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CRUSTAL MOTION
1.
Types of crustal motion and deformation
a.
Gravity anomalies and crustal heating
b.
Mountain-building
c.
Non-deforming tectonic uplift
d.
Folds – bending and warping of crustal layers
e.
Earthquakes – a sudden movement of the crust due to faults, fractures in the crust
2.
Types of crustal motion and deformation: Gravity anomalies
a.
Generate the “lumpy” earth. Areas of the earth bulge outward while other areas are deformed
downward.
b.
Earth surface is lumpy due to variations in the density of the rocks that comprise the earth’s
crust and subsequent variability in gravitational attraction.
c.
The force of gravity acting on crustal rocks is proportional to the density of the rocks
d.
Granitic rocks (continental crust) are less dense (more mass per unit volume) than basaltic
rocks (oceanic crust).
e.
Therefore areas underlain by basalts have a slightly greater gravitational force influencing them
and are deformed downward relative to granitic areas.
3.
Types of crustal motion and deformation: Crustal heating
a.
Hot magma near the surface may decrease its density and cause it to bulge upward. As rocks
cool they increase in density and are downwarped more by gravity
b.
Radioactive decay in the mantle heats up the overlying continental crust and causes it to bulge
upward. Thus the continental crust is higher not only because of its lower density, but because
it is heated up more efficiently by underlying radioactive decay in the mantle.
4.
Types of crustal motion and deformation: Mountain-building
a.
Slow deformation of crust. Rates of uplift and mountain building may approach 5-10 m/1000 yr
(on average a few centimeters a year)
b.
A mountain building episode is called an orogeny. Some mountains are new and have
undergone only a single orogenic event. Other mountain ranges reflect several orogenic
episodes, like the Apalachians
c.
Mountains are very small compared to the shape and size of the earth. In fact, the overall
topographic relief of canyons, mountains, oceanic trenches has little influence on the overall
shape of the earth. Consider these features to be like a coat of paint that varies in its thickness
i.
Mt. Everest - 29,000 ft altitude
ii.
Mariana Trench - 35,800 ft deep
iii.
Total maximum relief-12 miles versus the Earth circumference of 40,000 km/25,000
miles
d.
Mountain building is ultimately caused by tectonism that increases elevations through:
i.
Crustal uplifting and thickening from convergent plate motion
ii.
Thrust faulting from convergent plate motions
iii.
Accumulation of extrusive volcanic materials
iv.
Emplacement of batholiths, an intrusive igneous feature
v.
Accretionary wedging
vi.
Exotic terranes
5.
Types of mountain building: Crustal uplifting and thickening from convergent plate motion
a.
Himalayas
i.
Formed by collision of Asian and Indian plates. Entire Tibetan Plateau uplifted.
ii.
Thickening aided through the process of accretionary wedging. Sedimentary rocks can
be added to mountains, which can then be lifted to high altitudes. Thus it is not
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uncommon to find fossils at high altitudes in the Himalaya.
6.
Types of mountain building: Emplacement of volcanic extrusive and intrusive igneous rocks.
a.
Sierra Nevada Mountains
7.
Types of mountain building: Thrust faulting from convergent plate motions
a.
Rocky Mountains
i.
The Rocky Mountains are composed of several mountain belts separated by large
intermontane basins
ii.
Northern Rockies: persist into Canada
iii.
Middle Rocky Mountains
(1)
Yellowstone National Park
(2)
Grand Teton National Park
iv.
Southern Rockies
(1)
Numerous peaks above 14,000 ft (Pike’s Peak and Long’s Peak)
(2)
Rocky Mountain National Park in the Colorado Front Range
8.
Types of mountain building: Appropriation of exotic terrains
a.
California coastal mountains
9.
Types of mountain building: Accumulation of volcanic materials
a.
Hawaiian Islands
i.
Hawaii is the largest mountain in the world when measured from its base underwater to
the exposed peak
10.
Mountain ranges can be composed of a variety of rock types depending upon how they were formed
and the amount of weathering and erosion that has taken place
a.
Sierra Nevada - mostly uplifted intrusive igneous rocks, chiefly granite from batholiths.
Relatively less extrusive volcanics and sedimentary rocks.
b.
Himalayas - metamorphic, sedimentary rocks. Extrusive volcanics not as relatively abundant.
Continent-continent collisions do not typically produced subduction-driven volcanics.
c.
Cascades - relatively more extrusive volcanics than Sierra Nevada and Himalayas
11.
Types of crustal motion and deformation: Non-deforming tectonic uplift
a.
Continental crust can experience broad uplift from gentle warping. Gentle warping can raise
strata upward up to thousands of feet over long time periods without the deformation associated
with mountain building.
b.
Colorado Plateau in the American Southwest.
i.
Sedimentary rock units over a very wide area gradually lifted to over 10,000' above sea
level in southern Utah.
ii.
The Colorado River has cut down through these gently uplifted sedimentary layers to
form the Grand Canyon, which is a mile deep.
c.
Removal of immense weight from the crustal surface results in an upward adjustment of the
continental crust known as isostatic rebound. Remember, the crust floats in uppermost
mantle.
i.
When continental glaciers (ice over two miles thick in some spots) melted, crust rose by
isostatic rebound. Parts of the Atlantic coast, northern Europe, and Canada are still
rising from last pulse of Pleistocene glaciation
12.
Types of crustal motion and deformation: Folding
a.
In response to stress, rocks are slowly deformed (as opposed to sudden rapid movement along
faults)
b.
Two types of folds
i.
Anticlines-oldest rock in center
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c.
13.
ii.
Synclines-youngest rock in center
Anticlines do not necessarily form hills and synclines form valleys. Can have anticlinal valleys or
synclinal hills, depending on resistance of rock units and differential erosion. Type of rock
exposed to surface processes determines the shape of the land.
Types of crustal motion and deformation: Earthquakes
a.
Earthquake-vibrational shock waves transmitted through crustal material in association with
sudden release of stress along faults.
b.
Type of crustal motion shapes stress and type of fault
i.
Divergent motion results in tensional stress
(1)
Normal fault
(2)
Multiple sets of normal faults produce horsts and grabens.
(3)
Example: Basin and Range physiographic province in Nevada
ii.
Convergent motion results in compressional stress
(1)
Reverse fault (high-angle)
(2)
Thrust fault (low-angle). Contributes to mountain-building
iii.
Stress imposed on crust is lateral
(1)
Strike-slip fault (San Andreas Fault)
c.
Variables used to describe earthquakes
i.
Focus-subsurface point of origin
ii.
Epicenter-point on the surface above the focus
iii.
Magnitude
(1)
Measured with the Moment Magnitude scale, which is correlated with Richter
Scale and uses a similar 1-9 scale of intensity
iv.
Various mathematically defined motions, or waves originating out of release of energy
along faults
(1)
p waves
(2)
s waves
v.
Intensity - represents level of shaking at a given location. Shaking is a function of
distance to epicenter and type of rock. Measured by the Modified Mercalli scale
d.
Frequency
i.
Small earthquakes are common, literally thousands daily, and many are not even felt by
humans. Larger earthquakes like an 8.0 could be expected to occur somewhere in the
world each year
e.
Distribution
i.
Convergent plate boundaries (shallow and deep focus earthquakes)
(1)
Circum-Pacific belt (the Ring of Fire)
(2)
Trans-Eurasian belt (through Turkey into Iran)
ii.
Mid-ocean ridges (shallow focus earthquakes)
f.
Selected earthquakes
i.
1886 Charleston
(1)
Caused by offshore movement of sediments and possibly methane hydrates
ii.
1812 New Madrid, Missouri
(1)
Faults and earthquakes associated with former rift zone with possible
contribution from fluvial erosion of overlying sediments during Pleistocene
iii.
Megathrust earthquakes
(1)
1964 Alaska
(2)
2011 Japan
(3)
2004 Indonesia
iv.
Strike slip
(1)
1906 San Francisco (The Great Fire of San Francisco)
(2)
2010 Haiti
g.
Damage factors
i.
Damage intensified where there is a thick surface layer of unconsolidated (loose)
sediments. This results in liquification of surface sediments
ii.
Building codes also determine the amount of damage experienced. Lax building codes
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iii.
14.
result in more mortality
Case study: Bangladesh
h.
Non-tectonic earthquakes factors
i.
Methane hydrates
ii.
Human-caused earthquakes
(1)
Due to:
(a)
Injection of wastewater from oil drilling and fracking into deep rock layers
(b)
Construction of large artificial lakes
i.
Tsunamis, a consequence of crustal motion caused by earthquakes
i.
Motion from sudden plate movement transferred to ocean
ii.
Wave propogate very quickly in the open ocean
iii.
As tsunami approaches shallower water, wave slows and grows in height
iv.
Geological evidence for frequent tsunamis and megatsunamis
Surface features reflect the combined influence of these processes (gravity anomalies,
mountain-building, non-deforming tectonic uplift, folding, motions along faults) as well as volcanic
processes and the dynamics of weathering and erosion. Tectonics and associated volcanics create
primary landforms, the underlying rock types and their distribution. Weathering and erosion create
secondary landforms, the outward appearance of the these primary landforms.
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