In general, a rock is a group of minerals bound together

CHAPTER

6

Rocks

6.1 How Rocks Form
6.2 Igneous Rocks
6.3 Sedimentary Rocks
6.4 Metamorphic Rocks

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Rocks

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rock
igneous
magma
sedimentary
sediment

6.1 How Rocks Form
In general, a rock is a group of minerals bound
together.
Igneous, sedimentary, and metamorphic rocks are
formed, broken down, and reformed in a recurring
process called the rock cycle.

metamorphic
rock cycle

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felsic
mafic
pluton
batholith

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6.2 Igneous Rock
Igneous rocks form as molten rock solidifies, as
magma deep in the crust or lava at Earth’s surface
cool.
Felsic magmas form light-colored, silica-rich rocks.
Mafic magmas form dark-colored rocks rich in iron
and magnesiums. Igneous rock texture depends
mainly on the rate at which magma or lava cools.

Granite is igneous rock
formed from felsic magma.

Basalt is igneous rock
formed from mafic magma.


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mafic
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6.2 Igneous Rock
Igneous rocks are grouped into families by mineral
composition and texture.
Chemical
Composition

Texture
coarsegrained

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felsic

granite

felsicintermediate

granodiorite

intermediate

diorite

mafic

gabbro

ultramafic

peridotite,
dunite,
pyroxenite

finegrained

glassy

porous

rhyolite

obsidian

most pumice

andesite
diabase

basalt

basalt
glass

scoria


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felsic
mafic

6.2 Igneous Rock
Magma that solidifies underground forms various
types of igneous intrusions.

pluton
batholith

Volcanic neck

Laccolith

Volcano
Stock

Dike

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Sill

Batholith

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6.3 Sedimentary Rock
Sedimentary rocks form from sediments that result
from weathering and erosion of rock at Earth’s
surface. They often occur in layers, formed over
time as different sediments are deposited on top of
each other.


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6.3 Sedimentary Rock
Sedimentary rocks are grouped by the type of
sediment from which they form: clastic, chemical,
or organic.

Clastic: Sandstone

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Chemical:
Rock Salt Flat

Organic:
Limestone Cliffs


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6.3 Sedimentary Rock
Clastic sediments are often sorted by water action
before pressure and mineral cements turn them into
rock.
1. A river moves
sediment into a lake.

Sands and
Gravels

Conglomerate
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2. Particles are sorted by size. The
largest gravels are the first to be
deposited, followed by sands, and
then silt and clay.

Sands

Sandstone

Silt and

Clay

Shale

3. Over time, the
sediments are
buried, compacted,
and may be
cemented.


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6.3 Sedimentary Rock
Clastic sediments are often sorted by water action
before pressure and mineral cements turn them into
rock.
Fossils, ripple marks, mud cracks, nodules,
concretions, and geodes features associated with

sedimentary rocks.

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parent rock
metamorphism

6.4 Metamorphic Rock
Metamorphic rocks form when heat or pressure or
both alter parent, or preexisting, rocks.

deform
becomes

Shale

becomes

Slate

becomes
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Phyllite

Schist


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parent rock
metamorphism
deform

6.4 Metamorphic Rock
Metamorphic rocks form when heat or pressure or
both alter parent, or preexisting, rocks.
Metamorphism can occur across a region, as in
mountain building events, or it can occur in smaller
local areas.
A metamorphic rock may be described and
identified according to its parent rock, mineral
composition, and texture.

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