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Plate Tectonics
Plate Tectonics
• The Continental Drift Hypothesis
• Plate Tectonics – Overview
• Divergent Boundaries
• Convergent Boundaries
• Transform Boundaries
• Testing the Theory of Plate Tectonics
• Measuring Plate Motion
• What Makes Plates Move?
The Continental Drift Hypothesis
GEOL131: Plate Tectonics
Alfred Wegener (1880-1930)• German paleoclimatologist
– Studied evidence for ancient climates
• Traveled extensively, collected thousands of samples
• Published continental drift ideas in 1915
• Died during expedition to Greenland in 1930
GEOL131: Plate Tectonics: Continental Drift
Pangaea• “All lands”• 200 million years ago
GEOL131: Plate Tectonics: Continental Drift
Wegener’s Evidence• The “continental jigsaw puzzle”
• Trans-oceanic fossil matching
• Trans-oceanic rock matching
• Ancient climates
GEOL131: Plate Tectonics: Continental Drift
Wegener’s Evidence: Continental JigsawGEOL131: Plate Tectonics: Continental Drift
Wegener’s Evidence: Matching FossilsGEOL131: Plate Tectonics: Continental Drift
Wegener’s Evidence: Matching FossilsGEOL131: Plate Tectonics: Continental Drift
Wegener’s Evidence: Matching RocksGEOL131: Plate Tectonics: Continental Drift
Present
Pangaea
Wegener’s Evidence: Ancient ClimatesGEOL131: Plate Tectonics: Continental Drift
Present
Pangaea
• Other scientists were highly critical
• Major problem: no good explanation for how continents could move
• Wegener proposed– Moon’s gravity moved continents– Continents pushed through ocean crust like
icebreaker ships
Reaction to Wegener’s IdeasGEOL131: Plate Tectonics: Continental Drift
• Moon’s gravity too weak
• Ocean crust too dense and strong
• Most scientists concluded continental drift was incorrect
Reaction to Wegener’s IdeasGEOL131: Plate Tectonics: Continental Drift
Plate Tectonics: Overview
GEOL131: Plate Tectonics
• Based on composition– Crust, mantle, core
• Based on seismic wave behavior– Lithosphere, asthenosphere, mesosphere,
outer core, inner core
• Lithosphere, asthenosphere most important for plate tectonics
Earth’s Layers: Two schemesGEOL131: Plate Tectonics: Plate Tectonics Overview
Earth’s Layers: Two schemesGEOL131: Plate Tectonics: Plate Tectonics Overview
Asthenosphere
• Rigid, brittle
• Continents are “embedded” in them
Lithospheric PlatesGEOL131: Plate Tectonics: Plate Tectonics Overview
• Provides plausible explanation for how continents move
• Doesn’t require them to “plow” through ocean crust
Why is plate tectonic theory successful?GEOL131: Plate Tectonics: Plate Tectonics Overview
• Oceanic– Thin, dense, able to subduct into mantle– “asphalt”
• Continental– Thick, low density, buoyant, does not subduct– “styrofoam”
Two types of lithosphereGEOL131: Plate Tectonics: Plate Tectonics Overview
Plate MotionGEOL131: Plate Tectonics: Plate Tectonics Overview
Plate Boundaries: Divergent
GEOL131: Plate Tectonics
Plates separate at divergent boundariesGEOL131: Plate Tectonics: Divergent Boundaries
New crust created by undersea volcanism
Creation of Divergent Boundaries
GEOL131: Plate Tectonics: Divergent Boundaries
1. Continental lithosphere pushed up by upwelling magma
2. Lithosphere stretched & thinned, forming a continental rift
Creation of Divergent Boundaries
GEOL131: Plate Tectonics: Divergent Boundaries
3. Ocean water floods rift, creating narrow seaway
4. Seaway continues to widen as plates separate
Creation of Divergent Boundaries
GEOL131: Plate Tectonics: Divergent Boundaries
Stage 1: Yellowstone NP - Continental lithosphere being pushed up by magma below
Creation of Divergent Boundaries
GEOL131: Plate Tectonics: Divergent Boundaries
Stage 2: East African Rift - Lithosphere being stretched and thinned
Stage 3: Red Sea and Gulf of Aden- Narrow seaways
Creation of Divergent Boundaries
GEOL131: Plate Tectonics: Divergent Boundaries
Stage 4: Atlantic Ocean – wide ocean basin with divergent boundary at center
Plate Boundaries: Convergent
GEOL131: Plate Tectonics
• Three types, based on lithosphere involved– Oceanic-oceanic– Oceanic-continental– Continental-continental
Plates come together at convergent boundaries
GEOL131: Plate Tectonics: Convergent Boundaries
Oceanic-oceanic boundaries
GEOL131: Plate Tectonics: Convergent Boundaries
Oceanic-continental boundaries
GEOL131: Plate Tectonics: Convergent Boundaries
Continental-continental boundaries
GEOL131: Plate Tectonics: Convergent Boundaries
• A.k.a. “continental collision zones”
• Example: India and Asia
India-Asia Collision: Example of a continent-continent convergent boundary
GEOL131: Plate Tectonics: Convergent Boundaries
Plate Boundaries: Transform
GEOL131: Plate Tectonics
Plates slide past each other at transform boundaries
GEOL131: Plate Tectonics: Transform Boundaries
San Andreas Fault: A transform boundary
GEOL131: Plate Tectonics: Transform Boundaries
Los Angeles is moving northwest
San Francisco is moving southeast
San Andreas Fault: A transform boundaryGEOL131: Plate Tectonics: Transform Boundaries
Testing the Plate Tectonic Theory
GEOL131: Plate Tectonics
Much evidence comes from ocean drilling
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Research ships drill into ocean sediments and rock
• Drill cores are collected that show cross-section of ocean floor at that location
Major Lines of Evidence
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Sediment thicknesses
• Ocean crust ages
• Hot spot tracks
• Apparent polar wandering
• Paleomagnetic striping
Sediment Thickness and Crustal Age
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Both increase away from oceanic ridges
Hot Spot Tracks
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
Apparent Polar Wandering
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Earth has a magnetic field
• Some minerals are magnetic
• These minerals act like compass needles– They record where magnetic poles were
located at the time the minerals formed
Apparent Polar Wandering
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Rocks in North America and Eurasia seem to show that magnetic poles have moved thousands of miles
• Continents have moved, not the poles
Paleomagnetic Striping
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Earth’s magnetic field has reversed many times– Normal polarity: North magnetic
pole near north geographic pole
– Reversed polarity: N. magnetic near S. geographic
Magnetic field polarities over the last 4 million years. Periods of normal polarity shown in white.
Paleomagnetic Striping
GEOL131: Plate Tectonics: Testing the Plate Tectonic Theory
• Oceanic crustal rocks record the polarity in effect at the time they formed
• Symmetrical “stripe” pattern of normal and reversed polarity created in ocean crust
What Makes Plates Move?
GEOL131: Plate Tectonics
Three mechanisms: slab pull, ridge push, mantle drag
GEOL131: Plate Tectonics: What Makes Plates Move?
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End of Chapter
GEOL131: Plate Tectonics
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