Plate Tectonics

• It’s a pretty zany idea

• What brought it about?

• Geologic observations made by Wegener and first suggested in 1915

• Why is it important?

Geographic EvidenceFossil Evidence

Types of Evidence for Continental Drift

Types of Evidence for Continental Drift

Evidence from Glacial Deposits Evidence from Coal Deposits

Magnetic Evidence

Patterns of reversal are nearly mirror image

The Earth’s magnetic field can be:

Normal: Compass points North

Reversed: Compass points South

The direction of the magnetic field is recorded by rocks as they form

AnimationFrom Smith and Pun

Sea Floor Spreading andSea Floor Magnetism

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Whole Ocean Magnetics

How Plates Behave (Bean pot analogy)

Smith and PunPlate Motions Through Time

The PlatesThe plates are essentially RIGID pieces of the Earth’s crust

They move due to convection of the mantle

They can contain both OCEANIC and CONTINENTAL crust

Passive Margin

Active Margin

(Plate Boundary)

Isostacy

D=kT Depth below waterlineH=(1-k)T Height above waterlinek= B/W Fraction of block below water line

H

D

T

Thick Pine Thin Pine Thin Oak

Where does continental crust come from?

Continental Crust

Three Basic Types of Plate Boundaries

• Divergent - plates moving away from one another (Tension)

• Convergent - plates moving toward one another (Compression)

• Transform - plates moving past one another (Shear)

! Most plates include con/divergent and transform components !

Smith and PunCorrelating Processes and Plate

Boundaries

Smith and Pun

Sea Floor Spreading and Sea Floor Magnetization

Motion at Divergent Boundaries

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Formation of Oceanic Crust

Pillow lavas

Sheeted dikes

Sediments

Gabbro

Peridotite (Mantle rock)

Life at Midocean Ridges

midocean RIDGES

HotterLess Dense

Higher

CoolerMore Dense

Lower

CoolerMore Dense

Lower

Smith and PunForming a Divergent Boundary

ANIMATION20 - Plate Divergence

Smith and Pun

Motion at Convergent Boundaries

Convergent Boundaries

• Oceanic crust is denser and “rides lower” than continental crust

• Expect different things to happen– Ocean-Continent Collisions – Continent Continent Collisions

TrenchesMountains

Animations

• Subduction– Animation 20- Ocean-Continent

– Animation 20- Continent-Continent

Transform Boundaries

• San Andreas Fault in California– Animation 20-Transform Boundaries

• Transform Faults in Midocean Ridges– Motion at a transform plate boundary

The Complex Nature of Plate

Boundaries

Hotspots

– Smith and Pun: Hotspots and Plumes

What drives the plates?

What drives the plates, really?