The Rock Cycle
The Rock Cycle
 Through many different geologic
processes, the rocks that make up the
earth’s crust form, change, get
destroyed, and re-form over and over
again. This is why it is called the rock
CYCLE.
 There are constructive forces and
destructive forces that drive the cycle.
The Beginning
 Find “magma” on your rock cycle
diagram.
 ALL rocks start as magma, thus the rock
cycle begins in the earth’s interior where
magma is still in the liquid state.
 It is placed at the bottom of this diagram
because magma is found underground.
 Will not always be at the bottom of all
rock cycle diagrams!
The First Process
 As magma reaches the crust it is
exposed to cooler temperatures and will
harden.
 This is called crystallization
 Once crystallization has occurred, the
magma is “hard as a rock.”
 This is a constructive force of nature.
Igneous Rock
 Rocks that are formed directly from
cooled magma are classified as igneous
rocks.
 If the magma crystallized on the surface
of the crust (on land), it is called
extrusive igneous rock.
 If the magma crystallized while still in the
crust (underground), it is called intrusive
igneous rock.
Examples
 Some examples of igneous rocks are:

Intrusive:
 Granite,
gabbro, quartz (also a mineral)
 Can usually see crystals or “specks” in the
rock and mutlicolored

Extrusive:
 Basalt,
obsidian, scoria (“lava rock”)
 Usually one color without visible crystals,
sometimes have many holes or porous
Next Process
 Once igneous rocks are formed they
become exposed to the elements on the
surface of earth.

wind, rain, temperature changes, sun
exposure
 This exposure causes:


Weathering – breaking into pieces
Erosion – being moved (washed / blown
away) to another location
Lots and Lots of Little Pieces
 Once the destructive forces of weathering
and erosion have occurred, sediments
are formed.
 Dirt or soil, sand, silt, mud are all
examples of sediments that have piled up
after erosion
 Plants would not survive without this part
of the rock cycle! No plants = no animals!
The Third Process
 Sediments pile up in low areas after
erosional forces have slowed or stopped.
This is called deposition.
 As deposition continues, layers of
sediments continue to pile up, and they
compact the layers on the bottom. This
is called compaction.
 Once compaction occurs, the sediments
begin to “glue” together which is called
cementation. (like the word cement)
Third Process Continued
 These constructive forces of…



Deposition
Compaction
Cementation
 …are all steps in what is called
lithification- the process of loose
sediments turning into rock.
 Once sediments lithify, new rock forms
Sedimentary Rocks
 After lithification of the sediments, the
rocks that are formed are classified as
sedimentary rocks.
 Sedimentary rocks are the only type of
rock that fossils are found in because of
the way they form.
Examples
 Sedimentary Rocks are often coarse in
texture and have layers, and or visible
grains
 Common sedimentary rocks are:





Sandstone
Chert (flint rock)
Conglomerate
Limestone
Shale
The Fourth Process
 Because of the way sedimentary rocks
form, they usually get pushed further and
further down into the crust from
sediments continuing to layer on top.
 The deeper the sedimentary rocks get in
the crust, they are exposed to increasing
heat and pressure.
Fourth Process Continued
 This increase in heat & pressure
changes the rock, a lot like pressing 2
colors of playdough together…they’ll
eventually mix colors.
 This change in the rock due to high heat
and pressure is called metamorphism,
(both a destructive and constructive
force) and a new type of rock is formed.
Metamorphic Rock
 The process of metamorphism creates
the rocks classified as metamorphic
rocks.
 Only occurs deep in the crust or near
volcanoes where there is high enough
temperatures and pressure. Therefore
most metamorphic rock is found deep
underground or near volcano sites.
Examples
 All metamorphic rocks have a “parent
rock,” or the original igneous or
sedimentary rock it came from.
 Common metamorphic rocks:



Slate (parent rock = Shale)
Marble (parent rock = Limestone)
Gneiss (parent rock = Granite)
Last Process
 If metamorphic rocks remain deep
underground they eventually get pushed
so far down that they begin melting back
into the magma.
 This last destructive force completes the
rock cycle.
 The whole thing can then start all over!
Loops within the Rock Cycle
 If an igneous rock remains underground,
it will go through metamorphism and turn
into metamorphic rock.
 If sedimentary or metamorphic rocks are
exposed to the surface of the crust, they
will experience weathering and erosion
and turn into sediments.