Rocks:
Keepers of Earth’s History
1
What’s in your rock?:
Hornblende
Granite
Orthoclase
Quartz
aggregates of minerals
Biological material
Fossil fragments
Plant material
3
Understanding rocks is the basic foundation of
knowledge for earth scientists and materials that
make up the earth.
• Every rock “speaks” to the observer and gives
clues about where and how it was formed.
• Rocks are divided into 3 major groups:
• Igneous
• formed from solidifying hot molten
rock (magma)
• Sedimentary
• formed on the surface of the earth from
weathering processes
• Metamorphic
• pre-existing rocks are subjected to
various pressure and temperature
relationships
4
How does an earth scientist distinguish between
the three rock types (Ig, Seed, and Met)?
Texture:
• the appearance of the rock
• size, shape, and arrangement of mineral grains.
Composition (mineral assemblages)
• types and relative proportions of minerals
making up a rock
• composition, mineral make-up, chemistry--all
indicators of a rock’s composition
All rocks are identified based on their
texture and composition.
5
6
The Rock Cycle
The Sedimentary Cycle
The Rock Cycle:
7
• where the earth’s internal forces meet
the earth’s external forces (at the earth’s
surface)
Earth’s internal forces
Earth’s external forces
• moving continents (PT)
• earthquakes
• elevation of mountains
• volcanic eruptions
• the hydrosphere (HC)
• atmosphere
• erosion (wind, water, ice)
• biosphere activities
Produce a continuous cycle of rising mountains only to be
weathered down and uplifted repeatedly. As a result, igneous
sedimentary, and metamorphic rocks are created.
8
I
Earth Science, especially
the rock cycle !!!!!!!!!!!!!
Discuss with a friend:
1. Briefly identify each rock group and
its geologic environment.
2. Draw the rock cycle, and explain how the
rock cycle works to your neighbor.
3. Why is the rock cycle a “cycle”???
I will get an A on my exams and quizzes.
Geology – Chapter 3 – Igneous Rocks
Igneous Rocks
9
Magma
• the parent material for igneous rocks (if not all rock groups)
• typical temperature 1,200oC (2,200oF)
• forms about 250 km or 150 miles below the earth’s surface
Extrusive - igneous rocks – above the earth’s surface
volcanic rocks
lava flows
Example – Hawaiian Volcanoes
Intrusive - igneous rocks – below the earth’s surface
plutonic rocks
large granite mountains
Example – Sierra Nevada Mountain Range
10
Grain size determines if the ig rock is extrusive or
intrusive. Magma determines the rock’s composition.
When observing “frozen magma” (an igneous rock), how
does a geologist know if the rock is an Extrusive or
Intrusive igneous rock?
checking the crystal size - Does the rock contain visible
crystals, or are crystals non-existent?
Igneous Rock Textures
Factors that affect crystal size in an igneous rock:
The rate at which magma cools
•slow cooling – visible minerals
•rapid cooling – non-visible minerals
• non-visible minerals
• rapid cooling magma
• extrusive ig rock
• visible minerals
• slow cooling magma
• intrusive ig rock
Cooled above the
Cooled below the
surface (fine-grained) surface (coarse-grained)
11
Composition of Igneous Rocks
14
How much silica (SiO2) is in your igneous rock?
Felsic compositions (feldspar + silica)
• light-colored igneous rocks
• common minerals: feldspar and quartz
• 70% SiO2
Intermediate compositions:
• mixture of felsic and mafic minerals
• about 60% silica (SiO2)
Mafic compositions (magnesium + iron)
• dark-colored igneous rocks
• common minerals: olivine and pyroxene
• 50% SiO2
• ultramafic: composed entirely of olivine and pyroxene
and less than 50% silica (SiO2)
Igneous
rocks arerocks
classified
based on TEXTURE and
How
are igneous
classified?
COMPOSITION.
• Using their TEXTURE and COMPOSITION
COMPOSITION
Felsic (light color)
70% silica
Intermediate color
60% silica
COARSE
phaneritic
FINE
aphanitic
TEXTURE
Granite
Diorite
Rhyolite
Andesite
Mafic (dark color)
50% silica
Gabbro
Basalt
Igneous rocks have the same chemistry but
different textural characteristics due to the type
of geologic environment (cools quickly or slowly).
I
Earth Science, especially
igneous rocks.
1. What is the difference between extrusive
and intrusive igneous rocks?
2. How can one tell the difference between
an extrusive and intrusive rock texture?
3. The composition of a mafic ig rock is….
The composition of a felsic ig rock is ….
4. A fine-grained, light-colored ig rock is
called a ………………….
I will get an A on my exams and quizzes.
17
Sedimentary Rocks
18
How is a sedimentary rock formed???
Why is the earth’s surface 75% sedimentary rocks?
19
erosion
Pre-existing
Rock
sediment
water, wind,
ice
breaks down
pre-existing rock
Rivers
Lakes
lithified
transported
compacted
cemented
Sedimentary
Rock
21
Erosional processes – later (Chapter 4)
Sediments are separated into 3 broad categories:
• Clastic sediments:
inorganic grains or mineral fragments ranging in
size from boulders to clay particles (flour size)
• Chemical sediment:
formed by the precipitation of minerals dissolved
in lakes, rivers, or seawater environments
• Biogenic (bioclastic) sediment:
sediment composed of animal and plant remains or
material precipitated by biological processes
To be a sedimentary rock, it must be lithified.
Lithification: processes by which sediment is
transformed into sedimentary rock
Sediment can be lithified in three common processes:
Compaction: Overlying weight of the sediment “squeezes”
and compresses pore spaces, which pushes particles together.
A. loose, unconsolidated sediment with
abundant pore space
A
B
B. compacted, compressed sediment with
reduced pore space
Cementation: Pore water is expelled from voids, and rising
water carries iron, calcium carbonate, and silica which precipitates
as geological glue (cement) holding the grains together.
Grains are cemented together by three
types of cements:
•iron cement
•calcium carbonate cement
22
•silica cement
Lithification processes
Recrystallization: Overlying pressure causes less stable
minerals to convert to more stable minerals, producing new
substances that cement pre-existing grains.
Overlying pressure
Grain boundaries convert
to more stable mineral
compositions, “cementing”
grains together.
23
Clastic Sediment:
lithified
gravel sediment ranging
from pea-sized to larger
conglomerate
lithified
sand-sized particles (fine to coarse
sandpaper)
lithified
finer particles – size of table salt
finest sedimentary
particles (flour-size)
sandstone
siltstone
lithified
shale
24
Chemical Sediments and Sedimentary Rocks
Chemical Sediment: sediment formed by the precipitation
of minerals dissolved in a lake, river, or seawater
All surface water and groundwater contain
dissolved ions (chemicals), creating a venue for
precipitation of chemical sediment.
Precipitation of chemical rocks takes place in two ways:
• Plants and animals alter the chemical balance
of the water body (lake, ocean).
Increasing amounts of calcium carbonate
cause precipitation of limestone.
• evaporation of chemically saturated water
bodies
26
Evaporation of chemically saturated water creates
a chemical sedimentary rock (evaporite).
Na+ Cl- Na+ Cl
Cl
+
Na+ Cl-Na
a lake containing sodium (Na+)
and chlorine (Cl-)--free ions
Evaporation
Lake water is evaporated,
concentrating sodium and
chloride ions (saturation).
NaCl
Sodium and chlorine combine
to form halite (table salt).
27
The Great Salt Lake, Utah
28
(evaporite halite)
As lake becomes saturated,
density of water increases.
Utah
Chemical – sedimentary rocks that have been
precipitated or are the result of evaporation
Gypsum – evaporite CaSO4
Halite – evaporite
NaCl
Limestone-precipitated
calcium carbonate
29
Biogenic Sediments and Biogenic Rocks:
Biogenic sediment is composed of organic remains of plants and
animals (bioclastic rocks).
30
• shell, bones, teeth, plant fragments, wood, roots
Common Biogenic Rocks:
• Limestone
• most abundant biogenic sedimentary rock
• composed of calcium carbonate CaCO3(calcite, dolostone,
aragonite)
• lithified shells, skeletal material
• Chert
• composed of silica SiO2
• precipitated silica shells (made by sea animals) that protect
microscopic sea animals – Animals die, and silica shells
sink, creating layers of chert beds on the ocean floor.
“When the animal dies, it loses the chert off its back.”
Common Biogenic Rocks
31
Peat to Coal
• accumulated remains of terrestrial plants
• with time and pressure ---- peat
• continued pressure and lithification ------ coal
Peat
lithification
metamorphism
Anthracite coal
Bituminous coal
Biogenic (bio-clastic) rocks –
result from animal and plant secretions -- The term
“clastic” indicates that these rocks contain fossils or
parts of plants, shells, compacted plant material
Animal parts
Cemented shell fragments
Fossil Limestone
Microscopic fossils
32
Coquina
Chalk
What do sedimentary rocks tell the earth
scientist?
These rock “layers” were
deposited one layer at a
time and lithified.
strata
The banded appearance
is known as bedding or
groups of layers called strata.
The boundary between
each stratum is called a
bedding surface.
By observing the sedimentary rock type (clastic, chemical,
biogenic) and depositional patterns, an earth scientist can
decipher the geologic history – like reading pages in a book. 33
Sedimentary Rock Classification Summary Tree
Sedimentary Rocks are divided into 3-classification categories:
Clastic sediments
Chemical sediments
composed of inorganic
grains
classification based on
distribution of grain sizes
Conglomerate
Bio-clastic (chemical)
sediments
formed by precipitation
composed of
“animal parts”
from
plants and evaporation
animals
processes
Classification based on
lack of grains and chemical
composition
Classification based on
Observed fossils, animal parts,
Plant/animal secretions
Fossil-limestone
Gypsum (CaSO4)
Sandstone
Siltstone
Shale
Halite (NaCl)
Limestone (CaCO3)
Coquina
Chalk
36
I
sedimentary rock classification.
Discuss with a friend:
1. Describe at least 2 characteristics of
each sedimentary rock classification
(clastic, biogenic, chemical).
2. Give a rock example for each classification.
I will get an A on my exams and quizzes.
37
What is “metamorphism?”
• meta (change) morphism (form)
• transformation of pre-existing
rock to a metamorphic rock
through pressure and
temperature relationships
• All changes take place through
solid state transformation.
38
Types of Metamorphism
• takes place around 150oC (Temperature)
• typically at depths of 5 km below the
earth’s surface (Pressure)
At this temperature and pressure, new minerals
are formed – changed from one mineral to
another.
39
I
metamorphic rocks.
Discuss with a friend:
1. How do you define metamorphism? – Be
specific.
2. What two major factors are necessary
for pre-existing rocks to become
metamorphosed?
3. Where does metamorphism typically
take place within the earth?
I will get an A on my exams and quizzes.
40
2 Common types of metamorphism
• contact metamorphism
• regional metamorphism
44
What is contact metamorphism?
• Contact metamorphism:
magma “contacts” solid rock –
heat is transferred into overlying
rocks through conduction – changing
the pre-existing rock it intruded
• high heat/low pressure
• localized metamorphism
• “fast-acting,” like touching a
hot frying pan
45
Country rock (surrounding) is baked from conduction.
46
What is regional metamorphism?
Regional Metamorphism
Pre-existing rock is subjected to intense
stresses and strains (deformation) usually
from the forces of mountain building.
• high heat/high pressure
• very widespread geographically
• like experiencing a slow sunburn
47
Mountain building
widespread belts
48
I
the different ways to create
Metamorphism.
Discuss with a friend:
Describe the conditions necessary for each
type of metamorphism:
• contact metamorphism
• regional metamorphism
I will get an A on my exams and quizzes.
49
Metamorphic heat sources:
Heat sources:
contact with hot magma – heat penetrates
overlying rocks (conduction)
geothermal gradient – heat increases with
depth – rocks are hotter when buried
50
Geothermal Gradient
Steep gradient
0
1000
2000
3000
4000
• Temperature increases
as depth increases.
• Large thermal gradient
is at the surface.
• Gradient decreases
after 1000 km depth.
51
Pressure associated with metamorphism
Pressure – increases with depth,
various types of pressure rearrange
minerals – minerals compact or elongate
Confining pressure
Differential pressure
52
How does one know how much metamorphism
has taken place in a rock?
53
• reflected in the rock’s texture
Not Metamorphosed
Metamorphosed – mineral
grains are in a preferred
direction -- FOLIATION
54
I
to metamorphose.
Discuss with a friend:
1. Describe two heat sources associated with
metamorphism.
2. Define the differences between confining
and differential pressures.
3. Draw a diagram that shows mineral
orientation due to confining and
differential type pressures.
I will get an A on my exams and quizzes.