Glendale Community College
Fall
2004
Telecourse
EARTH REVEALED
Geol-101: Physical Geology
Course #7701: 3 units
Course Website: http://www.glendale.edu/~ppal
Instructor:
Poorna Pal MS MBA Ph.D.
Professor of Geology
Phone: (818)240-1000 ext 5517
Office: CS-262
e-mail:
[email protected]
Telecast :
Charter Communications Channel 15
TTh: 11PM-Midnight
and S: 11AM-Noon
Telecourse Office:
Location: AD 145
Phone: (818)240-1000 ext 5898
e-mail: [email protected]
Handout 4 (Oct 29, 2004)
Broadcasts and the corresponding chapters in the textbook:
14 (Sediments and Sedimentary Rocks) and
15 (Metamorphism, Metamorphic Rocks)
Oct 30 (11AM-Noon)
Nov 2 & 4 (11PM-Midnight)
117 (Sedimentary Rocks)
118 (Metamorphic Rocks)
Nov 6 (11AM-Noon)
Nov 9 & 11 (11PM-Midnight)
119 (Running Water I: Rivers, Erosion and Deposition)
120 (Running Water II: Landform Evolution)
Nov 13 (11AM-Noon)
121 (Ground Water)
Nov 16 & 18 (11PM-Midnight) 122 (Wind, Dust and Deserts)
16 (Streams and
Landscapes)
17 (Ground Water) and
19 (Deserts and Wind Action)
Quiz-4: Available Nov 13, due Nov 19
Supplemental Instruction Class (5-6 PM: CS-266)
Class Meeting and Test-4: Nov 19 (6-9 PM: CS-266)
Mike Strickler’s Video Study Guide
http://jersey.uoregon.edu/~mstrick/RogueComCollege/G100/StudyGuides/VSGindex.html
Episode 117: Sedimentary Rocks - The Key to Past Environments
1. How many years of earth history are represented in the Grand Canyon?
2. What are sedimentary rocks?
3. How are sediments formed?
4. What happens to a thick pile of sediments?
5. How do weathering and erosion influence the composition of sediments?
6. Describe mechanical weathering and the formation of clastic sediments.
7. Describe chemical weathering and the formation of solutions.
8. How can sediments be transported?
9. How does energy relate to the transportation and deposition of sediments?
10. Why do you usually find sand at the beach?
11. What is sorting?
12. What are "facies changes" and how do they relate to sedimentary rocks?
13. Describe the process of lithification.
14. Describe chemical sedimentation, and how it takes place in the ocean.
15. What are sedimentary structures, and why are they important?
16. What is a bedding plane, and what do they represent?
17. How can non-horizontal bedding form?
18. How is cross-bedding used by geologists to interpret geologic history?
19. Describe the formation of symmetrical and asymmetrical ripple marks.
20. Summarize the geologic history of the Ridge Basin.
21. What do "scour and fill" indicate?
22. How are sedimentary rocks important to economics?
23. Summarize the geologic history of the Grand Canyon.
Episode 118: Metamorphic Rocks
1. What happens during metamorphism?
2. Why are there so many different types of metamorphic rocks?
3. What is a protolith?
4. What temperature and pressure ranges are common in metamorphic environments?
5. What is directed pressure and what usually causes it to occur?
6. Describe how directed shear stress results in the formation of snowball garnets.
7. What happens to minerals as temperature and pressure increase?
8. What is foliation?
9. Why are foliated rocks often unstable when exposed at the surface?
10. Describe temperature increases and recrystallization.
11. What are migmatites and how are they formed?
12. What changes in composition often occur during the metamorphic process?
13. What factors affect the specific changes which take place during metamorphism?
14. Describe contact metamorphism.
15. Describe regional metamorphism.
16. How is metamorphism like cooking (called an "isochemical" process).
17. What are metamorphic facies, and how are they used to decipher geologic history?
18. Describe how metamorphism is a process of continual change.
19. Describe progressive regional metamorphic stages, using claystone as the protolith.
20. Why aren't metamorphic rocks 'unmetamorphosed' as temperatures drop?
21. Discuss the formation of blueschist.
22. What types of information are preserved in metamorphic rocks?
23. What is the assumed rate of growth of snowball garnets, and what does this suggest about the
metamorphic process?
24. Why are most metamorphic rocks associated with plate boundaries?
2
Episode 119: Running Water - Erosion and Deposition
1. What 3 things do all streams do?
2. What is one important factor in how rivers shape the land?
3. What factors can cause a river's velocity to increase?
4. What is discharge?
5. What are the 3 erosional processes of rivers?
6. What factors affect sediment transport?
7. What are river bars?
8. Describe a meander point bar and cut bank.
9. How often do most streams flood?
10. What is a floodplain, and what does a river store there?
11. How do streams work to maintain equilibrium?
12. What happens at "Red Eye Crossing" when water levels fluctuate?
13. What geologic factors contribute to the evolution of river systems?
Episode 120: Running Water - Landscape Evolution
1. What happens as a river deepens its channel?
2. What is base level, and what happens to a stream as it approaches base level?
3. Describe the youthful stage of stream development.
4. Describe the mature stage of stream development.
5. Describe the old age stage of stream development and the formation of peneplains.
6. Discuss the interplay between tectonics and erosion.
7. Describe the various patterns of drainage basin development.
8. Describe the process of rejuvenation.
9. What are stream terraces and how are they formed?
10. What are incised meanders and how are they formed?
11. Why do deltas form at the mouths of rivers?
12. How do distributaries contribute to the formation and evolution of river deltas?
13. How much sediment is added to the Mississippi delta every day?
14. What will it take to keep the Mississippi in its present channel?
15. Describe the Old River Control Project.
16. What happened in 1973 and what was the outcome?
17. How does sediment transport affect attempts to control the flow of the Mississippi?
18. What is the ultimate fate of any river control project?
19. Why are the processes which shape the land so poorly understood?
Episode 121: Groundwater
1. Why is groundwater such an important natural resource?
2. What is the source of most groundwater?
3. Describe porosity and permeability.
4. What is an aquifer?
5. What is an aquiclude?
6. What causes most groundwater to be slightly acidic?
7. What happens when groundwater reacts with limestone?
8. How do stalactites and stalagmites form?
9. What is the water table?
10. How does surface topography affect the shape of the water table?
11. What is effluence, and how does it help maintain year-round flow in many rivers?
12. Describe an open aquifer.
13. Describe a closed aquifer, and the formation of an artesian well.
14. What factors make Orange County well suited for groundwater storage?
15. Describe how the aquifer is being recharged.
16. What can happen when too much groundwater is removed from an aquifer?
3
17. What factors can lead to groundwater contamination?
18. How do engineers attempt to limit groundwater contamination from landfills?
19. How quickly does groundwater recharge in a natural setting?
20. What is Orange County doing to limit saltwater encroachment?
21. How is groundwater recharged?
22. What problems need to be addressed in terms of groundwater management?
23. What is the role of the hydrogeologist?
Episode 122: Wind, Dust, and Deserts
Instructor's note: In most cases, I feel that the Earth Revealed series does an excellent job of describing
the topic covered by the episode. However, it is my opinion that they have severely missed the point in
several issues related to "deserts." The first is in the definition of the term. The popular impression of a
desert is a place which is hot and dry. However, even by their own admission, a desert is classified by the
absence of water, and has nothing to do with temperature. Therefore, Antarctica and Point Barrow can
be considered deserts, just as easily as the Mojave or Sahara. A desert is dry, no matter where it is
located. Furthermore, this is not so much a lack of water, as it is a lack of moisture. I realize that this
sounds like the same thing, but the difference is profound. Consider this: A rich man may be concerned
with thousands (millions?) of dollars, but the loss of this amount, while painful, is of no great importance.
A poor man, on the other hand, is concerned with dimes and dollars, and any small fluctuation in the
available monetary supply can have significant effect! I much prefer the term "arid land" to "desert." In
this way, the reality of the influence of moisture (or actually the lack of moisture) is reflected in the name
we use to describe these most fragile of lands.
1. Where are most deserts located?
2. What is a desert?
3. Describe how proximity to the equator affects the formation of deserts.
4. Describe the rain shadow effect and the formation of deserts.
5. How does plate tectonics affect the formation of deserts?
6. What other factors influence the location and formation of deserts?
7. Describe the affect of precipitation on the desert.
8. What are alluvial fans?
9. Why do most deserts have interior drainage patterns?
10. What is loess?
11. How is sand transported by wind?
12. What are playas?
13. What are blow-outs and how can they contribute to the formation of an oasis?
14. Describe the formation of desert pavement.
15. Describe the formation of desert varnish.
16. What is desertification?
17. How can the removal of vegetation lead to desertification?
18. How did ranching and farming practices contribute to the Dust Bowl of the 1930s?
19. Describe recent developments in the Sahal.
20. How did modern technology contribute to the desertification of sub-Sahara Africa?
21. Discuss efforts to reduce desertification on a global scale.
22. Why do you think that desert lands are so fragile?
4
Sediments and Sedimentary Rocks
„ A sediment is a collection of loosened
particles of solid rock originating from
–
–
–
weathering and erosion of preexisting rocks
chemical precipitation
organic secretion or organic debris
„ SEDIMENTARY ROCKS form from
consolidation, compaction and lithification/
diagenesis of the sediments. This involves
–
–
–
PETROLOGY OF
SEDIMENTARY ROCKS
ROBERT L. FOLK
Hemphill Publishing Company
Austin, Texas 78703
Click at the URL below or go to
http://www.lib.utexas.edu/Libs/GEO
/FolkReady/TitlePage.html
to read this online book on this
subject
weathering and erosion
transportation and deposition
lithification and diagenesis
„ Other resources on the world wide web:
This USGS site has images and movies of crossbedding and bedform features in sedimentary
rocks: http://walrus.wr.usgs.gov/seds
This University of Oregon site,
http://darkwing.uoregon.edu/~dogsci/dorsey/SedResources.
html
has a comprehensive list of web resources on Sedimentary Geology.
„ CLASSIFICATION OF SEDIMENTARY ROCKS
– CLASTIC SEDIMENTARY ROCKS lithify from the sediments, e.g.,
conglomerates and breccia, sandstones, shales
–
ORGANIC, BIOGENOUS or BIOCLASTIC SEDIMENTARY ROCKS consolidate
from the organic remains, e.g., coal, limestones (such as oolites, coralline
limestones), chert etc.
–
CHEMICAL or HYDROGENOUS SEDIMENTARY ROCKS either
a. precipitate from solution (e.g., carbonates like limestones and dolomites) or
b. form as evaporates (e.g., gypsum, rock salt).
„ The common sedimentary structures include bedding, crossbedding, ripple marks, folds, faults and unconformities.
5
Metamorphism and the Metamorphic Rocks
„
Metamorphism is the solid-state
physicochemical (i.e., the textural
and/or mineralogical) alteration of
preexisting rock by changes in the
pressure and/or temperature
conditions.
„
These factors affect metamorphism:
–
–
–
–
This USGS website on North Cascades Geology
http://wrgis.wr.usgs.gov/docs/parks/noca/nocageol2c.html
has an interesting presentation on metamorphic rocks
parent rock’s mineral composition;
the temperature changes that affect
mineral stability;
pressure that may be (a) confining
or static pressure which squeezes
or visit the following University of British Columbia website
http://www.science.ubc.ca/~geol202/meta/metamorphic.html
grains together; so reducing the
to learn about the metamorphic rocks and processes.
porosity and producing a denser
and finer grained rock, or (b) directed (or dynamic) pressure
which aligns platy minerals and produces shearing and
foliation; and
often water vapor, and sometimes CO2, aid mobility and the
reconfiguration of ions.
„ The classification of metamorphic rocks
– FOLIATED rocks are
(a) slaty,
–
(b) phyllitic,
NONFOLIATED rocks
are named on the basis
of composition.
(a) quartzite and marble have coarse interlocking grains of
quartz and marble, respectively;
(b) hornfels are fine-grained with (i) micaceous minerals if
they are derived from shales, (ii) mafic and plagioclase
minerals when they are derived from basalts, and (iii)
metaconglomerates
„ Types and facies
of metamorphism
–
–
–
–
–
(c) schistose and
(d) gneissic
named texturally as
Contact or thermal metamorphism
Regional or dynamothermal metamorphism
Metasomatism (involves extraneous chemical
changes)
Hydrothermal alterations/vein mineralization
Metamorphic facies is the grade concept of
metamorphism that includes the effects of pressure
and temperature, using indicator mineral
assemblages, to infer the P/T conditions.
Read more about this concept at:
http://duke.usask.ca/~reeves/prog/geoe118/geoe118.029.html
„ Granitization (follows migmatization) as
the ultimate stage of metamorphism and
completes the rock cycle.
Learn about the granites from Rob’s Granite page at
http://uts.cc.utexas.edu/~rmr/definition.html
6
Streams and Landscapes
„
The hydrologic Cycle
Running water is
–
–
the run-off of excess precipitation from
land to the oceans that completes the
hydrological cycle;
the most important geologic agent for
(a) erosion, transportation and
deposition of sediments, and (b)
landscape development.
Evaporation
= 85 x 109
million
gallons
Precipitation
= 75 x 109
million gallons
Typically, 15-20% of rainfall in the
hydrologic cycle becomes surface runoff,
usually through streams, but also as sheet
wash under favorable conditions (e.g., in
the deserts).
„
–
–
„
Land
World Ocean = 362 x
1012 million gallons
Run-off
= 1010
million
gallons
Following USGS sites offer excellent introduction to
and information on surface water resources:
Stream, a gravity driven channel
flow,
–
Evaporation
= 15 x 109
million
gallons
Precipitation
= 25 x 109
million gallons
http://ga.water.usgs.gov/edu/
removes water from a drainage basin
http://water.usgs.gov/data.html
(internal or external) separated from
other basins by the divides;
is typically antecedent in Southern California; and
can have dendritic, radial, trellis etc. drainage patterns that reflect rock type and structure.
The longitudinal profile of a stream reflects its gradient:
It is steep at headwaters (juvenile phase) with V-shaped valley, gentler when it enters the
plains (adult phase) where it meanders in a broad valley with a flood plain, and nearly flat
towards its mouth (mature phase) where a delta forms.
„
Stream erosion, transportation and deposition are controlled by
(a) velocity (governed by channel shape and roughness, volume of water) and
(b) discharge (= volume per unit time = flow velocity × area of channel’s cross-section)
Specifically, (a) erosion involves hydraulic action, solution and abrasion; (b) transportation
occurs through bed load/saltation (sand and gravel), suspension (silt and clay) and solution;
while (c) deposition occurs with drop in velocity (e.g., formation of flood plains, channel-fills
and levees, meander loops and placer beaches, deltas and alluvial fans ⎯ sediment
supply, waves and shoreline currents control the shape of a delta).
„
Regional erosion by streams is controlled by
–
–
–
climate (angular landforms in dry climates and rounded landforms in wet climates),
rock type (slope angle decreases with the overburden grain size) and
structure (folded and faulted rocks depart from the staircase character of horizontal
beds).
Landscapes reflect either a reduction of slope angles from erosion approaching base level
or parallel retreat of slopes across a region.
„
Stream terraces reflect
–
either regional uplift which lowers the base level and promotes down-cutting or
change from dry to wet climate (which increases a stream’s erosional capacity).
7
Deserts and Wind Action
Deserts
„
„
are “arid” or dry regions (i.e., regions that receive
<25 cm (or <10 inches) of annual precipitation and
commonly occur at
–
–
–
–
–
about 30° N and S latitudes and the poles,
i.e., where air pressure is usually high
the rain shadows of mountains,
continental interiors,
the proximity to cold ocean currents, and/or
high altitude
„
Of these, atmosphere plays the most important
role. Deserts worldwide are found at ~30° N
and S latitudes, to-wards the western margins
of land, and at the South Pole. Note that ~30°
N and S are also the latitudes at which sea
surface waters are particularly salty, because
evaporation exceeds precipitation. Air pressure
is high at these latitudes.
A separate handout is being provided on
atmospheric circulation and weather in the attached
handout. The model shown below is from the URL:
http://pubs.usgs.gov/gip/deserts/atmosphere/
source: http://pubs.usgs.gov/gip/deserts/what/world.html
„ Desert landforms
–
–
Visit http://pubs.usgs.gov/gip/deserts/contents/
to read this USGS online publication on deserts.
Dunes typically characterize the deserts and form under winds
that blow from one direction.
Crescentic or crescent-shaped mounds that are generally wider
than long, with slipface on the concave side, are the most
common dune form on Earth and on Mars.
The other types of dunes are linear, star, dome, and parabolic.
–
„ Desert streams tend to be
– intermittent rather than perennial, and
– characterized by (a) internal drainage (e.g., Colorado, Niger,
Amu and Syr darya); and (b) flash flooding.
Sahara once received almost 18 inches of
rain in a 3-hour period, for instance.
„ Desert of Southwestern U.S.
–
–
reflect partly the latitude effect and partly
the rain-shadow effect; and
are characterized by flat lying sediments
in Colorado plateau and (b) block faults in
the Basin and Range province.
Visit the above USGS site at
http://terraweb.wr.usgs.gov/TRS/projects/eolian/eolianmp.html
for information on aeolian mapping and related issues
or go to http://www.desertusa.com/ to browse multimedia desert
life related articles in the online magazine “desertUSA”
„ Wind action
–
–
–
also creates the erosion, transportation and deposition of sediments (aeolian
sediments), depending on wind speed (and air temperature), topography and geology;
typically produces LOESS deposits (e.g., the U.S. Midwest and China) and sand dunes;
and
combines with Southern California Sun to produce the Los Angeles SMOG through
thermal inversion.
8
Atmospheric Circulation and Weather
Ocean-atmosphere interactions (a) moderate surface temperatures, (b) shape the earth’s overall weather and climate, and
(c) create ocean waves and currents.
z Atmosphere is the solid earth’s ~110 km thick gaseous envelope; weather
is the atmosphere’s state at a given point in time and space; and climate is
the weather’s yearly averaged seasonal composite.
z In the near-surface region that we are interested in, i.e., the troposphere (it
extends to 10-15 km above earth’s surface and carries ~90% of the
atmospheric mass), average temperatures decrease as we go up.
z The atmosphere
— is uniquely rich in N2 (78%) and O2 (21%) - Moon and Mars lack an
atmosphere, atmosphere of Venus is CO2-rich (~96%), while Jupiter and
Saturn have H and He dominated atmospheres;
— evolved in three phases: (1) H, He rich early phase of ~4.5 Ga ago, (2) CO2, N2 and H2O rich middle
phase of ~3.7 Ga ago and (3) N2 and O2 rich present phase since ~1.25 Ga ago.
z Where did Earth’s CO2 ― Atmosphere has ~0.03% CO2, Seawater ~60 times as much.
― Carbonates (e.g., limestones) precipitate in the ocean bottom. Indeed, most of this C is
go?
now locked up in limestones and marble.
Oceans have thus depleted the earth’s atmosphere of its CO2 content.
z Three forces mainly drive atmospheric circulation: (a) differential solar
heating of earth’s surface, (b) gravity, i.e., equatorial bulge versus
polar flattening, and (c) rotation.
z As solar heating of the earth’s surface varies with latitude,
— earth’s elliptical orbit and 23½° tilt of spin axis create seasons; and
― evaporation dominates radiation in the tropics while water freezes at the polar latitudes to create icecaps.
z Moisture–laden warm air that rises at the equator cools down and
must sink but (a) travels polewards as gravity at the poles exceeds
that at equator and (b) begins equatorward return as cold surface air
after sinking at the pole, so creating a convective cell.
z Being basically unstable, this single cell breaks down into three, at 30°
and 60° (N and S) latitudes. This creates (a) high pressure zones of
sinking air masses at the 30° N and S latitudes and poles (these
latitudes thus have deserts on land and high surface water salinity in
the oceans), and (b) low pressure zones of rising air masses at the 60°
N and S latitudes and equator (these latitudes thus have rain forests
on land and low surface water salinity in the oceans).
z The Coriolis deflection of
these convection cells in the
direction of Earth’s spin now
creates:
°
― westward surface winds (“trade winds”) at 0°-30°N and° 0°-30
S,
― eastward surface winds (“prevailing westerlies”) at° 30° -60°N & S, and
― westward surface winds (“polar easterlies”) at 60 -90 N and S.
z Equatorial surface air thus flows against earth’s spin
direction. Warm surface waters thus stack up on the
western margins of tropical/semi-tropical oceans, with the
following results:
― Warm surface waters stack up on the western margins of
tropical/semitropical oceans. This (a) deepens the thermocline on
the western coasts, so producing generally humid climatic
conditions on one hand and poor fishing conditions on the other;
and (b) creates breeding grounds for tropical storms/cyclones on
these western margins.
― Upwelling of cold deep waters on the opposite eastern coasts
produces arid climates on one hand but excellent fishing on the
other.
9