They are formed by a long

process. First step, erosion, here small rocks and pebbles from
the land get eroded and fall in any water body. Then,
sedimentation takes place. Now,
the small rocks and pebbles (also
known as sediment) settle down
at the water body’s bed. Then,
compaction happens. At this
point many layers of sand and silt
are formed over the sediment.
These layers put pressure and
compress the sediments. The last
step is cementation, here the salt,
that was left by the water, glues
the sediments while they are
under pressure. Thus, they form
Sedimentary rocks. Some
sedimentary rocks are used as tiles and building material like
Sandstone and Limestone.

3. After this, there comes the Metamorphic Rocks. They usually

form when an igneous or sedimentary rock is subjected to high
heat and pressure. Due to this, the igneous or sedimentary rocks
get disassembled and then, they change their chemical
composition and turn into metamorphic rocks. And yes,
metamorphic rocks are mineral rich and very durable. E. g.
Marble, Slate etc.
ROCK CYCLE –
Rock Cycle is a continuous cycle that depicts the relations between
different types of rocks. And, yes, Ores are not included in the Rock
Cycle. When Igneous or Metamorphic or Sedimentary rocks erode,
they leave sediments which turn into Sedimentary Rocks. When
Sedimentary, Igneous or Metamorphic Rocks melt they turn into
Magma (Lava inside the volcano) which cools to become Igneous
rocks. When Sedimentary or Igneous rock are exposed to pressure
and heat, they turn into Metamorphic Rocks. This cycle shows us that
how rocks are dependent on each other. It also clears our doubt about
how different types of rocks are formed.
CHART –
Sl. no. Name of the Type of the
rock rock
1. Obsidian Igneous

2. Pumice Igneous

3. Sandstone Sedimentary

4. Shale Sedimentary

5. Slate Metamorphic

6. Marble Metamorphic

7. Conglomerate Sedimentary

8. Gneiss Metamorphic

9. Limestone Sedimentary

10. Granite Igneous

11. Diorite Igneous

12. Andesite Igneous

13. Bauxite Ore (Aluminium),

Sedimentary
14. Hematite Ore (Iron), Igneous

15. Pyrite Ore (Copper, Iron etc.),

Igneous/Sedimentary/
Metamorphic
DETAILS –
Obsidian -

Obsidian is often regarded as a stone even though it is a naturally

occurring volcanic glass formed when lava thrown out from a
volcano, cools down rapidly with minimal crystal growth. It is an
igneous rock. Obsidian is rich in light elements such as silicon,
oxygen, aluminium, sodium, and potassium. It is deep black or
blackish green in colour. It is lustrous like normal glass. It is
translucent in nature.

Pumice –

Pumice is a type of igneous rock, produced when lava with a very

high content of water and gases is discharged from a volcano. When
this lava cools and hardens, the result is a very light rock material
filled with tiny bubbles of gas. This is the only type of rock that can
float.
Sandstone –

It is a type of stone that is formed of grains of sand tightly pressed

together and that is used in building creations. This is a weather
resistant stone. As the stone is porous, the water absorption is very
higher than other stone materials. Sandstone is a type of sedimentary
rock.

Shale –

Shale is a fine-grained, laminated sedimentary rocks consisting of

silt- and clay-sized particles. Shale is the most abundant of the
sedimentary rocks, accounting for roughly 70 percent of this rock
type in the crust of the Earth. It is used to make bricks, pottery, tiles,
and cement. Natural gas and petroleum may be extracted from oil
shale.
Slate –

Slate is a fine-grained, clayey metamorphic rock that splits readily

into thin slabs having great strength and durability. It can be used for
roofing and similar purposes. It is quite brittle. It is also used in
Blackboards.

Marble –

Marble is a course, crystalline, metamorphic rock, which is formed

when limestone is recrystallised under conditions of high
temperature and pressure, often a result of the movement of the
earth's crust. Marble Cancer is caused by Acid Rain. In it the
monuments’ marble’s colour degrades. It is usually formed from
Limestone.
Conglomerate –

Conglomerate is a sedimentary rock that is composed of a substantial

fraction of rounded to subangular gravel-size clasts. A conglomerate
typically contains a matrix of finer grained sediments, such as sand,
silt, or clay, which fills the interstices between the clasts. The clasts
and matrix are typically cemented by calcium carbonate, iron oxide,
silica, or hardened clay.

Gneiss –

Gneiss is a type of metamorphic rock formed at high pressure and

temperature deep in the ground. It can be made from shale, granite
or impure sandstone. Gneiss has many uses as a building material
such as flooring, ornamental stones, gravestones, facing stones on
buildings and work surfaces.
Limestone –

Limestone is a common type of carbonate sedimentary rock. It is

composed mostly of the minerals, calcite and aragonite, which are
different crystal forms of calcium carbonate (CaCO3). Limestone
forms when these minerals precipitate out of water containing
dissolved calcium. It is called so because it yields lime when burnt. It
is used to construct bricks and crushed to make Cement.

Granite -

Granite is a coarse-grained intrusive igneous rock composed mostly

of quartz, alkali feldspar, and plagioclase. It forms from magma with
a high content of silica and alkali metal oxides that slowly cools and
solidifies underground. Granite is the best-known igneous rock.
Many people recognize granite because it is the most common
igneous rock found at Earth's surface. It is used in flooring and
making Kitchen Countertops. Granite is also the most abundant rock
in the continental crust and is the hardest rock in the World.
Diorite –

Diorite is an igneous rock composed principally of silicate minerals.

It is used as a base material in the construction of roads, buildings,
and parking areas. It is also used as a drainage stone and for erosion
control. In the dimension stone industry, diorite is often cut into
facing stone, tile, ashlars, blocking, pavers, curbing, and a variety of
dimension stone products.

Andesite –

It is used for making tiles because it is resistant to slipping. It is used

as filling material in construction and road construction. It is used in
landscaping, garden designs and the construction of sculptures and
monuments.
Bauxite -

Bauxite is a sedimentary ore with a relatively high aluminium content.

It is the world's main source of aluminium and gallium. In 1821 the
French geologist Pierre Berthier discovered bauxite. Bauxite is used
to produce alumina, which is then used to produce aluminium.

Hematite -

Hematite, also spelled haematite, is heavy and relatively hard oxide

mineral. It is used to obtain iron because of its high iron content (70
percent) and its abundance. In spite of its iron constituency, it is really
fragile. This is an igneous rock.
Fun Fact!
Ores can be of two types, Metallic Ores and Non-Metallic Ores.
Metallic Ores are from which Metals are obtained. and Non –
Metallic Ores provide us with minerals and gemstones

Pyrite –

Pyrite (CuFeS2) is a strange ore. It is also known as chalcopyrite. It is

strange because it is of different types – Copper Pyrite, Iron Pyrite,
Gold Pyrite, Sulphur Pyrite etc. All of these types help in the
production of sulphur dioxide for the paper industry and the
production of sulphuric acid for the chemistry industry and the
fertilization industry. Some varieties of Pyrite are igneous, whereas
others are sedimentary and, some are even metamorphic.

Fun Fact!
All the ores of metals have a name but the ores of
gemstones don’t have any names. Ex – Ruby Ore,
Diamond Ore, Emerald Ore, Sapphire Ore etc.

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