Plate Tectonics. The Continental Drift Hypothesis Plate Tectonics – Overview Divergent Boundaries Convergent Boundaries Transform Boundaries Testing the

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


Wegener’s Evidence• The “continental jigsaw puzzle”

• Trans-oceanic fossil matching

• Trans-oceanic rock matching

• Ancient climates


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


• 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


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



3. Ocean water floods rift, creating narrow seaway

4. Seaway continues to widen as plates separate



Stage 1: Yellowstone NP - Continental lithosphere being pushed up by magma below



Stage 2: East African Rift - Lithosphere being stretched and thinned

Stage 3: Red Sea and Gulf of Aden- Narrow seaways



Stage 4: Atlantic Ocean – wide ocean basin with divergent boundary at center

Plate Boundaries: Convergent


• 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


Oceanic-continental boundaries


Continental-continental boundaries


• A.k.a. “continental collision zones”

• Example: India and Asia

India-Asia Collision: Example of a continent-continent convergent boundary


Plate Boundaries: Transform


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


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


• Sediment thicknesses

• Ocean crust ages

• Hot spot tracks

• Apparent polar wandering

• Paleomagnetic striping

Sediment Thickness and Crustal Age


• Both increase away from oceanic ridges

Hot Spot Tracks


Apparent Polar Wandering


• 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


• 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


• 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


• 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?


Three mechanisms: slab pull, ridge push, mantle drag

GEOL131: Plate Tectonics: What Makes Plates Move?

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End of Chapter


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Plate Tectonics. The Continental Drift Hypothesis Plate Tectonics – Overview Divergent Boundaries Convergent Boundaries Transform Boundaries Testing the