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Rocks
Three main types, and their
formation/classification.
Igneous Rocks
• Igneous rocks cooled and hardened from
magma/lava
• They are further classified by their rate of
cooling (texture), and their color
(composition.)
• You can tell by looking at a rock how
quickly it cooled, and where it formed.
• You can tell by looking at the rock’s color,
what the chemical composition is.
Igneous Rocks: Where they form
gives them texture.
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• Extrusive Igneous Rocks
If the magma/lava cooled FAST, it
would not have time to develop
crystals.
If it cooled on the surface of the
earth’s crust, the crystals would be
small. Because the surface of the
earth is cooler than the inside of
the earth.
From lava
An example of small crystal
igneous rock would be BASALT.
An example of no crystals would
be SCORIA, OBSIDIAN, or
PUMICE. (SEE NEXT SLIDE FOR
PICTURE)
Said to have a fine grained texture
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• Intrusive Igneous Rocks
If the magma/lava cooled SLOW,
it would have the time for the
crystals to develop.
It would usually cool on the inside
of the earth’s crust, where it is
warmer than the surface. The
crystals would be large to very
large because they had time to
develop.
From magma
An example of large crystal rock
would be GRANITE
An example of very large crystal
rock would be a PEGMATITE.
(SEE NEXT SLIDE FOR
PICTURES)
Said to have a coarse grained
texture
Extrusive Igneous Rock Pictures:
Black basalt, typical of oceanic crust
Scoria, a dark form of Pumice, it
formed the same way
Pumice, cooled so quickly it has
holes where the gas escaped.
This rock can float because of
those trapped gas bubbles.
Extrusive Igneous Rock Pictures:
•This rock cooled so
quickly on the earth’s
surface, even the gas
bubbles didn’t have time
to form.
•It is natural volcanic
glass.
The grey spots make it “snowflake” obsidian instead of
just plain obsidian.
The “snow” flakes are volcanic ash that fell on the cooling
rock.
• Giorgio Armani actually
grinds this stuff up, puts it
in a moisturizer and sells it
for over $500.00 an
ounce. (See this is why
you need earth science!!!)
It is called Crema Nera,
and you need to be a VIP
to actually get it!!!
Intrusive Igneous Rock Pictures:
Granite, you can see
the individual crystals, it
must have cooled inside
the earth.
This is granite pegmatite, the
crystals are much larger, and
giving it the name
“pegmatite.”
Weird Textured Igneous Rocks:
Porphyritic Igneous Rocks
• An igneous rock can undergo two separate
coolings.
• This results in a very strange looking rock.
• It has both large crystals and small crystals
• This rock cooled both quickly and slowly.
• It is called a Porphyry.
• Rocks with this texture are said to have a
porphyritic texture.
Porphyritic Rocks: Pictures
•The big crystals in this rock are called
“Phenocrysts” or “Xenoliths”
•The term “xenolith” means “strange
guest”
•The term is appropriate because it
looks strange to have these coarse
grains in the fine grained rock.
•Lovingly referred to as the “puke” rock
by some, because of the chunky
texture.
Xenolith/Phenocryst
Igneous Rock’s color/composition
• Three types of color:
• Felsic: Light colored, contains feldspar minerals, and
quartz minerals. It traps gas, therefore it would come
from an explosive volcano.
• Intermediate: Medium colored, contains a balance of
feldspar and quartz minerals, but minerals high in
iron and magnesium. Iron and Magnesium are dark
colored minerals that do not trap gas. Don’t be fooled
these rocks still come from violent volcanic eruptions.
• Mafic: Dark colored, all made from “ferro-magnesian”
minerals. (ferro=iron) and magnesian, meaning
magnesium. These minerals do not trap gas, and are
not from very explosive volcanoes.
Color vs. Texture Examples of
Igneous Rocks:
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Intrusive- Felsic~> Granite
Extrusive- Felsic~> Rhyolite
Intrusive- Intermediate~> Diorite
Extrusive- intermediate~> Andesite
Intrusive- Mafic~> Gabbro
Extrusive- Mafic~> Basalt
Igneous Felsic Rocks
• These two rocks have
the same chemical
composition, but
differing textures.
• One is coarse grained
(Intrusive, cooled
slowly)
• One is fine grained
(Extrusive, cooled
quickly)
Granite
Rhyolite
Igneous Intermediate Rocks
• These two rocks have
the same chemical
composition, but
differing textures.
• One is coarse grained
(Intrusive, cooled
slowly)
• One is fine grained
(Extrusive, cooled
quickly)
Diorite, the
way I always
remember it is
black and
white is diorite,
like a little
rhyme.
Salem is famous for its
“Salem grano-diorite”
Andesite, named
after the Andes
mountains
Igneous Mafic Rocks
• These two rocks have
the same chemical
composition, but
differing textures.
• One is coarse grained
(Intrusive, cooled
slowly)
• One is fine grained
(Extrusive, cooled
quickly)
Gabbro, big
black crystals
Basalt, dark
oceanic rock.
Igneous Rock Structures
Igneous Rock Structure Key: All are intrusive
structures unless otherwise noted.
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A = Stock~>Less than 100 square km, usually part of a batholith, but the batholith
cannot be seen, so they named this stucture as a way of classification 
B & D = Batholith~> At least 100 square km, means “deep” rock, usually a core of a
mountain.
C = Laccolith~> (My favorite, because it is the cutest!!!) It is a mushroom shaped
igneous intrusion. Usually the core of a domed mountain.
E & G = Dike/Dyke~> Discordant (which means it cuts across the layers of rock)
F = Sill~> Concordant (which means it goes along with the layers of rock, like a
window sill, it is parallel to the rock layers)
H = Volcanic Mountain ~> Made from previous lava flows (extrusive)
I = Lava Flow ~> Lava from volcano that has spread down the volcanic mountain
(extrusive)
J = Cinder ash/cone ~> Made from a different type of lava it is a type of volcano
(extrusive)
K = Volcanic Neck ~> The remnant (or leftovers) of an old volcano. The rest of the
cone wore away, and all that is left is the pipe that the lava flowed out from.
(extrusive)
****Be Prepared to fill in the blanks for this type of drawing on the “Rock
Examination.”
Sedimentary Rocks
• Formation:
– They are made out of pre-existing rocks.
– Usually formed from sediments.
– Sediments are usually from igneous, sedimentary or
metamorphic rock (sometimes minerals too!!!)
– The sediments from by weathering (the breaking down of
a materials) or erosion (the carrying away of the
materials) and then the cementation and compaction (or
lithification of these sediments.
Sedimentary Rock: Process of
Lithification
• First the sediments get laid down flat (law of original horizontality
states that, because of gravity all sediments are originally laid down
flat.) Usually in water.
• These sediments pile on top of each other. Oldest on the bottom,
youngest on top (law of superposition.)
• As the sediments get laid down, they squish the water and the air
out of the spaces in between the particles. (compaction)
• Then the particles/sediment get cemented (glued together) by the
solution left behind in the remaining water, as it evaporates, all that
is left behind is the cement.
• Lithification~> The process of the sediments getting compacted and
cemented.
• The resulting rock is now considered to be a sedimentary rock
Sediments
• Sediment size will tell you a great deal about the origin of
the rock.
• Small particles will usually be older, and more worn
down, or rounder in shape.
• Angular or sharp sediments are usually younger and
have note been around for a long time.
• The sediment usually settles out in water according to
size.
– The heaviest/largest at the bottom and the smallest/lightest at
the top. This is again due to gravity.
– Look at my beautiful drawing to see “Joey” to see how this
settles out. I know, you are in awe of my talent, lol 
Sedimentary Rock Structures
Varves-
•Alternating
bands of
sediment laid
down in a calm
water
environment, like
a lake.
Ripple Marks•When wind or water “wave” over
sediment.
•Usually happens in a beach
environment, or in a desert with sand
dunes.
•The layers show
seasonal change.
Sedimentary Rock Structures
Mudcracks-
•These form when
mud/silt/clay have
water evaporate
quickly, the mud
then dries out an
shrinks.
•You can observe
this after a
summer rain
storm, with
puddles, or in the
desert, with flash
rains
Sedimentary Rock Structures
Glacial Erratic•Ever see a rock in the middle of a place, and you can’t
quite imagine anyone actually putting it there on purpose?
•Of course you have!!! It was not put there by a human,
however.
•We live on glaciated land. This means that our area was
once covered in glaciers.
•Glaciers are like big snow plows, and when they come
through, they pick up rocks, and sediment and other such
things an move them far away from where they started.
•When the glacier starts to melt, it drops all this material.
•These huge rocks are deposited that way.
•It is how you can tell that there was once a glacier there!!!
Sedimentary Rock Structures
Fossils•Fossils are any evidence of
pre-existing life.
•Fossils can tell you a lot
about the area in which they
are found
•If you find a clam shell, like
the one in the picture, on top
of a mountain, what does that
tell you about the mountain?
•It tells you that, that area was
once under water, many
millions of years ago.
•Cool huh???
Classification of Sedimentary
Rocks: Three types
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1st Type: Clastic
Def~> sediment thatr has been
lithified to form a rock
Igneous rock fragments make up
Breccia and Conglomerate (gravel
size pieces) Indicates a slow
moving river.
Sand size particles (63 microns)
make up sandstones Usually
found at ancient beaches and
deserts
Mud & clay make up shales (very
small particles), Usually fresh
water environments.
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2nd Type: Organic
Def~> Formed from living things
or their remains
Limestone: From sea critters. It is
made up of CaCO3 (like calcite, it
fizzes with HCl) it looks like
sparkly cement. Marine
environment.
Chalk: Little microscopic sea
critters that died. Marine
environment.
Coquina: (one of my faves!!!)
Cemented shell fragments. Marine
environment.
Coal: Made from dead and
decayed organic matter from 350
MYA. Ancient swamp areas.
Classification of Sedimentary
Rocks: Three types
• 3rd Type: Chemical
• Def~> When minerals come out of a solution and
crystallize.
• Most form from a solution with a heavy water content.
• These are called “evaporites”
• Most common are cementing agents in the process of
lithification.
• Halite: NaCl or more commonly known as salt. This was
an ancient ocean.
• Gypsum: Ancient ocean
• Calcite: Ancient ocean
Clastic Rocks
Breccia: angular sediments
Conglomerate: rounded sediments
Shale: with
pyrite (the
gold stuff!!)
Sandstone
Organic Rocks
Limestone
Chalk
Coquina
See the
shell?
Coal
Chemical Rocks
Halite
Gypsum
Calcite
Metamorphic Rocks
• Def~> Changes in the structure or
appearance and partial composition of
rock beneath the surface is called
metamorphism
• Metamorphism- is caused by intense heat
and pressure, acting on a rock.
Metamorphic Rocks
• Contact MetamorphismWhen magma intrudes
into “country rock” (rock
that was already there)
the magma/lava’s heat
and alters the rock.
• The rock that comes into
the contact with the
magma/lava/heat “cooks”
that part of the rock.
– When this happens
minerals in the magma mix
and a new rock is created.
• Regional Metamorphism• This occurs at the margin
of the continents, (the
coastlines) where large
areas of rock are buried
deep.
• Pressure and heat work
on a larger scale and
effect more rocks.
• Larger scale of contact
metamorphism
Metamorphic Rock Classification: Classified
according to mineral grains…two types
• 1st Type Foliated:
• Metamorphic rocks that are arranged in parallel
bands, giving the rock a striped appearance.
– Examples:
• Gneiss- originally granite
• Slate- originally shale 1st stage of metamorphism
• Phyllite- originally shale 2nd stage of
metamorphism
• Schist- sparkly, originally shale 3rd stage of
metamorphism
Foliated Metamorphic Rocks
Slate
Schist- Silvery and sparkly
Phyllite- Silvery shine, like
eyeshadow
Foliated Metamorphic Rocks
Gneiss- Pronounced
“Nice”
Wanna hear a bad
geology joke?
“it’s a gneiss day, don’t
take it for granite”
Get it? Granite is the
parent rock (where the
rock originated…)
Ha ha ha ha ha
Metamorphic Rock
• Layers from sedimentary rocks and the
foliated layers are very different.
• In sedimentary rocks the particles are not
all of the same size (usually)
• In foliated metamorphic rocks the mineral
bands are all re-crystallized.
Metamorphic Rocks
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2nd Type Non-Foliated:
They do not have bands.
They just look like they have been re-heated.
Quartzite, is re-crystallized sandstone, in other
words sandstone is the parent rock.
• Marble, is re-crystallized limestone, in other
words limestone is the parent rock
• All parent rock means is that it was the original
un-changed rock.
Non-Foliated Metamorphic Rocks
Quartzite
Marble