The Rock Cycle
[Mohan Basnet, Gauhati University, Guwahati-781014 Assam, India]
RESOURCE
ENCYCLOPEDIC ENTRY
The Rock Cycle
The rock cycle is a series of processes that create and transform the types of
rocks in Earth’s crust.
GRADES
5-8
SUBJECTS
Chemistry, Earth Science, Geology
IMAGE
Reunion Island Volcano
Active volcanoes like this one on Reunion Island—east of Madagascar, in the
Indian Ocean—forms a type of igneous rock. Extrusive, or volcanic, igneous
rocks are formed when molten hot material cools and solidifies.
PHOTOGRAPH BY STEVE RAYMER
ENCYCLOPEDIC ENTRYVOCABULARY
There are three main types of rocks: sedimentary, igneous, and metamorphic.
Each of these rocks are formed by physical changes—such
as melting, cooling, eroding, compacting, or deforming—that are part of
the rock cycle.
Sedimentary Rocks
Sedimentary rocks are formed from pieces of other existing rock
or organic material. There are three different types of sedimentary
rocks: clastic, organic (biological), and chemical. Clastic sedimentary rocks,
like sandstone, form from clasts, or pieces of other rock. Organic sedimentary
rocks, like coal, form from hard, biological materials like plants, shells, and
bones that are compressed into rock.
The formation of clastic and organic rocks begins with the weathering, or
breaking down, of the exposed rock into small fragments. Through the
process of erosion, these fragments are removed from their source and
transported by wind, water, ice, or biological activity to a new location. Once
the sediment settles somewhere, and enough of it collects, the lowest layers
become compacted so tightly that they form solid rock.
Chemical sedimentary rocks, like limestone, halite, and flint, form
from chemical precipitation. A chemical precipitate is a chemical compound—
for instance, calcium carbonate, salt, and silica—that forms when the solution
it is dissolved in, usually water, evaporates and leaves the compound behind.
This occurs as water travels through Earth’s crust, weathering the rock and
dissolving some of its minerals, transporting it elsewhere. These dissolved
minerals are precipitated when the water evaporates.
Metamorphic Rocks
Metamorphic rocks are rocks that have been changed from their original form
by immense heat or pressure. Metamorphic rocks have two classes: foliated
and nonfoliated. When a rock with flat or elongated minerals is put under
immense pressure, the minerals line up in layers, creating foliation. Foliation is
the aligning of elongated or platy minerals, like hornblende or mica,
perpendicular to the direction of pressure that is applied. An example of this
transformation can be seen with granite, an igneous rock. Granite contains
long and platy minerals that are not initially aligned, but when enough
pressure is added, those minerals shift to all point in the same direction while
getting squeezed into flat sheets. When granite undergoes this process, like at
a tectonic plate boundary, it turns into gneiss (pronounced “nice”).
Nonfoliated rocks are formed the same way, but they do not contain the
minerals that tend to line up under pressure and thus do not have the layered
appearance of foliated rocks. Sedimentary rocks like bituminous coal,
limestone, and sandstone, given enough heat and pressure, can turn into
nonfoliated metamorphic rocks like anthracite coal, marble, and quartzite.
Nonfoliated rocks can also form by metamorphism, which happens when
magma comes in contact with the surrounding rock.
Igneous Rocks
Igneous rocks (derived from the Latin word for fire) are formed when molten
hot material cools and solidifies. Igneous rocks can also be made a couple of
different ways. When they are formed inside of the earth, they are called
intrusive, or plutonic, igneous rocks. If they are formed outside or on top of
Earth’s crust, they are called extrusive, or volcanic, igneous rocks.
Granite and diorite are examples of common intrusive rocks. They have a
coarse texture with large mineral grains, indicating that they spent thousands
or millions of years cooling down inside the earth, a time course that allowed
large mineral crystals to grow.
Alternatively, rocks like basalt and obsidian have very small grains and a
relatively fine texture. This happens because when magma erupts into lava, it
cools more quickly than it would if it stayed inside the earth, giving crystals
less time to form. Obsidian cools into volcanic glass so quickly when ejected
that the grains are impossible to see with the naked eye.
Extrusive igneous rocks can also have a vesicular, or “holey” texture. This
happens when the ejected magma still has gases inside of it so when it cools,
the gas bubbles are trapped and end up giving the rock a bubbly texture. An
example of this would be pumice.
chemical
Noun
molecular properties of a substance.
clast
Noun
fragment of a rock, often broken off through weathering.
compact
Verb
to pack tightly together.
cooling
Verb
to become cool : lose heat or warmth
cycle
Noun
regularly occurring event that repeats over a period of time.
deform
Verb
to put out of shape or distort.
erosion
Noun
act in which earth is worn away, often by water, wind, or ice.
extrusive igneous rock
Noun
any rock derived from magma that was poured out or ejected at the Earth’s
surface.
foliation
Noun
layering that occurs when pressure squeezes flat or long minerals so they
become aligned.
igneous rock
Noun
rock formed by the cooling of magma or lava.
intrusive igneous rock
Noun
plutonic rock; formed from magma forced into older rocks at depths within the
Earth’s crust, which then slowly solidifies below the Earth’s surface.
melting
Verb
to become altered from a solid to a liquid state usually by heat
metamorphic rock
Noun
rock that has transformed its chemical qualities from igneous or sedimentary.
organic
Adjective
produced according to standards using limited amounts of chemical additives.
rock cycle
Noun
processes that explain the relationship between the three rock types: igneous,
sedimentary, and metamorphic. Any rock type can become any other.
sedimentary rock
Noun
rock formed from fragments of other rocks or the remains of plants or animals.
weathering
Noun
the breaking down or dissolving of the Earth's surface rocks and minerals.
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ENCYCLOPEDIC ENTRY
Metamorphic Rocks
Metamorphic rocks start as one type of rock and—with pressure, heat, and time—
gradually change into a new type of rock.
413
ENCYCLOPEDIC ENTRY
Sedimentary Rocks
Sedimentary rocks are one of three main types of rocks, along with igneous and
metamorphic. They are formed on or near the Earth’s surface from the compression of
ocean sediments or other processes.
165
ENCYCLOPEDIC ENTRY
Igneous Rocks
Igneous rocks are one of three main types of rocks (along with sedimentary and
metamorphic), and they include both intrusive and extrusive rocks.
421
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