Transcript Geology of the Hawaiian Islands

Dynamic Earth
Class 17
7 March 2006
Homework, 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.
Experimental Deformation of Marble
Brittle Deformation
Ductile Deformation
(low confining pressure)
(high confining pressure)
Factors that affect deformation

Temperature

Pressure

Strain rate

Rock type
The variation of these factors
determines if a rock will fault or fold.
Homework, 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.
Geometry of Anticlines & Synclines
Homework, 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.
Homework, 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.
The drooling lithosphere
Exam 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

How do we know about the
Earth’s Interior?
By studying Meteorites
 Direct observation (rocks originating
from depth)
 Experiments at high pressure
 By studying earthquake waves
(Seismology)

Structure 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.
Most 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

Changes in
P- and S- wave
Velocity Reveal
Earth’s Internal
Layers
Velocities generally
increase in each
layer
Types of Seismic Waves
P-wave
Shadow
Zone
S-wave
Shadow
Zone
S wave
shadow zone
P wave
shadow zone
Earth’s 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
Isostasy:
Another key to Earth’s 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
The less dense crust “floats” on
the less buoyant, denser mantle
Mohorovicic
Discontinuity
(Moho)
Mantle 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
Basics of Tomography
Hotspots

Areas with volcanic activity NOT
explained by plate tectonics

Mantle beneath may be hot, wet, or
chemically different

Commonly active for long time
Hotspot tracks
Flood basalts
Oceanic
plateaus
Stress
The force that acts on a rock unit to
change its shape and/or its volume
 Causes strain or deformation
 Stress

 Compression
 Tension
 Shear
Strain
Any change in original shape or size of an
object in response to stress acting on the object
Ductile (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
Brittle 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
Factors that affect deformation

Temperature

Pressure

Strain rate

Rock type
The variation of these factors
determines if a rock will fault or fold.
Folds

Most common ductile response to
stress on rocks in the earth's crust
Symmetrical, Asymmetrical
and Overturned Folds
Faults




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
Stacked Sheets of Continental Crust
Due to Convergence of Continental
Plates
Overlapping Thrust Faults,
e.g. the Himalayas
Tilted Normal Fault Blocks,
e.g. Basin and Range Province
Structures 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).
Age of the Continental Crust
Blue areas mark continental crust
beneath the ocean
Stages in the
formation of
the
Southern
Appalachians
Fig. 17.30
India has collided with Asia
Next Tuesday and Thursday
Coastal Processes
(March 14th)
Coastal Management
(March 16th)
Chip Fletcher
(No reading or homework)