Rocks Tell Stories
Records of Geologic Past
Rock Classification – 3 major
groups based on their methods of
formation or origin
1. Igneous rocks … from cooling and
solidification of lava or magma
2. Sedimentary rocks … from compacted
and cemented sediments, or chemical
precipitates or evaporites
3. Metamorphic rocks …
meta (change) morphic (form) … rocks
changed by heat and pressure –but
remain solid
IGNEOUS ROCKS
Formation of Igneous Rocks
COOLING AND SOLIDIFICATION OF
MAGMA/LAVA
INTERGROWN OR INTERCONNECTED
SILICATE CRYSTALS IN A RANDOM
PATTERN – NOT ORGANIZED
INTERGROWN SILICATE
CRYSTALS - RANDOM PATTERN
MAGMA VS. LAVA
Magma – molten material
BELOW the surface of the
Earth
Lava – molten material
AT OR ABOVE earth’s
surface
ENVIRONMENT OF FORMATION
COOLING HISTORY
MAGMA
LAVA
Intrusive – below the surface
Extrusive - above the surface
A pluton is a body or
chamber of magma deep
inside the crust.
Ejected from volcano
Rocks formed when magma
Rocks formed when lava
cools and solidifies are called cools and solidifies are called
INTRUSIVE (PLUTONIC)
EXTRUSIVE (VOLCANIC)
IGNEOUS ROCKS ARE
CLASSIFIED BASED ON 2 MAIN
CHARACTERISTICS
• TEXTURE – MINERAL GRAIN SIZE
• MINERAL COMPOSITION (COLOR)
Classification of Igneous Rocks
by Texture
• WHAT IS TEXTURE?
It is NOT how it feels!
It’s the SIZE OF MINERAL
CRYSTALS
What controls texture?
How fast an igneous rock
cools.
What controls how fast an igneous
rock cools?
Where the rock is formed.
slower cooling
coarser crystals
deep in ground
ESRT – TOP OF THE IGNEOUS
ROCK SCHEME TELLS THE
TEXTURE / GRAIN SIZE
Classification of Igneous Rocks –
Texture or Grain Size
Extrusive
Extrusive
Intrusive
Intrusive
Volcanic LAVA
VolcanicLAVA
Plutonic MAGMA
Plutonic MAGMA
Extremely
fast cooling
Fast Cooling
Slow
Cooling
Extremely
slow cooling
NonFine-grained
crystalline
Glassy
vesicular (gas
pockets)
Coarsegrained
Very Coarsegrained
EXTRUSIVE – VOLCANIC - LAVA
• When lava cools extremely fast crystals have
no time to form – JUST FROZEN IN PLACE
RANDOMLY WHEN LAVA ENTERS
ATMOSPHERE OR WATER
• GLASSY – NON-CRYSTALLINE
Non-vesicular
Vesicular
gas pockets
EXTRUSIVE – VOLCANIC - LAVA
Lava that cools fast at or near the
Earth’s surface
Produces small crystals
FINE-GRAINED TEXTURE
not easily seen with the naked eye.
INTRUSIVE – PLUTONIC - MAGMA
Magma that cools slowly deep underground
produces large crystals
COARSE-GRAINED TEXTURE
easily seen with the naked eye.
INTRUSIVE – PLUTONIC - MAGMA
EXTREMELY SLOW COOLING HAS
ABNORMALLY LARGE CRYSTALS
AND IS VERY COARSE.
CHARACTERISTICS OF IGNEOUS ROCKS
IN THE MIDDLE OF THE IGNEOUS
ROCK CHART ARE CHARACTERISTICS
BASED ON WHICH MINERALS ARE IN
EACH ROCK.
NOTICE 2 NEW WORDS
FELSIC AND MAFIC
FELSIC VS. MAFIC
FELSIC
MAFIC
“FEL” = FELDSPARS
“MA” = MAGNESIUM
“SIC” = SILICA
“F” = IRON
High % of quartz,
feldspars, mica
“IC” = SILICA
High % of pyroxene,
feldspars, amphibole
DARKER IN COLOR
MORE DENSE
LIGHTER IN COLOR
LESS DENSE
Continental
FELSIC
mixed oceanic
MAFIC
mantle
ULTRAMAFIC
THE BOTTOM OF THE IGNEOUS ROCK
SCHEME TELLS YOU THE COMPOSITION
TEXTURE + COMPOSITION CAN NAME ROCK
FELSIC
COMPOSITION
COLUMN
+
COARSE
TEXTURE
ROW
INTRUSIVE
EXTRUSIVE
Granite
Rhyolite
Figure 4.7
A
MINERAL
COMPOSITION
THE SAME – TEXTURE ONLY
DIFFERENCE
Intrusive vs. Extrusive
INTRUSIVE
EXTRUSIVE
MINERAL COMPOSITION THE SAME ---- TEXTURE
ONLY DIFFERENCE
Andesite
Diorite
Intrusive vs. Extrusive
INTRUSIVE
Gabbro
EXTRUSIVE
Basalt
MINERAL COMPOSITION THE SAME – TEXTURE ONLY DIFFERENCE
Intrusive vs. Extrusive
WHAT GRAPH BELOW SHOWS AS
TIME INCREASES THE SIZE
INCREASES?
A
B
SEDIMENTARY ROCKS
Sedimentary Rocks
• 80 % of Earth’s surface is covered
with sediments and Sedimentary
Rocks (only a thin layer on top)
What is sediment????
 Broken pieces of rocks called Sediments
 Chemical precipitates ( dissolved solids
come out of solution)
 Organic fragments – shells, decayed
plant matter – this is called BIOCLASTIC
Source of Sediment
When Igneous, Metamorphic Rocks, and
Sedimentary Rocks are uncovered and exposed
to the atmosphere and hydrosphere they start to
break down
Weathering is the general term for all actions
that break down or wear away rock
Erosion is the moving/transport of the sediment
Sedimentary Rocks form when
sediments are transported to large
bodies of water and are …
Deposited in horizontal layers
called beds or strata
Buried
Compacted – particles forced
extremely close together because
of the pressure of the overlying
layers and water
Cemented – glued together
Sedimentary Rocks
Sediments that are
eroded settle to the
bottom of the rivers,
lakes, and oceans
Layer after layer of
eroded earth is
deposited on top of
each other
Most Sedimentary Rocks form
under large bodies of water, such
as lakes, seas, and oceans
Horizontal layers deposited under water
Classification of Sedimentary
Rocks
• We classify sedimentary rocks depending
whether the sediments are
LAND DERIVED (came from)
CHEMICALLY - ORGANICALLY DERIVED
INORGANIC LAND DERIVED ROCKS NAMED BY GRAIN SIZE
Sediments
are
pebbles,
cobbles,
and
boulders
rounded
Breccia
angular
Silt (0.006 to 0.0004 cms.
SILTSTONE
Sand (0.2 to 0.006 cms.)
SANDSTONE
Clay (less than 0.0004 cms.)
SHALE
2 TYPES OF TEXTURE
 CRYSTALLINE FOR CHEMICALLY
FORMED SEDIMENTARY ROCKS
 BIOCLASTIC FOR ORGANICALLY
FORMED SEDIMENTARY ROCKS
 HOW USEFUL IS GRAIN SIZE????
CHEMICAL Sedimentary Rocks
Sediment From Solution
 All liquid water contains dissolved
minerals.
 These minerals precipitate (dissolved
solids come out of solution/water)
 Usually happens because of evaporation
Characteristics of Chemically
Formed Sedimentary Rocks
They are monominerallic –
consisting of intergrown
crystals of just 1 mineral.

 Form in shallow seas
ROCK SALT
Intergrown
crystals of just
1 mineral
DOLOSTONE
ROCK GYPSUM
ORGANIC Sedimentary Rocks
Bioclastic Sediment
LIMESTONE AND CORALS
• Dissolved minerals taken out of solution by
marine organisms for their hard parts (shells)
can pile up on the seafloor when they die.
• These fossil remains accumulate into a thick
enough layer to form an “organic” sedimentary
rock
• Cement is commonly calcite ( bubbles with
acid)
COAL
• Decayed plant remains
BIOCLASTIC LIMESTONE
COAL
CHEMICALLY PRECIPITATED
LIMESTONE
Summary
 Inorganic Land-Derived Sedimentary
Rocks are identified by Texture or Grain
Size
 Chemically and/or Organically Formed
Sedimentary Rocks are identified by
Composition
HIGH
ENERGY
LOW
ENERGY
Environment of Formation
Ocean Derived Sedimentary Rocks
Sedimentary Rocks
help us understand our past
geologic history
There is evidence of climate
change as well as lost species
of plants and animals in
Sedimentary Rocks
During the
Cretaceous Period
(about 120 mya) a
shallow inland sea
split North
America.
We know this by
the Sedimentary
Rocks deposited
in the former
shallow sea
SALT AND GYPSUM
EVAPORATION
 Bedrock of Salt and Gypsum around
Buffalo
 What type of environment would cause
salt and/or gypsum to evaporate?
 Ocean environment with hot temperatures
CHARACTERISTICS OF
SEDIMENTARY ROCKS
1. Horizontal layers called beds or strata
2. Clasts are rounded and sorted because
they have been moved by water
3. Ripple Marks
4. Mud Cracks
5. Fossils
6. Interconnected mineral crystals of 1
mineral (monominerallic)
HORIZONTAL LAYERS
A. SHALLOW WATER B. LOW ENERGY C. DIRECTION OF FLOW
MUDCRACKS
ALTERNATING WET AND DRY
CONDITIONS
Limestone - FOSSILS
SEDIMENTARY ROCK MANTRA
1.COMPACTED AND
CEMENTED SEDIMENTS
FOSSILS
RIPPLE MARKS
MUDCRACKS
METAMORPHIC ROCKS
METAMORPHIC ROCKS
META = CHANGE
MORPH = FORM
Start with pre-existing rock
and change to another rock –
but these changes occur in the
SOLID STATE
METAMORPHISM
 Process
by which igneous,
sedimentary, or another
metamorphic rock called the
“parent rock” undergoes
changes in minerals and
texture
METAMORPHISM
Occurs in rocks due to the
effects of
• High temperature
• High pressure
HEAT EFFECTS
Temperature increases with depth or
contact with magma
 Recrystallization – a process that
forms new mineral crystals because
the mineral is no longer stable at the
new temperature

• Changes to a new (high temperature)
mineral
PRESSURE EFFECTS
 Pressure increases with depth –
weight of overlying rock (see page 10
ESRT)
 Mineral crystals grow larger
 Mineral crystals are more dense
 Mineral crystals grow perpendicular
to pressure
 Mineral crystals are arranged in
parallel layers – PRODUCES
FOLIATION
FOLIATION
 Pressure actually rearranges the minerals
into parallel layers.
 More pressure – minerals grow larger and
recrystallize and separate into layers
 Even more pressure – alternating layers of
light and dark minerals called
Banding
Random
arrangement
of minerals
Organized
arrangement of
minerals - banding
METAMORPHIC ROCK MANTRA
• INTERGROWN CRYSTALS
IN AN ORGANIZED
PATTERN
Granite
Intergrown crystals
random pattern
Gneiss
Intergrown crystals
Organized Pattern
2 Types of Metamorphism
1. Regional Metamorphism – Occurs over
large areas where there is active
mountain building
a) Convergent plate boundaries
2. Contact Metamorphism – areas in
contact with magma intrusions and/or
lava extrusions
MOUNTAIN
BUILDING
Classification of Regional
Metamorphic Rocks
 Composition is basically the
same for all foliated rocks –
parent rock is usually listed in
comments section.
 Type of texture and grain size
determined by the amount of heat
and pressure
GRADES OF METAMORPHISM
LOW GRADE – LESS HEAT AND PRESSURE – SLATE
HIGH GRADE – MORE HEAT AND PRESSURE - GNEISS
Temperature
Low-grade metamorphism:
Occurs at about 2000 C to 5000 C.
and pressure greater than 1,000
atmospheres
High-grade metamorphism:
Occurs at > 5000 C
FOLIATED METAMORPHIC ROCKS
MICA SCHIST WITH GARNET
MICA
CRYSTALS
BECOME
LARGE
AND
SPANGLY
METAMORPHIC ROCKS WILL
OFTEN BE DEFORMED
BECAUSE OF INTENSE HEAT
AND PRESSURE
LOOK AT ROCK SYMBOL FOR GNEISS
DEFORMED GNEISS
A metamorphic rock exposed
to too much heat will melt
and become ???
MAGMA
CONTACT METAMORPHISM
Contact Metamorphic
• Metamorphism
means "changed
form".
• The Changes occur
because of:
• Heat from magma
intruding on the
surrounding rock
• Very little pressure
 Near magma chambers (plutons)
 Rocks in “contact” with the
magma are heated
 New minerals are formed –
recrystallization
Contact Metamorphism – heating
the adjacent rocks
Symbol for Contact Metamorphism
is a line with tick marks
When limestone undergoes contact
metamorphism what rock will it metamorphose
into? Sandstone??
PARENT ROCK IN THE COMMENTS
LIMESTONE METAMORPHOSES INTO MARBLE
SANDSTONE METAMORPHOSES INTO QUARTZITE