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Rocks
And the
Rock Cycle
Created by Mrs. Hutchins
Sedimentary Rocks
There are three different
types of sedimentary
rocks: clastic, chemical,
and organic.
Clastic sedimentary rocks
are the rocks that most
people are familiar with.
Forming clastic sedimentary
rock is a four-step process.
Breaking Rocks Apart - Step 1
Feldspar chemically
changes into clay
minerals.
Chemical Weathering
Minerals from rock are
dissolved into rainwater
or changed from one
type of mineral to
another.
Quartz does not
weather easily.
Chemical weathering
happens quickly in warm,
moist environments. The
water needed for
chemical weathering is
present, and the heat
speeds up the process.
Physical Weathering
Physical weathering
occurs from water flowing
in rivers and streams.
Water freezing and
expanding can crack
rocks apart.
Physical weathering also
occurs because of wave
action on the shoreline.
Large temperature
changes cracks rock.
Tree and plant roots push
rocks apart, especially in
areas where there is little
topsoil.
Biological Weathering
Weathered rock moved by
water, wind, or plants and
animals become part of the
soil.
Soil is formed when rocks
are weathered at the surface
and then combine with plant
and animal remains.
The ridges of rock here are more resistant to
weathering than the surrounding rock.
Rocks on the Move – Step 2
Large pieces of rock are
carried in strong currents.
They settle quickly to the
bottom when the current
slows down.
Small particles (silt, clay)
are carried by slow currents.
They settle very slowly onto
the bottom of quiet waters
like lakes, swamps, and
lagoons.
Sediments Settle Down – Step 3
Larger pieces of rock settle more quickly than
small pieces of rock.
The settlement of bits of rock to the bottom of the
water is called deposition.
Sediment to Solid Rock – Step 4
Cement holds the sediments together. Cement is a
mineral that forms when seawater or groundwater
travels through empty spaces between the sediments. If
the water contains chemicals that are needed, the
mineral crystals will form in between the sediments. The
minerals hold the sediments together. The mineral
crystals and the water form the “cement” to hold the
sedimentary rock together.
Common minerals include calcite, quartz, and hematite.
Chemical Sedimentary Rocks
Chemical sedimentary
rocks are not made from
sediments. Instead, they
are comprised of mineral
crystals made from
elements and dissolved in
water.
Calcite is
typically found
in limestone.
Through evaporation the
dissolved elements form
crystals.
Halite is rock salt and used
to season your food.
This picture shows the formation of salt through
evaporation in Death Valley, California.
Organic Sedimentary Rocks
Sediments are made of fossils.
Animal parts that become cemented
together make limestone.
Organic rock made from tiny silica
skeletons is called chert.
Plant remains that go through the
lithification process becomes coal.
Igneous Rocks
• Igneous rocks form either above ground or
underground.
• An igneous rock that forms above the
ground is called an extrusive igneous rock.
• An igneous rock that forms under the
ground is called an intrusive igneous rock.
Intrusive Igneous Rocks
• There are many cracks and crevices deep within
the earth.
• Magma from volcanoes often flow through these
cracks and crevices into pockets within the
earth. The magma is under a tremendous
amount of pressure and is very hot.
• The magma in the pockets cools very slowly.
This gives the minerals in the rock time to form
large crystals. These crystals are large and you
do not need a microscope to see them.
A large pocket of magma underground is called a
basolith.
Minerals in Igneous Rocks
• There are primarily 8 different types of minerals
in rocks. The minerals are
–
–
–
–
–
–
–
–
Olivine
Pyroxene
Amphibole
Biotite (mica)
Muscovite (mica)
Orthoclase (feldspar)
Plagioclase (feldspar)
Quartz
Granite is the most commonly
known intrusive igneous rock.
Can you see the crystals that have formed in the rock?
What is granite used for?
Extrusive Igneous Rock
• Extrusive rocks are formed when magma
from the upper mantle of the earth (50 to
150 km below the surface) makes its way
to the surface.
• The magma may explode from a volcano,
or flow just under the surface in channels
and tubes.
Extrusive Igneous Rocks
• If the magma (lava) cools very quickly there is
very little, if any, crystal formation in the rock.
Instead, the magma cools into glass.
• Magma that cools very quickly creates pumice,
obsidian and scoria.
• Pumice has many air pockets and is very light. It
can be used in polishes and soap. It floats!
• Scoria has some air holes in it and is fairly
rough. It is a kind of glass. Obsidian is a very
smooth black glass.
Pumice
Obsidian
Scoria
The magma from an exploding volcano
tends to cool very quickly. Pumice
forms with the air as the volcano
explodes. Scoria and obsidian form
more slowly as the magma cools.
Magma that flows in large
channels or tubes cools
more slowly still. The rock
from this slower cooling
magma tends to have very
small crystals formed in it.
The rock formed is called
basalt.
Metamorphic Rocks
• One of the places metamorphic rocks are
formed is where tectonic plates come
together. At that location there is very hot
temperatures and very high pressure.
Metamorphic Grade
• The amount of metamorphism that takes
place in a rock is called the metamorphic
grade.
• Little metamorphism takes place at
relatively low temperatures – about 200
ºC, a little higher than the oven
temperature to bake chocolate chip
cookies. Mica minerals crystallize. These
are found in the igneous rock
granite and the metamorphic
rock schist.
Metamorphic grade, continued
• Intermediate grade metamorphic rocks
form between 200ºC and 800ºC. These
rocks contain mica and garnet.
• High grade metamorphic rocks form in
places where most rocks almost melt. It is
800ºC or above.
Foliation
• Sometimes the minerals in the rock form
crystals that are lined up in right angles to
the direction of the pressure. This is called
foliation.
• At low temperatures shale morphs into
slate.
• At medium temperatures schist forms.
• At high temperatures gneiss forms.
Non-foliation
• Often the minerals do not form layers. Rocks
that morph without layers are said to be nonfoliated.
• Marble morphs from limestone at medium to
high temperatures.
• Quartzite morphs from sandstone at medium to
high temperatures.
• Metabasalt (greenstone) morphs
from basalt at low temperatures.
Sometimes greenstone is foliated.
It is then called greenschist.
Contact Metamorphism
• Contact metamorphism occurs when rocks
come into contact with hot magma.
• Underground the rock that surrounds the
basolith area can be altered because of
the high temperature of the magma and
the length of time it takes to cool.
• Above ground the rock is altered on the
surface from the lava that flows onto it and
over it.
The Rock Cycle – Explained
All rocks on the surface are exposed to weathering –
mechanical, chemical, and biological
The weathered rock is transported by water and wind,
gradually settling out in a process called deposition.
Deposition means the rock settles into layers.
The layers are placed under pressure and go through a
process of forming together called lithification.
The new rock might be uplifted to the surface and be
weathered once more. Or the new rock may become more
buried and become exposed to additional pressure and
heat, thereby becoming a metamorphic rock.
Metamorphic rock may be uplifted to the surface through
plate movement or earthquakes. If it is uplifted to the surface
it becomes weathered once more and starts into the rock
cycle once again.
Metamorphic rock may also be placed under additional heat
and pressure, melting into magma.
If the magma is trapped under the earth and cools
slowly, large crystals will form in the rock as the magma
cools. This magma becomes an intrusive igneous rock.
The rock may eventually come to the surface and
become weathered, or it may be buried deeper and go
through metamorphosis again.
The magma may come to the surface in a volcanic
eruption. When the magma comes to the surface and
cools quickly it turns into extrusive
igneous rock. There is very little
crystallization of mineral crystals
that occurs. Weathering occurs.
How long does the rock cycle take?
• The rock cycle is an on-going process.
Weathering is always happening,
volcanoes are always erupting,
earthquakes occur on a regular basis, and
the tectonic plates are always moving
causing uplift and subduction.
Key Words to Know:
• Igneous Rocks
– Intrusive
– Extrusive
Sedimentary Rocks
Clastic
Organic
Chemical
Metamorphic Rocks
Foliated
Non-foliated
Weathering
Mechanical
Chemical
Biological
Rock cycle
Transportation
Deposition
Lithification
Crystallization
Uplift
Credits
• University Corporation for Atmospheric
Research (UCAR), University of Michigan
• Rock Hounds
• Encarta