Title: Dynamic Earth
1Dynamic Earth
2Homework, Chapter 5
- Why do some rock layers fold and others break
into faults when they are subjected to crustal
forces?
All other things being equal, experimentation
indicates that rocks subjected to low confining
pressures and low temperatures, such as exist
near Earth's surface, will tend to break under
deforming pressure. Rocks subjected to conditions
that simulate those deep below the surface will
bend or fold when a similar force is applied.
3Experimental Deformation of Marble
Brittle Deformation (low confining pressure)
Ductile Deformation (high confining pressure)
4Factors that affect deformation
- Temperature
- Pressure
- Strain rate
- Rock type
The variation of these factors determines if a
rock will fault or fold.
5Homework, Chapter 5
- If you found tilted beds in the field, how would
you tell if they were part of an anticline or a
syncline?
One could not usually make a determination at the
site but would need to look for other outcrops of
the same rocks in the surrounding area. If the
outcrops define a strip of rocks that becomes
older as one progresses toward its center, it is
an eroded anticline. If the rocks become younger
toward the center of the feature, it is an eroded
syncline.
6Geometry of Anticlines Synclines
7Homework, Chapter 5
- Evidence for vertical crustal movements is often
found in the geologic record. Give some examples
of such evidence.
Areas that are lifted above the surrounding
terrain are targets for increased erosion. Thus,
the extensive erosion of such areas as the Black
Hills is evidence that the area has been lifted.
The tilt in adjacent sedimentary beds that were
revealed by erosion indicates uplift as well.
8Homework, Chapter 5
- It has been suggested that the Himalaya Mountains
and the Tibetan Plateau were uplifted 2,000 m
about 10 million years ago. What caused this
sudden uplift?
Mountains are pushed up when the continental
lithosphere is compressed in the process, both
the crustal and mantle parts of the lithosphere
are thickened, creating a deep root beneath the
mountains. The mantle portion of the root is
denser than the underlying asthenosphere and
eventually drops off, allowing the mountains to
rise higher.
9The drooling lithosphere
10Exam Review
- Second Exam Thursday March 9th
- Exam will be fill in the blank (15 x 2 points)
and short answer questions (9 x 5 points) 75
points total - Questions will come from Lectures (including
videos), reading and homework
11How do we know about the Earths Interior?
- By studying Meteorites
- Direct observation (rocks originating from depth)
- Experiments at high pressure
- By studying earthquake waves (Seismology)
12Structure of the Earth
- Seismic velocity (how fast earthquake waves
travel through rocks) depends on the composition
of material and pressure. - We can use the behavior of seismic waves to tell
us about the interior of the Earth.
13Most common types of earthquake waves
- P-waves and S-waves Body waves
- Primary waves travel the fastest in the crust and
usually are the first waves to arrive - Secondary (or Shear) waves are slower and
therefore take longer to arrive
14Changes in P- and S- wave Velocity Reveal
Earths Internal Layers
Velocities generally increase in each layer
15Types of Seismic Waves
16P-wave Shadow Zone
17S-wave Shadow Zone
18P wave shadow zone
S wave shadow zone
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20Earths CORE
- Outer Core - Liquid Fe, 2200 km thick, No
S-waves transmitted - S- P-wave Shadow Zones - Inner Core - solid Fe (some Ni, Co, S, C), 2500
km thick - How do we know? Meteorites, Seismology, Magnetic
field
21IsostasyAnother key to Earths Interior
- Buoyancy of low-density rock masses floating on
high-density rocks accounts for roots of
mountain belts - First noted during a survey of India
22The less dense crust floats on the less
buoyant, denser mantle
Mohorovicic Discontinuity (Moho)
23Mantle Tomography
- Uses numerous seismic data
- Uses small changes in speed of seismic waves
- Faster wave motion may correspond to denser or
colder regions - Slower wave motion may correspond to buoyant or
warmer regions
24Basics of Tomography
25Hotspots
- Areas with volcanic activity NOT explained by
plate tectonics - Mantle beneath may be hot, wet, or chemically
different - Commonly active for long time
26Hotspot tracks
Flood basalts
Oceanic plateaus
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28Stress
- The force that acts on a rock unit to change its
shape and/or its volume - Causes strain or deformation
- Stress
- Compression
- Tension
- Shear
29Strain
Any change in original shape or size of an object
in response to stress acting on the object
30Ductile (Plastic) Deformation
- Permanent change in shape or size that is not
recovered when the stress is removed - Occurs by the slippage of atoms or small groups
of atoms past each other in the deforming
material, without loss of cohesion
31Brittle Deformation (Rupture)
- Loss of cohesion of a body under the influence of
deforming stress - Usually occurs along sub-planar surfaces that
separate zones of coherent material
32Factors that affect deformation
- Temperature
- Pressure
- Strain rate
- Rock type
The variation of these factors determines if a
rock will fault or fold.
33Folds
- Most common ductile response to stress on rocks
in the earth's crust
34Symmetrical, Asymmetrical and Overturned Folds
35Faults
- Occur when large stresses build up in the crust
- Most common brittle response to stress on rocks
in the earth's crust - Classified according to the kind of movement that
has occurred along them - Know the types of faults especially if they are
important in mountain building
36Stacked Sheets of Continental Crust Due to
Convergence of Continental Plates
37Overlapping Thrust Faults,e.g. the Himalayas
38Tilted Normal Fault Blocks,e.g. Basin and Range
Province
39Structures of continents
- 1) Continents are made and deformed by
plate motion. - 2) Continents are older than oceanic crust.
- 3) Lithosphere floats on a viscous layer below
(isostasy).
40Age of the Continental Crust
Blue areas mark continental crust beneath the
ocean
41Stages in the formation of the Southern
Appalachians
Fig. 17.30
42India has collided with Asia
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44Next Tuesday and Thursday
- Coastal Processes
- (March 14th)
- Coastal Management
- (March 16th)
- Chip Fletcher
- (No reading or homework)