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22.3 Rocks and the Rock Cycle
In 1993, Lynn Hill
climbed the granite
face of El Capitan
without the aid of a
rope except to
provide safety in
case of a fall. El
Capitan is one of
the largest and
most impressive
chunks of granite
on Earth.
22.3 Rocks and the Rock Cycle
Classifying Rocks
What are the three major groups of rocks?
Rocks are classified into three major
groups—igneous, sedimentary, and
metamorphic—based on how they form.
22.3 Rocks and the Rock Cycle
Classifying Rocks
The size, shape, and arrangement of the
crystals and other particles that make up a
rock give the rock its texture.
A rock’s texture reveals what the rock is made
from and how and where it formed.
22.3 Rocks and the Rock Cycle
Igneous Rock
How do igneous rocks form?
Igneous rock forms when molten material
cools and solidifies either inside Earth or at
the surface.
22.3 Rocks and the Rock Cycle
Igneous Rock
An igneous rock is a rock that forms from
magma.
Magma is a mixture of molten rock and
gases, including water vapor, which forms
underground.
Magma that flows out of volcanoes is called
lava.
22.3 Rocks and the Rock Cycle
Igneous Rock
An igneous rock that forms underground from
hardened magma is called an intrusive rock.
An igneous rock that forms at Earth’s surface
is called an extrusive rock.
22.3 Rocks and the Rock Cycle
Igneous Rock
Basalt is a
fine-grained,
extrusive rock.
As this basalt
cooled, the
rock formed
into hexagonal
columns.
22.3 Rocks and the Rock Cycle
Igneous Rock
Extrusive rocks and intrusive rocks have
differences in texture caused by differences in
how the rocks formed.
• Intrusive rocks cool slowly underground,
allowing their crystals to grow large. Large
crystals give intrusive rocks a coarse-grained
texture.
• Extrusive igneous rocks cool very quickly at the
surface. Their crystals do not grow much before
the rock cools. This gives extrusive rocks a finegrained texture.
22.3 Rocks and the Rock Cycle
Igneous Rock
An igneous rock’s color gives a clue to its
mineral composition.
• Gabbro, an intrusive rock, and basalt, an
extrusive rock formed from magma rich in iron
and magnesium. The rocks are dark and dense.
• Granite is a coarse-grained, intrusive rock with a
high silica content. Granite is less dense and
lighter in color than basalt and gabbro.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
How are sedimentary rocks classified?
A sedimentary rock is a rock that forms over
time as sediment is squeezed and cemented
together.
Geologists classify sedimentary rocks into
three main groups according to how they
form: clastic rocks, chemical rocks, and
organic rocks.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Sediment consists of small, solid pieces of
material that comes from rocks or living
organisms.
• The process of weathering breaks down rock at
Earth’s surface, turning it into smaller pieces.
• Minerals dissolved in water are also sediment.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Sediment is often carried away by running
water or wind to a new location, where it is
deposited in layers.
As sediment piles up, the pressure causes
the deeper sediment to be compressed.
Dissolved minerals in the water seep into the
space between particles of sediment and form
a kind of cement.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Colorful layers of
sandstone, like
those in this Utah
canyon, formed
over millions of
years as water and
wind laid down
sediment.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Clastic Rock
Sedimentary rocks that form from the broken
fragments of other rocks are called clastic rocks.
The fragments that make up clastic rocks are
usually held together by cement.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Clastic rocks are classified mainly based
upon the average size of the fragments that
they contain.
•
•
•
•
•
Conglomerate is made of gravel and pebbles.
Breccia is made up of sharp-edged fragments.
Sandstone is formed from grains of sand.
Mudstone is made primarily of mud or silt.
Shale is mudstone made of flat grains aligned
so that the rock can split into sheets.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Conglomerate is
a sedimentary
rock in which
rounded pieces
of other rocks
are cemented
together.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Chemical Rock
Chemical sedimentary rocks form when minerals
precipitate out of solution.
• Rainwater dissolves many minerals on the land.
These dissolved minerals are then carried into the
ocean.
• As water evaporates from the ocean surface, the
concentration increases until the minerals precipitate.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
The strangely shaped rocks in Mono Lake,
California, are made of tufa, a chemical rock
composed of calcium carbonate.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
Organic Rock
Some rocks form as the result of organic
processes.
• Marine animals extract calcium carbonate from
ocean water to form their shells and skeletons.
• The shells and skeletons sink to the ocean floor.
• The fragments compact and cement together,
forming limestone.
22.3 Rocks and the Rock Cycle
Sedimentary Rock
The cliffs of Dover on the southern coast of
England are composed of chalk, a type of
fine-grained organic limestone.
22.3 Rocks and the Rock Cycle
Metamorphic Rock
How do metamorphic rocks form?
Metamorphic rock is rock that has been
changed by temperature, pressure, or reactions
with hot water.
Metamorphic rock forms when a rock is
transformed by heat, pressure, or chemical
reactions. Most metamorphic rocks form
under high temperatures and pressures
deep underground.
22.3 Rocks and the Rock Cycle
Metamorphic Rock
Metamorphism can result in a rock with a
mineral content that is different from that of
the original rock.
• Heat deep inside Earth allows the minerals to
recrystallize, and small crystals to enlarge.
• Chemical changes occur, and new minerals
may replace the original minerals.
22.3 Rocks and the Rock Cycle
Metamorphic Rock
Heat, pressure, and chemical reactions
change the form of rocks. In this sequence,
shale changes to slate. Additional heat and
pressure may change slate into schist or
gneiss.
Shale
Slate
Schist
Gneiss
22.3 Rocks and the Rock Cycle
Metamorphic Rock
Metamorphism also changes the texture of
rocks.
When pressure is applied from one direction,
the particles may line up and lock together in
layers or bands.
Metamorphic rocks with crystals arranged in
parallel layers or bands are foliated rocks.
22.3 Rocks and the Rock Cycle
Metamorphic Rock
Some foliated rocks have a striped
appearance. For example, when schist is
subjected to heat and pressure, the result is
gneiss.
Nonfoliated rocks don’t have bands. Their
crystals are arranged randomly.
22.3 Rocks and the Rock Cycle
The Rock Cycle
How can one type of rock change into
another?
The rock cycle is a series of processes in
which rocks continuously change from one type
to another.
In the rock cycle, forces within Earth and at
the surface cause rocks to change form.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Constructive forces form new igneous rock.
Destructive forces break down rock, forming
sediment.
Other forces push rock deep beneath the
surface, where heat and pressure form
metamorphic rock.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Depending on
their pathway
through the
cycle, rocks can
wear away,
undergo
metamorphism,
or melt and
form new
igneous rock.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Depending on
their pathway
through the
cycle, rocks can
wear away,
undergo
metamorphism,
or melt and
form new
igneous rock.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Depending on
their pathway
through the
cycle, rocks can
wear away,
undergo
metamorphism,
or melt and
form new
igneous rock.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Depending on
their pathway
through the
cycle, rocks can
wear away,
undergo
metamorphism,
or melt and
form new
igneous rock.
22.3 Rocks and the Rock Cycle
The Rock Cycle
Depending on
their pathway
through the
cycle, rocks can
wear away,
undergo
metamorphism,
or melt and
form new
igneous rock.
22.3 Rocks and the Rock Cycle
Assessment Questions
1. What type of rocks require weathering and erosion
to form?
a.
b.
c.
d.
sedimentary
metamorphic
igneous
sedimentary, metamorphic, and igneous
22.3 Rocks and the Rock Cycle
Assessment Questions
1. What type of rocks require weathering and erosion
to form?
a.
b.
c.
d.
sedimentary
metamorphic
igneous
sedimentary, metamorphic, and igneous
ANS: A
22.3 Rocks and the Rock Cycle
Assessment Questions
1. Igneous rocks are formed when rock is
transformed by heat, pressure, or chemical
reactions.
True
False
22.3 Rocks and the Rock Cycle
Assessment Questions
1. Igneous rocks are formed when rock is
transformed by heat, pressure, or chemical
reactions.
True
False
ANS:
F, Metamorphic