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Chapter 2
Rocks
Introduction to the world
directly beneath our feet
Why study rocks?
Three reasons…
To find and utilize
natural resources
Prepare for and
maybe prevent
natural disasters
Learn about
earth’s natural
history
Natural Understand Earth’s
Resources Disasters History
Coal Earthquakes All rocks tell
Petroleum Volcanoes an individual
Gravel Landslides story about
Uranium their past
Tsunami’s
All metals If we look and
used in Sink Holes listen closely,
construction Foundation we can gain
Aquifers Problems knowledge of
our planet’s
history
Three basic rock types
We classify rocks into three categories
These categories are distinguished by how
each rock type is formed
Igneous Sedimentary Metamorphic
Rocks Rocks Rocks
The Three Rock Types
Igneous– Rocks Born of Crystallized
Fiery Molten Magma or Lava
Sedimentary– Rocks Born of Layered
Water or Wind Deposits
– Rocks Born of a
Metamorphic
Change Deep Within the Earth
Magma or Lava….which is it?
Magma – hot
molten rock that is
beneath the surface
of the earth
Lava– hot molten
rock that has
reached the surface
of the earth
How does an Igneous Rock Form?
Crystallization – the solidifying of magma
either at depth or on the exposed surface
of the earth
To become Sedimentary---
1. Weathering – rocks
exposed at the surface
undergo disintegration
and decomposition from
natural forces
Water, wind, glaciers,
waves, heat, etc…
All these forces will
break down rocks and
transport the smaller
broken down pieces
These broken down
pieces are now called…
SEDIMENTS
Sediments – the eroded particles of
rock that have been transported
away from the original rock
2. Transportation
3. Lithification (compaction and
cementation)– literally means
“conversion into a rock”
Lithification is when compacted and
cemented sediments get buried and
formed into a new rock
Hence, sedimentary rocksaddition
More pressure is applied due to the
of overlying layers
Sediment grains are fused and cemented
together as the water content is reduced
Check out all these sedimentary layers
Sky Walk
Two things can happen to
sedimentary rocks:
They either get exposed at the
surface as sedimentary rocks for us
to find, or…
They continue to get buried deeper
and deeper within the earth’s crust
To become Metamorphic
1. rocks get buried very, very deep
2.The intense heat and pressure of
deep burial will ultimately change the
rocks into something new
Metamorphosis
Metamorphic Rock – a rock that has
been changed from its original
parent rock to a new form due to
intense heat and pressure
And if you want to go even deeper?
Rocks that reach depths where the
heat is extreme enough will turn
back into molten rock material...
We have come full circle
Time, we need more time
All the processes of the rock cycle
happen over incredible time spans
We cannot see the entire cycle in
action, it takes much too long
We see the results of different stages
of the cycle
Evidence for the cycle is found
everywhere, in every rock
What Cycle are we Talking about?
ROCK CYCLE – The series of geologic
processes that forms igneous,
sedimentary, and metamorphic rocks and
then breaks them down only to form
new rocks again at a later time and place
Developed by James Hutton
THE ROCK CYCLE
ROCK CYCLE with Diagrams
To Review:
There are 3 main Heat & Pressure form
reasons to study rocks metamorphic rocks
There are 3 rock types: The Rock Cycle is not
igneous, sedimentary, & just a one way street
metamorphic There are varying
Magma & Lava are not paths, and rocks can
quite the same thing reform or skip whole
Erosion must occur to sections of the cycle
form sediments Geologic processes
Deposition occurs to take incredible amounts
form sedimentary rocks of time
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IGNEOUS ROCKS
Fire Rocks
Yosemite
Igneous formations
PikesPeak in the Rockies--Colorado
Mount Rushmore, South Dakota
White Mountains of New Hampshire
Stone Mountain in Georgia
Yosemite-- California
Igneous Rock Facts
Form from the slow cooling of magma or
the quick cooling of lava on the surface
Most abundant rock on earth
Magma can rise from depths of 200 km or
more
This cross section
of the earth shows
how magma can
crystallize under
the surface or how
lava can erupt and
then cool on the
surface
Aboveground or Underground?
Igneous rocks can be subdivided into two
main categories: intrusive or extrusive
Intrusive – igneous rocks formed inside the
earth at depth from cooling of magma
Extrusive – igneous rocks formed on the
earth’s surface from cooling of lava
A note about magma: it’s mostly hot
molten rock, but it does contain some solid
material and some dissolved gasses
Classifying all the Igneous Rocks
Two criteria are used when
classifying igneous rocks:
TEXTURE – overall appearance of the
rock based upon the size and
arrangement of the crystals
MINERAL COMPONENTS - Chemistry
Textures
Four textures used for all igneous rocks
Coarse-grained texture (large)
Fine-grained texture (small)
Porphyritic texture (mixed)
Glassy texture (none)
Igneous Textures
Coarse-grained –
rocks that cool
slowly at depth and
crystals are large
enough to be seen
with the naked eye
Granite is a very common
This texture is example of a coarse-
inherently intrusive grained igneous rock
Rhyolite is the fine grained version of granite – same mineral different cooliing
history
Igneous Textures
Fine-grained – rock
cools more rapidly on
or near surface and
crystals are small
and can only be seen
with a microscope.
This texture is
inherently extrusive
Basaltic lava flows are
Hawaiian Islands and very common and are
good examples of fine-
Iceland composed grained igneous rocks
primarily of basalt.
Igneous Textures
Porphyritic – rock
that has large
visible crystals
embedded in a
matrix of fine-
grained crystals
Porphyritic rocks usually
This texture is both have the appearance of
intrusive & extrusive chocolate chip cookies
Igneous Textures
Glassy – rocks that
cool so rapidly that
crystals do not form
This Volcanic Glass
is the result of a
violent eruption
Obsidian is volcanic glass
and is the result of rapidly
This texture is cooling lava (no crystals)
inherently extrusive
The Chemistry of Magma
Magma has different chemicals in it
The chemicals determine what minerals will form
The same magma can actually produce a wide
variety of minerals and therefore a wide variety
of different igneous rocks
N.L. Bowen came up with a way to predict which
minerals will form in a cooling magma body
He charted mineral formation in the BOWEN
REACTION SERIES
The Sliding Scale of Igneous Rocks
Granitic Light-colored Rich in SiO2 Least Dense
Sparse Fe & Mg
(Felsic)
Andesitic Medium-colored
(Intermediate)
Basaltic Dark-colored
(Mafic)
Rich in Fe & Mg
Ultramafic
Very-dark Sparse in SiO2 Highest Density
Chemistry of Magma & Minerals
The Bowen Reaction Series (pg 39)
How do we know the names?
Igneous rocks are classified by their
texture and their chemistry right?
And Bowen figured out which minerals will
form when right?
So we make a chart that correlates all of
these characteristics
Then we simply give each rock a name
Igneous Rock Names (pg 41)
To Review:
Igneous Rocks form from There are 4 textures that
the cooling of either you need to know
magma or lava The four chemistry types
Igneous rocks can form are: granitic, andesitic,
on the surface or deep basaltic, & ultramafic
within the earth – Know how to read and
extrusive and intrusive understand the Bowen’s
We classify igneous reaction series
rocks by two criteria – Know how to read and
texture & chemistry understand an igneous
naming chart
SEDIMENTARY
ROCKS
Rocks of Wind & Water
Sedimentary Rock Facts
Remember, sediments are broken
down rock fragments from other
rocks due to weathering & transport
Sedimentary rocks are only 5-10% of
all rocks in the crust
But, sed rocks are over 75% of all
visible land rocks
Sed Rocks are great story tellers
Sedimentary rocks tell the story of the past
They show what kind of climate and area
the rock formed in
They show what kind of depositional
environment the rock was formed in
Ex. dunes, streams, ocean bottoms, salt
flats, etc…
And most importantly, they are the only
rocks that contain fossils
Two types of Sed Rocks
DETRITAL CHEMICAL
Sed rocks that Sed rocks that
form from the solid form from material
weathered particles that precipitates
of other existing out of an aqueous
rocks solution
DETRITAL ROCKS
All minerals can be weathered down into
sediments and transported
Therefore you will find all types of minerals in
sedimentary rocks
However, quartz & clay minerals are predominant
Quartz because it is very durable and,
Clay because they are weathered silicates
DETRITAL ROCKS
Names are based upon sediment particle size
Sediments can range from microscopic dust
specks to large house size boulders
Sand grains Boulder size sediments
DETRITAL ROCKS
Conglomerate – a
sed rock with large
rounded particles of
varying sizes
Breccia – a sed
rock that has large
jagged-edge
particles of varying
sizes
DETRITAL ROCKS
Sandstone – sed rock
that has uniform sand
grain size particles
only
Shale – sed rock that
has very fine grained
particles, the particles
are usually flat and
platy
Energy of Transport
The size of the sediment will tell how much
energy was present when the rock was formed in
the past
The larger the sediment sizes = the higher the
energy required to transport the particles
Examples: conglomerate and breccia rocks could
only have formed in high energy environments;
whereas, shale and sandstone will usually form in
much lower energy environments
Energetic Environments
Low energy depositional
environments might
include lakes, ponds, bays,
and small streams
High energy environments
might include raging
rivers, flood prone areas,
and glaciers
Zones of Deposition
High energy environments might include the steep mountain slopes,
rushing rivers, and the rugged continental shelf
Low energy zones would include lakes, lagoons, swamps, and the deep
ocean floor
CHEMICAL ROCKS
These are different from detrital rocks in that
the sediments are already dissolved in water
The sediments then precipitate out of that
water to form a rock
Three types:
EVAPORITES BIOCHEMICAL COAL
CHEMICAL ROCKS
Evaporites – rocks
form when a sea or
lake dries up and
leaves behind
minerals
– Sometimes these
are called “salt flats”
Usually halite and Rocksalt is a common name
for the evaporite rock halite
gypsum are the
minerals
CHEMICAL ROCKS
Biochemical – plants
and animals in water
excrete CaCO3 as
waste or they make
their shells out of it
Over time the CaCO3
will settle and be
deposited to make
LIMESTONE
Most abundant
chemical sed rock!
CHEMICAL ROCKS
Coal – is nothing more than buried
swamps or buried peat bogs
Plant material is “cooked” over long time
spans and transformed into a fossil fuel
Examples of modern day swamps
CHEMICAL ROCKS
Coal can differ depending upon how much it has
been cooked. The more it is cooked the harder it
becomes, the more dense it becomes, the less
water it will have in it, and the cleaner it burns.
Swamp Peat Lignite Bituminous Anthracite
These two charts show how the
different stages of coal develop
CHEMICAL
ROCKS
We have a lot of
Lignite in Texas...
The four
stages of coal
are shown in
progressive
order in this
diagram
How do seds become Sed Rocks?
There are two main ways that sediments
can be lithified into sedimentary rocks.
Remember, “lithified” literally means
turned into a rock
Compaction Cementation
Shale is the frock used in fracking
Lithification Part I
Compaction – seds
accumulate over
time, the weight of
overlying seds
compress the
deeper ones, pore
space in between
seds reduces and
the rock is
squeezed together
Lithification Part II
Cementation –
water percolates
through pores,
minerals dissolved
in the water are left
behind in the pore
spaces, these
minerals now act as
a glue holding the
sediments together
It Looks Like a Sed Rock!
The most characteristic feature of sedimentary
rocks are the strata or beds
Strata – layers of sed rocks that represent a
specific depositional period --most striking
feature of sed rock
Bedding Planes – these separate strata from each
other and represent the end of one depositional
period and the beginning of a new one
1000s of feet of strata
To Review:
Sed rocks are made up of The particle size gives
weathered particles from you clues to the energy of
other pre-existing rocks the past environment
Sed rocks can tell a vivid Chemical rocks are
named by the process
story of the past
that forms them
The two types of sed Lithification can occur
rocks are Detrital & either by compaction or
Chemical by cementation
Detrital rocks are named Strata is the most
by the particle size distinguishing feature of
sedimentary rocks
METAMORPHIC
ROCKS
Rocks of Change
The Keys to Metamorphic Rocks
Remember, “metamorphism” literally means to
change form
How does the original parent rock change??
The size, the shape, the texture, and even the
chemistry of the minerals can change!
Which kind of rocks can be changed into a
metamorphic rock? All Types:
– Igneous rocks can be changed
– Sedimentary rocks can be changed
– Even metamorphic rocks can be changed into other
kinds of metamorphic rocks
Meta-why?
What causes a rock to change?
Three factors:
1. Pressure from deep burial
2. Heat from deep burial or a nearby
magma body
3. Chemically active fluids percolating
through the rock (we don’t get into this)
Meta-how much?
2 levels of Metamorphism
Low-grade metamorphism
– hardly changed from the
original parent rock
High-grade metamorphism
– cannot even recognize the
original parent rock
The hotter and deeper the
rocks go the greater the
metamorphism will be
Different types of Change
There are 3 types of metamorphism:
– Burial Contact Regional
Burial Metamorphism
Burial
Metamorphism –
usually a low grade
change due to
pressure caused by
shallow to medium
burial; heat is not
a major factor Burial Metamorphism will
not drastically change the
parent rock
Contact Metamorphism
Contact Metamorphism – also usually a
low grade change; the parent rock comes
in contact with a magma body and gets
cooked enough to metamorphose
Contact
Metamorphism
Rising magma bodies will metamorphose nearby rocks
Regional Metamorphism
Regional Metamorphism – both heat and
pressure change rock on a grand scale;
this usually occurs during major tectonic
events such as mountain building
Regional
Metamorphism
The highest grade of change will occur during regional metamorphism
Texture You Say?
2 types of texture for metamorphic
rocks
Foliated Non-Foliated
Foliated Texture
In Foliated rocks
mineral grains align
parallel because of the
pressure & heat
This recrystallized
mineral alignment
causes the rock to
have a banded look
Rocks with multiple
mineral types will
exhibit this texture
Notice the foliation of
this gneiss sample
Non-Foliated Texture
In Non-Foliated rocks
the minerals do not
line up parallel
These rocks have a
uniform appearance
and no banding
Rocks that have only
one kind of mineral
will usually exhibit this
metamorphic texture
Non-Foliated rocks do not
have banding in them
Naming & Classifying
Metamorphic Rocks
We can group these into the two
categories based upon the types of
texture the rocks exhibit
Foliated Rocks Non-Foliated Rocks
•Slate •Marble
•Schist •Quartzite
•Gneiss •Anthracite
Naming Foliated Rocks
Slate – a low grade
metamorphism of the
parent rock shale
The thin platy mica
flakes line up and give
the rock foliation
Slate has great “rock
cleavage” and it used
as roof & floor tile, Slate is a low grade
metamorphic rock which
chalkboards, and was once used as hand held
billiard table tops chalk boards in schools
Naming Foliated Rocks
Schist – higher grade
metamorphism of the
rock slate
Schist is “platy” due to
the mica minerals,
and it can be split into
flakes & slabs
Schist is a high grade
metamorphic change Schist is a high grade
metamorphic rock with
obvious foliation
Naming Foliated Rocks
Gneiss (“nice”) – A
foliated rock from the
parent rock granite
Minerals are banded,
but instead of being
platy like schist, they
are granular and
rounded
Gneiss is a very high Notice the good
grade metamorphic foliation in this
sample of gneiss
change
Naming Non-Foliated Rocks
Marble – coarse
grained crystalline
rock from the parent New reduced
rock limestone price! It’s Jesus!!!
It has large crystals of
interlocking calcite
Calcite has a hardness
of 3 so it is relatively The calcite crystals
that make up
soft and great for limestone do not
change drastically
carving and sculpting when marble is
made; if anything,
Marble is a low-med they will grow in
metamorphic change size as they fuse
together
Naming Non-Foliated Rocks
Quartzite – very hard
rock made from the
parent rock sandstone
With heat/pressure
the quartz grains
simply fuse together
Quartzite is a
The rock quartzite is
medium-high grade not too different than
metamorphic change a large quartz crystal
Naming Non-Foliated Rocks
Anthracite – the
highest grade of coal
It forms from the
continued cooking of
bituminous coal
Anthracite is a high
grade metamorphism,
it is extremely hard,
and it burns almost
completely clean
Both of these anthracite
samples are hard and shiny
Naming Metamorphic Rocks
Metamorphic Parent Grade of Foliated or
Rock Name Rock Metamorphism Non-Foliated
Slate Shale Low Foliated
Schist Slate High Foliated
Gneiss Granite Very high Foliated
Marble Limestone Low-Med Non-Foliated
Quartzite Sandstone Med-High Non-Foliated
Anthracite Bit. Coal High Non-Foliated
To Review:
All aspects of a rock There are two types
can change during of textures in
metamorphism metamorphic rocks:
All rock types can foliated & non-foliated
undergo metamorphism Foliated rocks look
Heat, pressure, and banded due to mineral
fluids do the changing alignment
There are three types Non-foliated rocks do
of metamorphism: burial, not look banded
contact, & regional Learn the six rocks!
