Download Exam Block #3

Exam Block #3
• Chapter 6 – Weathering ONLY
• Chapter 7 – Sedimentary Rocks
• Chapter 8 – Metamorphic Rocks
How to study for this class:
1. Read the chapter and answer the Review
Questions. As you read, follow along with
the Chapter Outline (download from our
class site below) and these PowerPoint
Notes.
2. Review each chapter using the GEODe:
Earth CD-ROM in your book.
3. Go online and take the Concept Quizzes
and Chapter Test for each chapter at:
www.prenhall.com/tarbuck
4. Check your grades online at:
http://pages.sbcglobal.net/solanogeo/index.htm
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 1 of 20
Chapter 6 - Weathering
1
2
Rates of Weathering
Q: Why do some gravestones weather at a
greater rate?
Slate
Arches National Park, Utah
Weathering
3
Limestone
The Earth’s surface is dynamic – volcanic forces
and mountain building elevate portions of the
surface – while opposing processes move
material from higher Æ lower elevations. By:
ƒ Weathering (This Chapter) – the physical
breakdown and chemical alteration of rocks.
ƒ Chemical Weathering
– involves a chemical
transformation of rock.
Q: How do I chemically
weather a piece of
paper?
ƒ A:
ƒ Mass Wasting (Chapter 15) – the transfer of
rock down-slope by forces of gravity.
ƒ Erosion (Chapter 16) – physical removal of
material by water, wind, or ice.
1. Mechanical Weathering
5
1. Frost Wedging –
repeated cycles of
freezing and
thawing; water
expands about 9%
when frozen; this
exerts great
pressure that
cracks the rock.
Talus slopes are
piles of loose rocks
at base of cliffs.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
4
Weathering
ƒ Mechanical
Weathering –
physical processes
that break rock
into smaller
pieces. Q: How do
I mechanically
weather a piece of
paper?
ƒ A:
2. Mechanical Weathering
6
2. Unloading – the
great reduction in
pressure when the
overlying rock is
eroded away;
generate onionlike layers called
sheeting; erosion
creates
exfoliation
domes. Ex: Half
Dome, Yosemite.
Page 2 of 20
7
2. Mechanical Weathering
2. Mechanical Weathering
8
Jointing – tectonic
mountain building
events can
fracture rocks and
create patterns
called joints.
These allow water
to penetrate and
start the process
of weathering. Ex:
Moab, Utah.
9
3. Mechanical Weathering
3. Thermal
Expansion –
repeated heating
and cooling of
rocks. This
process is not
very effective but
more readily
seen when rock
has already
weakened by
chemical
weathering.
1. Chemical Weathering
4. Mechanical Weathering
10
4. Biological
Activity – plants
(roots);
burrowing
animals; and
humans (road
cuts). Tree roots
promote further
weathering of the
rock.
11
Complex processes that break down rock to new
minerals and products. Water is the most
important agent in these three processes:
1. Dissolution – dissolving of minerals in water,
just as sugar does in water; water molecules
are polar – that is – they have a small amount
of charge and attract ions from minerals.
12
1. Chemical Weathering
Weak acids form as groundwater travels through
soil; these acids dissolve large quantities of
limestones (mineral calcite) to form caves.
Acid rain is formed from sulfur and nitrogen
oxides released into the atmosphere by the
burning of fossil fuels.
The granite
headstone (left)
has not weathered
as much as the
marble one about
the same age.
Dissolving of salt (halite) in water.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 3 of 20
2. Chemical Weathering
13
3. Hydrolysis – reaction with water to form new
products (like free iron).
2. Oxidation – the process of rusting:
4Fe + 3O2
Æ
2Fe2O3
Iron
Oxygen
14
3. Chemical Weathering
Iron Oxide (Hematite)
Oxidation is important in
decomposing of
ferromagnesian minerals:
olivine, pyroxene, and
hornblende, but first iron
must be freed from the
silicate structure by
hydrolysis.
Chemical Weathering of Pyroxene
Notice: Quartz – is very resistant to chemical
weathering and can be transported great
distances to the sea to become beach sand.
15
16
Spheroidal Weathering
Corners of rocks have more surface
area and are weathered to a
greater extent, resulting in a
spherical shape (Joshua Tree, CA).
Hydrolysis
Oxidation
Spheroidal Weathering
17
Rates of Weathering
18
Several factors influence the rate of rock weathering.
1. Rock Characteristics – weathering rates follow Bowen’s
Reaction Series: the first formed ferromagnesian minerals
are the least resistant to chemical weathering (iron
oxidizes) and the last formed minerals are silica rich and are
the most resistant the chemical weathering.
Basalt
Granite
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 4 of 20
Rates of Weathering
19
2. Climate – warm temperatures and abundant
moisture increase chemical weathering.
3. Differential Weathering – different rates of
weathering due to rock type or jointing; Ex:
Arches National Park.
✓REVIEW QUESTIONS
20
1. Describe the 3 processes of moving rock in the rock cycle.
2. Contrast mechanical and chemical weathering.
3. Where is frost wedging most effective?
4. Describe formation of an exfoliation dome.
5. How does mechanical weathering increase rates of
chemical Weathering?
SKIP SOIL SECTION
✓REVIEW QUESTIONS
21
6. Granite and basalt are exposed at the surface in a hot wet
region.
a) Which type of weathering will predominate?
b) Which of these rocks will weather most rapidly? Why?
7. Heat speeds up a chemical reaction. Why then does
chemical weathering proceed slowly in a hot desert?
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 5 of 20
Chapter 7 – Sedimentary Rocks
1
Detrital & Chemical Sedimentary Rx
Detrital – solid pieces
from both mechanical
and chemical
weathering.
2
Chemical –
soluble material
that precipitates.
White Chalk Cliffs of Dover, England
% by Volume Minerals in Earth’s Crust
3
1. Detrital (Clastic) – accumulation of material (solid
particles) derived from mechanical and chemical weathering.
2. Chemical – precipitation of dissolved substances from
solutions produced by chemical weathering.
3. Organic – carbon from plants & animals that form coal.
What are the
chemical
weathering
products?
Erosion
Resulting
sedimentary
rocks?
Turning Sediment Into Sedimentary Rock
4
Sedimentary Rocks
Weathering
Deposition
Diagenesis
5
6
Cementation
Diagenesis – refers to changes that take place after
the sediments are deposited. Lithification is the
process of turning unconsolidated sediments into
solid rock by two processes: (1) Compaction –
volume may be reduced by 40%. (2)
Cementation – minerals precipitate in the pore
spaces, such as calcite, silica, and iron oxides.
Bedding Planes – The end of
one deposit and the
beginning of another create
flat surfaces separating two
beds of sedimentary rock. A
very distinguishing
characteristic of sed. rocks.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Photomicrograph of sandstone showing quartz
grains and calcite cement.
Page 6 of 20
Detrital Sedimentary Rocks
ƒ
ƒ
7
Clay minerals are the most abundant product of chemical
weathering from feldspars. Quartz is also abundant because it
is extremely durable and resistant to chemical weathering.
Particle Size – is the primary basis for classification of
detrital rocks: mud (or clay), sand, and gravel size.
Shale (Fine Particle Size)
9
ƒ Shale – splits into thin layers along well
developed closely spaced planes (fissility).
ƒ Mudstone – breaks into chunks or blocks.
ƒ Siltstone – silt-sized particles.
Weathering of Shale
8
Mud or Clay (Fine Particle Size)
Mud is Lithified into SHALE:
ƒ About 50% of all sedimentary rocks are
shales. The size of the grains provides
important information about the
environment of deposition. The stronger
the current (of water or air), the larger the
particle size carried.
ƒ Therefore, the tiny grains indicate that
settling must have occurred in a quiet
deposition setting, such as: lakes, river
floodplains, lagoons, and deep-ocean
basins.
10
Shales Create Oil Traps
The clay minerals have
sheet-like structures like
micas. They settle nearly
parallel and become
tightly packed. This
results in water not
being able to penetrate;
thus shale often forms a
barrier to fluid flow. Ex:
Oil and gas reservoirs
are often capped by
shales.
11
Sand (Medium Particle Size)
12
Sand is Lithified into SANDSTONE: 20% of
sedimentary rocks are made of this size.
1. Sorting on Particle Size:
ƒ (a) Well-sorted grains are all about the same size:
Ex: wind blown sand; beach sand.
ƒ (b) Poorly sorted – large & small particles
together. Ex: mountain streams.
Grand Canyon, Arizona. Weaker shales crumble and
produce debris in which some vegetation grows.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 7 of 20
Sandstone (Medium Particle Size)
13
2. Particle Shape:
ƒ (a) Rounded grains indicate the distance
or time involved in the transportation of the
sediment; likely involves air & water.
ƒ (b) Angular grains indicate material was
transported a short distance; likely involves
other transport medium, like glaciers.
Sandstone (Medium Particle Size)
14
3. Particle composition – long transport leads
to the destruction of weaker and less stable
minerals (feldspars, ferromagnesians). Quartz
is usually the only mineral to survive long trips.
Q: Feldspars grains never become well rounded. Why?
A:
X
Types of Sandstone
15
ƒ Quartz Sandstone – mostly well-rounded quartz
grains; indicates long transport & substantial
weathering. Forms at beaches, sand dunes.
ƒ Arkose – mostly feldspar grains with quartz and
micas; source was a granite with short transport
distance; usually poorly sorted and angular grains.
Forms at alluvial fans.
4 TYPES OF SANDSTONE
17
16
Types of Sandstone
ƒ Graywacke – quartz, feldspars with silt and
clay matrix; also short distance of transport
and rapid burial. Forms at offshore turbidity
currents (deep sea fans).
ƒ Lithic Sandstone – many other fragments
besides quartz. Forms at deltas.
18
Quartz Sandstone
Burial & Lithification
Great Sand
Dunes
National
Park,
Colorado
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Navajo
Sandstone,
Zion
National
Park, Utah.
Page 8 of 20
Gravel (Coarse Particle Size)
19
Gravel sized particles are Lithified into:
CONGLOMERATE: Rounded
gravel sized particles. Ex:
mountain streams, glacial
deposits.
✓REVIEW QUESTIONS
20
2. List the three basic sedimentary rock categories.
BRECCIA: Angular gravel sized
particles; rock formed near
the source of weathered
material. Ex: fault zone.
3. What minerals are most common in detrital sedimentary
rocks? Why are these minerals so abundant?
4. What is the primary basis for classifying detrital
sedimentary rocks?
6. How are the degree of sorting and the amount of rounding
related to the transportation of sand grains?
Chemical Sedimentary Rocks
21
22
Limestone
1. Limestone – about 10% of sedimentary rocks;
make of calcite (CaCO3).
(a) Coral Reefs – secrete a calcareous skeleton.
(b) Coquina (kō-kē-na) – a course rock made of
shells and shell fragments.
Mineral Composition – is
the primary basis for
classification of chemical
sedimentary rocks. They
form from the precipitation
of chemicals from solution
by two methods:
1. Inorganic
2. Organic (biochemical)
ƒ
Cave deposits (travertine) are
created by the precipitation of
calcium carbonate from solution
(inorganic).
(a)
Coral Reef Formation
23
Modern coral reef (left). Exposed
Permian coral reef Texas (right).
Limestone
(b)
24
(c) Chalk – very fine microscopic marine organisms.
(d) Inorganic limestones – direct precipitation
from sea water.
(d)
(c) The White Cliff
of Dover, England.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 9 of 20
Chemical Sedimentary Rocks
25
2. Dolostone – magnesium replaces calcium in an
already formed limestone.
3. Chert – microcrystalline silica (SiO2); it can be both
organic and inorganic in origin. (a) Flint – dark
(organic matter present); (b) Jasper – red (iron
oxides present); (c) Agate – banded form.
26
Chemical Sedimentary Rocks
4. Evaporites – Halite or Rock Salt (NaCl); Gypsum
(hydrous calcium sulfate, CaSO4 • 2H2O) – Plaster of
Paris used for wallboard and interior plaster.
(a)
Halite
(b)
(c)
Chemical Sedimentary Rocks
Evaporites form from evaporation of bodies of water.
27
Coal – The
accumulation of
large quantities of
plant remains in a
stagnant swamp
with little oxygen.
Decomposition and
burial of organic
matter changes
form with depth
and temperature
of burial.
Bonneville salt flats, Utah.
Modern Swamp
Coal – Organic Sedimentary Rock
29
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
28
Peat
Lignite
Bituminous
Anthracite
Review – Classification System
30
Page 10 of 20
✓REVIEW QUESTIONS
31
32
ƒ Sedimentary rocks are important in the interpretation
of Earth history.
ƒ By understanding the conditions under which
sedimentary rocks form, geologists can often deduce
the history of a rock, including information about the
origin of its component particles, the method and
length of sediment transport, and the nature of the
place where the grains eventually came to rest.
ƒ That is the geographic setting called the
sedimentary environment.
ƒ ‘The present is the key to the past’. Present-day
sedimentary environments are similar to those of the
past.
7. Distinguish conglomerate and breccia?
8. Distinguish between the 2 categories of chemical
sedimentary rocks.
9. What are evaporite deposits? Name some sedimentary
rocks that are evaporites.
12. Chemical sedimentary rocks are classified on…?
Sedimentary Environments
Sedimentary Environments
33
Sedimentary Environments
34
Where do these
sedimentary
rocks form?
• arkose
Where do these
sedimentary
rocks form?
• shale
• evaporates
• quartz sandstone
• conglomerate
• graywacke
• coal
• limestone
• lithic sandstone
35
Facies
Facies is a term used to describe a unit with different types
of sediments next to one another in a single sedimentary
layer. When a stream reaches a delta, sand, the coarsest
size, deposits at the beach, but the finer clay stays in
suspension and is carried further offshore and eventually
deposits a shale layer. In warm water regions, the
skeletal remains of a coral reef may deposit limestone.
36
Facies
Stratigraphic Section
ss
Regression:
fall in sea level.
sh
ls
shoreline
sh
ls
sh
ss
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
ss
Transgression:
rise in sea level.
Page 11 of 20
Grand Canyon Stratigraphic Section
37
38
Sedimentary Structures
Cross-Bedding (wind or water).
Cross-Bedding Formation
39
Ripple Marks (wind or water).
Modern sand dune (left). Exposed
ancient cross-bedded sandstone (right).
Sedimentary Structures
40
Sedimentary Structures
41
Graded Bedding (water).
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
42
Sedimentary Structures
Mud Cracks (wet & dry conditions).
Page 12 of 20
Sedimentary Structures
43
Fossils (Geology 3 & 4 at Solano).
✓REVIEW QUESTIONS
44
11. Name the sedimentary rock described below:
a) An evaporate used to make plaster =
b) A fine-grained detrital rock that exhibits fissility =
c) Dark-colored sandstone containing angular rock particles
as well as clay, quartz, and feldspar =
d) The most abundant chemical sedimentary rock =
e) A dark-colored hard rock made of microcrystalline quartz =
20. What is probably the single most characteristic feature of
sedimentary rocks?
Gastropods.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 13 of 20
Chapter 8 – Metamorphic Rocks
1
2
Metamorphic Rocks
ƒ Metamorphism – “changed form”.
Metamorphism takes place where rocks are
subjected to conditions unlike those in which it
formed. Rocks form new textures or new minerals.
• Every metamorphic rock
has a parent rock – the
rock from which it was
formed.
• Metamorphism progresses
in increments from lowgrade to high-grade, but
the rock remains solid!
Metamorphic Rocks, Canadian Shield
Settings of Metamorphic Rocks
3
Metamorphism occurs in three settings:
4
1. Heat - The most important agent of
metamorphism is heat. It causes the ions in a
mineral to vibrate more rapidly and migrate to
form new minerals. There are 2 sources of heat:
(a) contact metamorphism with intruded
magma produces a “baked” zone – high
temp./low pressure.
(b) geothermal gradient increases with
depth; minerals become unstable and form new
minerals that are stable at that temperature.
This may occur at convergent plate boundaries
or thick sequences of sediment.
1. Contact or Thermal Metamorphism –
rocks near a magma chamber “bake”.
2. Hydrothermal Metamorphism – involves
chemical alterations as hot water circulates
through fractures in rocks.
3. Regional Metamorphism – extensive areas
subject to high temperature and pressure due
to tectonic forces.
** All three may occur in mountain belts. **
Geothermal Gradient
Agents of Metamorphism
5
Agents of Metamorphism
6
2. Pressure & Stress
Buried rocks are subjected to confining pressure
and forces are applied equally in all directions.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 14 of 20
7
Agents of Metamorphism
2. Pressure & Stress
Rocks subjected to directed pressure (such at
convergent boundaries) deform unequally in different
directions referred to as differential stress.
8
Agents of Metamorphism
3. Chemically Active Fluids
As temperature and pressure break up crystal
structures, atoms and ions move into solution and
migrate through the rock to form new minerals or
ore deposits.
Quartz vein deposit with gold & silver.
✓REVIEW QUESTIONS
9
1. What is metamorphism? What are the agents that change
rocks?
2. Why is heat considered the most important agent of
metamorphism?
Metamorphic Textures (Foliated)
10
1. Foliated Texture – flat arrangement of
mineral grains within a rock. Pressure is
important in the realignment of particles to
change the texture of a rock.
3. How is confining pressure different than differential stress?
4. What role do chemically active fluids play in
metamorphism?
Metamorphic Textures (Foliated)
11
Metamorphic Textures (Foliated)
12
There are three types of foliated textures:
1. Rock or Slaty
Cleavage – platy
minerals, such as
micas, align along
planes.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 15 of 20
13
Slaty Cleavage
Metamorphic Textures (Foliated)
14
2. Schistosity – “fish scales” – larger pieces of
micas have formed; may also contain
porphyroblasts (large mineral grains).
3. Gneissic Texture –
segregation of light and
dark minerals into bands.
Slate quarry near Alta, Norway.
Metamorphic Textures (Nonfoliated) 15
Foliated Metamorphic Rocks
16
Slate (Left) & Phyllite (Right)
18
There are no alignment of mineral grains. They
exhibit interlocking equidimensional crystals.
Marble
Quartzite
Nonfoliated Metamorphic Rocks
17
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 16 of 20
Mica Schist
19
Gneiss
20
Migmatites
21
Common Foliated Rocks
22
Migmatites – ‘Mixed Rock’ – partial melting has
occurred. As a granite approaches 750° C, the lightcolored silicates (quartz & potassium feldspar) will
melt, but the mafic silicates will remain a solid.
M
Granite
d
te
el
m
Un
Migmatite
(Mixed Rock)
d
te
el
✓REVIEW QUESTIONS
23
6. What is foliation? Distinguish between slaty cleavage,
schistosity, and gneissic textures.
7. Describe the mechanisms by which minerals develop a
preferred orientation.
8. List some changes that might occur to a rock in response
to metamorphic processes.
Metamorphic Environments
24
1. Contact
Metamorphism
Aureole (or-ē-ōl) is a
zone of alteration around
emplaced magma. Most
are fine-grained, dense,
baked clay minerals
called hornfels.
9. How can you tell slate and phyllite apart?
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Page 17 of 20
Contact Metamorphism
25
Metamorphic Environments
26
2. Hydrothermal
Metamorphism –
hot fluids circulate
ion-rich solutions to
move and deposit
heavy minerals.
Good example are
black smokers at
mid-ocean ridges.
Metamorphic Environments
27
3. Regional Metamorphism
The greatest quantity of metamorphic rock is produced by this
method and associated with mountain building.
Metamorphism Along Fault Zone
29
Metamorphic Environments
28
4. Other Metamorphic Environments
ƒ Burial Metamorphism – as basins subside, thick
accumulations of sediment cause low-grade
metamorphic conditions.
ƒ Along Fault Zones – near the surface, rocks
behave like a brittle solid and fracture creating a
fault breccia. At greater depth along the fault, temp.
and pressure are higher and rocks are ductile and
flow creating a sheared rock called mylonites.
ƒ Impact Metamorphism – meteorites striking the
surface subject rocks to metamorphic conditions.
Tektites are beads of silica-rich glass that cool to
teardrop shape as they fall.
30
Impact Metamorphism
Upper crust - brittle
Lower crust - ductile
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
Meteor Crater, Winslow, Arizona
Page 18 of 20
Metamorphic Zones
31
ƒ There exists systematic variations in the mineralogy
and texture of rocks in each metamorphic zone.
Index Minerals
33
Higher grade metamorphism is located near the
core of ancient mountain ranges.
Metamorphic Environments
32
Index Minerals
Index Minerals: the grade of metamorphism
can be determined by the minerals present.
34
Metamorphic Environments
ƒ Metamorphic rocks containing the same assemblage
of minerals belong to the same metamorphic
facies – implying they formed in the same
metamorphic environment.
35
ƒ Blueschist is a low temp & high-pressure
environment found locally in the coast range.
ƒ Eclogite facies is high temp & high pressure.
ƒ Blueschist is a low temp & high-pressure
environment found locally in the coast range.
ƒ Eclogite facies is high temp & high pressure.
Glaucophane
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
36
Metamorphic Environments
Garnets
Page 19 of 20
Ancient Meta. Environments
37
✓REVIEW QUESTIONS
38
10. Name the metamorphic rock described:
Most continental shields are composed of meta/igneous
rocks indicating periods of ancient mountain building.
a) Calcite-rich and often nonfoliated =
b) Loosely coherent rock composed of broken fragments that
formed along a fault zone =
c) Represents a grade of metamorphism between slate and
schist =
d) Very fine-grained and foliated; excellent rock cleavage =
e) Foliated and composed predominately of platy minerals =
f) Composed of alternating bands of light and dark silicate
minerals =
g) Hard, nonfoliated rock resulting form contact
metamorphism =
✓REVIEW QUESTIONS
39
11. Contrast contact and regional metamorphism. Which
generates the greatest quantity of metamorphic rock?
✓REVIEW QUESTIONS
40
16. How are gneisses and migmatites related?
12. Where does most hydrothermal metamorphism occur?
17. Which type of plate boundary is regional metamorphism
associated?
13. Describe burial metamorphism.
18. Why do the cores of major mountain chains contain
metamorphic rocks?
14. How to geologists use index minerals?
15. Describe textural changes from slate through gneiss.
BLOCK EXAM #3 - CHAPTERS 6, 7 & 8
19. What are shields? How are these relatively flat areas
related to mountains?
Page 20 of 20