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Rocks

Rock: A rock is any solid mass of mineral or
mineral-like matter that occurs naturally as
part of our planet.
 Most rocks contain multiple types of minerals.
Each mineral retains its properties within the rock.

There are 3 major types of rocks: Igneous,
Sedimentary, and Metamorphic.
Interactions among
Earth’s water, air,
and land can cause
rocks to change
from one type to
another. This
continuous process
that causes rocks
to change is the
Rock Cycle.
Sedimentary Rock: A rock
formed by the compaction and
cementation of sediments.
Weathering: Process where
rocks are physically and/or
chemically broken down by
water, air, and living things.
The weathered pieces are
called sediments.
Weathering and Erosion can
change any rock into a
sedimentary rock.
Metamorphic
Rock: A rock
altered by heat
and pressure.
Heat and Pressure
and can change
any rock into a
metamorphic
rock.
Igneous Rock: A rock formed by
the solidification of magma.
Magma-molten material that
forms deep beneath the Earth’s
surface.
Lava-Magma that reaches the
surface
In order to become an igneous
rock, magma must cool beneath
the surface or as a result of a
volcanic eruption becomes an
igneous rock.

Processes driven by heat from the Earth’s
interior are responsible for forming both
igneous and metamorphic rocks.

Weathering and the movement of weathered
materials by water, gravity, glaciers, and wind
are external processes powered by the
energy from the sun. External processes
produce sedimentary rocks.
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1. What is a rock?
2. What are the three major types of rocks?
3. What is the rock cycle?
4. What powers the Earth’s rock cycle?
5. How might a sedimentary rock become an
igneous rock?

Igneous Rocks forms from the solidification
and crystallization of magma or lava.

There are two types of igneous rocks.
 Intrusive
 Extrusive

Intrusive Igneous Rocks form when magma
solidifies under the Earth’s surface.

Magma is less dense than the surrounding rocks,
so it slowly works its way toward the surface.

As it rises, it cools it allows elements to combine
and form minerals. Since the magma cools
slowly, the minerals are able to grow in size.
Diorite (Left) and Granite (Right) are both examples of intrusive
igneous rocks. The slow cooling of magma beneath the Earth’s
surface allows the growth of large mineral crystals.

Extrusive Igneous Rocks form when lava
solidifies above the Earth’s surface.

Since the lava cools much quicker above the
Earth’s surface, the minerals in extrusive
igneous rocks are not able to grow as large.
Rhyolite (Left) and Basalt (Right) are both examples of extrusive
igneous rocks. The rapid cooling of lava resulted in a rock with
small, interconnected mineral grains.

Texture and composition are two characteristics
used to classify igneous rocks.

The rate of cooling strongly affects the type
texture.

Texture Classifications: Coarse-Grain, FineGrain, Glassy, and Porphritic

Composition Classifications: Granitic, Basaltic,
Other Compositions
COARSE-GRAIN TEXTURE:
THE SLOW COOLING OF
MAGMA ALLOWS THE
FORMATION OF LARGE
CRYSTALS.
FINE-GRAIN TEXTURE: THE
RAPID COOLING OF MAGMA
OR LAVA RESULTS IN
SMALL, INTERCONNECTED
MINERAL GRAINS.
GLASSY TEXTURE: WHEN LAVA
IS SPEWED ONTO THE
SURFACE, IONS DON’T HAVE
TIME TO ARRANGE
THEMSELVES INTO CRYSTALS
AND THE RESULT IS A GLASSY
TEXTURE.
PORPHYRITIC TEXTURE: MINERALS DO
NOT FORM AT THE SAME RATE SO IT IS
POSSIBLE FOR SOME MINERALS TO
BECOME QUITE LARGE BEFORE
OTHERS BEGIN TO FORM. THESE LARGE
CRYSTALS THAT ARE SURROUNDED BY
FINE GRAINED MINERALS ARE CALLED
PHENOCYRSTS.

Granitic
Composition:
Igneous rocks
that are made
almost entirely
of the lightcolored silicate
minerals quartz
and feldspar.

Granitic rocks
are the major
rocks of the
continental
crust.
Rhyolite is an extrusive granitic rock.
Notice that is light in color’

Basaltic
Composition:
Rocks that
contain many
dark silicate
minerals and
plagioclase
feldspar.

They are rich in
iron which
makes them
darker and
Gabbro is an intrusive igneous rock
denser than
granitic rocks. with basaltic composition.

Other compositional groups: Rocks with a
composition between granitic and basaltic have
an andesitic composition. They contain at least
25% dark silicate minerals.

Ultramafic: Peridotite is an important igneous
rock that is composed mostly of olivine and
some pyroxene. It is composed almost entirely
of dark silicate minerals so its composition is
referred to as ultramafic.

All sedimentary rocks form when existing
rocks are broken down into sediments.

These sediments are compacted and
cemented together.

Weathering: Chemical or physical breakdown
of rock into smaller pieces called sediments.

Erosion: The carrying away of the sediments
by agents such as water, wind, ice, or gravity.

Deposition: When the agent loses energy
and drops the sediments. Sediments are
deposited according to size.
 Large sediments are deposited first.

Compaction and cementation turns
sediments into sedimentary rock.

Compaction: is a process that squeezes
sediments. It is caused by the weight of the
sediments.

Cementation: takes place when dissolved
minerals are deposited in the tiny spaces
among the sediments.

Sedimentary rocks can be classified into two
main groups according to the way they form.
 Clastic Sedimentary Rocks: Made of weathered
bits of rocks and minerals.
 Chemical/Biochemical Sedimentary Rocks:
Forms when dissolved minerals precipitate from
water solutions.

Grouped according to the size of the sediments in the
rocks.
Conglomerate is
a clastic
sedimentary rock
that contains
large (greater
then two
millimeters in
diameter)
rounded particles.
Siltstone is a
clastic
sedimentary
rock that forms
from silt-size
(between 1/256
and 1/16
millimeter
diameter)
weathering
debris.
Sandstone is a
clastic
sedimentary
rock made up
mainly of sandsize (1/16 to 2
millimeter
diameter)
weathering
debris
Shale is a
clastic
sedimentary
rock that is
made up of
clay-size
(less then
1/256
millimeter in
diameter)
weathering
debris

The precipitation that forms chemical
sedimentary rocks occurs when the water
evaporates or boils off leaving a solid
product.
Rock Salt
is a
chemical
sedimentary
rock that
forms from
the
evaporation
of ocean or
saline lake
waters

The many unique features of sedimentary
rocks are clues to how, when, and where the
rocks formed.

In undisturbed sedimentary rocks, the oldest
layer of deposition is at the bottom and the
youngest is at the top.

Ripple marks and mud cracks can give clues
as to the environment that the rock formed in

Fossils: traces or remains of ancient life are
unique to some sedimentary rocks such as
limestone.
 Fossils play a key role in matching up rocks from
different places that are the same age.
 Fossils can be used to help answer questions
about the rocks that contain them such as did the
rock form in land or water? Was the climate hot or
cold? When did the rock form?

Form when existing rocks are changed by heat and
pressure, usually a few kilometers below the Earth’s
surface and extend to the upper mantle.

Often the changed rock looks different and has a
different chemical composition than the parent rock
(parent rock=original rock).

There are two types of metamorphism: Contact and
Regional

Contact
Metamorphism: Hot
magma moves into
rock causing contact
metamorphism to
take place.
 Produces low-grade
metamorphism which
are minor changes in
the rock. Example:
Marble

Regional
Metamorphism:
During mountain
building, large areas
of rocks are
subjected to extreme
pressure and
temperatures.
 Produces high-grade
metamorphism and
large-scale
deformation.

The agents of metamorphism are heat, pressure, and
hydrothermal solutions.

Heat: The most important agent because it provides
energy to drive chemical reactions.
 Two sources of heat: Magma and depth

Pressure: Just like temperature, pressure increases with
depth. Pressure causes the spaces between mineral grains
to close.

Hydrothermal Solutions: Hot, water based solutions that
escape from magma. Promote recrystallization by
dissolving minerals and depositing new ones.

Like igneous rocks, metamorphic rocks are
classified based on texture and composition.

There are two groups: foliated and
nonfoliated

When rocks undergo contact metamorphism,
they become more compact and dense. The
pressure causes minerals to align in a similar
direction. This gives the rock a layered or
banded appearance.
 Under extreme conditions, minerals recrystallize
with a preferred orientation-at a right angle to the
direction of force.
Gneiss is
foliated
metamorphic
rock that has a
banded
appearance
and is made up
of granular
mineral grains.
It typically
contains
abundant
quartz or
feldspar
minerals

A metamorphic rock that does not have a
banded texture.

Most nonfoliated rocks contain only one
mineral.
Quartzite is a non-foliated
metamorphic rock that is produced by
the metamorphism of sandstone. It is
composed primarily of quartz
Marble is a non-foliated
metamorphic rock that is produced
from the metamorphism of
limestone. It is composed primarily
of calcium carbonate