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Sections 3.2, 3.3 and 3.4
TYPES OF ROCKS
IGNEOUS ROCKS
How are igneous rocks classified?
 Igneous rocks (ig nee us) are formed from
magma or lava. Igneous means fire-formed.
 Igneous rocks are classified according to
their crystal size and silica content!
 The origin of a rock is where the rock was
formed.

Extrusive Rocks: igneous rocks which formed
from lava which erupted on the earth’s surface.
 Ex. Basalt: forms most of the crust which
includes the crust on the ocean floor.

BASALT
IGNEOUS ROCKS


Intrusive Rocks: igneous rocks that formed
when magma hardened beneath the earth’s
surface.
Ex. Granite: the most abundant intrusive rock in
the part of the crust that makes up the
continents. Granite forms the core of many
mountain ranges
GRANITE
IGNEOUS ROCKS

Crystal Size
 The
texture of an igneous rock depends on
the size and shape of its mineral crystals.
 The texture may be fine grained, coarsegrained, glassy, or porphyritic.
 Rapid cooling lava forms fine-grained igneous
rocks with small crystals.
 Slow cooling magma forms coarse-grained rock
with large crystals.
IGNEOUS ROCKS
A rock with large crystals scattered on a
background of much smaller crystals has a
porphyritic texture
 Porphyritic rocks form when intrusive rocks
cool in two stages. As the magma begins to
cool, large crystals form slowly. The
remaining magma, however, cools more
quickly, forming small crystals.


Extrusive rocks have a fine grained or glassy
texture.
IGNEOUS ROCKS

Mineral Composition
 Most
of Earth's minerals contain silica, a
material formed from oxygen and silicon.
 The silica content of magma and lava affects
the types of rock they form.
 Lava that is low in silica usually forms darkcolored rocks such as basalt.
 Magma that is high in silica usually forms
light-colored rocks, such as granite.
IGNEOUS ROCKS
Observing
You can learn about a rock's mineral
composition by looking at a thin slice.
1. The diagram shows a thin slice of an
igneous rock. The key identifies
different minerals. Which mineral makes
up most of this rock? How did you
decide?
2. Which mineral is present in the
smallest amount?
SEDIMENTARY ROCKS
From Sediment to Rock
 Sediment means “something that settles”

 Sedimentary
rocks form from particles
deposited by water and wind. These particles
are called sediment.
 Sediment are small, solid pieces of material
that come from rocks or living things.
SEDIMENTARY ROCKS
Erosion occurs when running water or wind
loosen and carry away these fragments of
rock.
 The moving water or wind slows and
deposits the sediment

SEDIMENTARY ROCKS

After sediment has been deposited, the
processes of compaction and cementation
change the sediment into sedimentary
rock.
 Sediment
may include shells, bones, leaves,
stems, and other remains of living things.
 Over time, any remains of living things in the
sediment may slowly harden and change into
fossils trapped in the rock.
Sedimentary Rock Formation Sedimentary rocks form
through the deposition, compaction, and cementation of
sediments.
A. Water or wind deposits sediment.
B. The heavy sediments press down on the layers
beneath.
C. Dissolved minerals flow between the particles and
cement them together.
SEDIMENTARY ROCKS
At first the sediments fit together
loosely. But gradually, over millions of
years, thick layers of sediment build up.
These layers are heavy and press down on
the layers beneath them.
 Then compaction occurs. Compaction is the
process that presses sediments together.

SEDIMENTARY ROCKS
While compaction is taking place, the
minerals in the rock slowly dissolve in the
water.
 The dissolved minerals seep into the
spaces between particles of sediment.
 Cementation is the process in which
dissolved minerals crystallize and glue
particles of sediment together.

TYPES OF SEDIMENTARY ROCKS
There are three major groups of
sedimentary rocks: clastic rocks, organic
rocks, and chemical rocks.
 Clastic Rocks

 Clastic
rocks form when rock fragments are
squeezed together.
 Ex. shale, sandstone, conglomerate and
breccia
 Usually occurs under water.
Shale
Conglomerate
Sandstone
Breccia
SEDIMENTARY ROCKS
Organic Rocks
 Forms where the remains of plants and
animals are deposited in thick layers.
 The term “organic” refers to substances
that once were part of living things or
were made by living things.
 Two important organic sedimentary rocks
are coal and limestone.

SEDIMENTARY ROCKS
Coal
Coal forms from the
remains of swamp
plants buried in
water. As layer upon
layer of plant
remains build up, the
weight of the layers
squeezes the
decaying plants.
Over millions of
years, they slowly
change into coal.
SEDIMENTARY ROCKS
Limestone
How does limestone form? In
the ocean, many living things,
including coral, clams,
oysters, and snails, have
shells or skeletons made of
calcite. When these animals
die, their shells pile up as
sediment on the ocean floor.
Slowly, the pressure of
overlying layers compacts the
sediment. Some of the shells
dissolve, forming a solution
of calcite that seeps into the
spaces between the shell
fragments. Later, the
dissolved material comes out
of solution, forming calcite.
The calcite cements the shell
particles together, forming
limestone.
SEDIMENTARY ROCKS
Chemical Rocks
 Chemical rock forms when minerals that
are dissolved in a solution crystallize.
 Chemical rocks can also form from mineral
deposits left when seas or lakes
evaporate. Rock salt is a chemical rock
made of the mineral halite, which forms by
evaporation.

 Gypsum
also forms this way!
Examples of chemical rocks
R
o
c
k
S
a
l
t
Gypsum
METAMORPHIC ROCKS
The word metamorphic comes from the
Greek words meta, meaning “change,” and
morphosis, meaning “form.”
 Heat and pressure deep beneath Earth's
surface can change any rock into
metamorphic rock.
 When rock changes into metamorphic
rock, its appearance, texture, crystal
structure, and mineral content change.

METAMORPHIC ROCKS
With the added heat and pressure, the bonds
between atoms can break
 When new bonds form and the atoms join
together differently we call this recrystallization
 This is one way that metamorphic rocks form

METAMORPHIC ROCKS
While metamorphic rocks are forming,
high temperatures change the size and
shape of the grains, or mineral crystals, in
the rock.
 In addition, tremendous pressure squeezes
rock so tightly that the mineral grains may
line up in flat, parallel layers.
 Geologists classify metamorphic rocks by
the arrangement of the grains that
make up the rocks.

Metamorphic Rocks Great heat and pressure
can change one type of rock into another.
Granite becomes gneiss, shale becomes slate,
and sandstone changes to quartzite.
Metamorphic rocks that have their grains
arranged in parallel layers or bands are said to
be foliated. Ex. Slate, schist, gneiss
Gneiss
Slate
Schist
Sometimes metamorphic rocks are
nonfoliated. The mineral grains in these
rocks are arranged randomly. Metamorphic
rocks that are nonfoliated do not split into
layers.
Marble
Quartzite
NONFOLIATED
One reason that metamorphic rocks do not
show foliation is that it is made up of only one
type of mineral, so the different minerals
cannot separate and line up in layers
 Another reason that a metamorphic rock may
lack foliation is that it has not been subjected
to high pressure!

SUMMARY






Igneous rocks form from magma or lava.
Igneous rocks are classified according to their origin,
texture, and composition.
Most sedimentary rocks form from sediments that are
compacted and cemented together.
The three types of sedimentary rocks are clastic rocks,
organic rocks, and chemical rocks.
In a process that takes place deep beneath the surface,
heat and pressure can change any type of rock into
metamorphic rock.
Geologists classify metamorphic rock according to
whether the rock is foliated or nonfoliated.