platetectonics.pdf

8/11/2019 PlateTectonics.pdf

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World Map


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Evidence for Continental Drift(Wegener)

Geometrical fit of continental margins

Similarity of fossil record in distant lands

Alignment of geological features across ocean basins


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Pacific Sea Floor


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Arctic Sea Floor


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quake distribution

Epicenters 29,0000 quakes 1961-67

Barazangi and Dorman (1969)


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Volcano distribution


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Plate Tectonics: ConceptThe Earth can be divided into layers based on the strength of the

rocks. The outer layer is called the lithosphere. It is cool,

strong and much of the deformation in response to stress iselastic or brittle. Below this layer is the asthenosphere, a hotter

weak layer that behaves plastically.

The lithosphere is broken into units called plates, and each plate

moves coherently. Plates move in response to stresses created

by the action of gravity on density contrasts. Dense materialssink into the Earth interior, while less dense materials rise.

Density contrasts develop due to heating in the interior and

cooling at the surface. Earthquakes are generated at plate

boundaries, where adjacent plates move in different directions.


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Observations explained by Plate Tectonics

Geometrical fit of continental margins

Similarity of fossil record in distant lands

Alignment of geological features across ocean basins

Surface topography (mts, trenches, fault patterns)

Localized distribution of earthquakes and volcanoes

(most on plate margins)

Paleomagnetic polar wander patterns

Magnetic bands on sea floor


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Plates and motion

Plate Names


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China Plate


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Plates and velocities


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Plates/volcanoes


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GPS Absolute Plate Velocities(Press et al.)


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Earth cross section


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Plate Boundaries

Divergent (constructive)

Extension = normal faultsexample = Mid-Atlantic Ridge


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Mid-Atlantic Ridge


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Divergent Boundary(Press et al.)


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Africa


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Asia minor/east Africa


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Divergent Boundary(Press et al.)


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Rift valley volcanoes


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Plate Boundaries

Divergent (constructive)

Extension = normal faultsexamples = Mid-Atlantic Ridge, Red Sea

Convergent (destructive)

Compression = reverse faults)examples = Aleutian Trench, Peru-Chile Trench


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Andean suubduction zone

How do we form rocks

like those in CoastRanges and Sierras?

Subduction Zones!

(drawing from Press et al.)


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subduction zone


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Pacific Sea Floor


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Africa, Europe and Asia


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Himalayas


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Plate BoundariesDivergent (constructive)

Extension = normal faultsexamples = Mid-Atlantic Ridge, Red Sea

Convergent (destructive)

Compression = reverse faults)examples = Aleutian Trench, Peru-Chile Trench

Conservative (no lithosphere made or lost)

Shear = transform or lateral faults

examples = San Andreas Fault, Mendocino Fracture zone


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608 Juan de Fuca Plate


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Map view of Transform Fault(conservative boundary)

ridge axis

(quakes)

ridge axis

fracture zone

(no quakes)

green arrows

show motion

red = seismic

activity


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Pangea schematic

Pangea about 240 my ago


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Early Triassic Plates


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Late Jurassic Plates


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Uses of PaleomagnetismPolar Wander (on continents) = apparent motion of North Pole

Rocks record magnetic vector aligned with fieldDeclination relative to North Pole = rotation

Inclination relative to level surface = latitude

Magnetic Anomalies of Earth FieldMagnetic Field flip flops every 1-2 my

Ocean crust records field orientation when it cools

Positive anomaly = rocks formed in normal times

Negative anomaly = rocks formed in reversed timesTransitions = rocks formed when field flipped


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Magnetic stripes

Color defines positive an

negative anomalies

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