Earth Sciences 240A Lecture 12

Plate Tectonics II

Heat Plume > Spreading Center

Hot spot: Domed oceanic lithosphere; 3-pronged fracture; 1 failed arm: aulacogen

Red Sea

Subduction

Push? Friction of convection cycle?

Pull by gravity

Dense cold oceanic crust

Seismic Tomography

Seismic velocity: Fast = cold; Slow = hot

Inhomogeneous asthenosphere

Settling of old lithosphere fragments

Hot Spots

Surrounding rock: +100^oC; Plume center: + 3000^oC

Life: est. 100 my

Location: anywhere

Documented: approx. 122

Concentration beneath Africa

Superplume branches?

Hawaii

Yellowstone caldera

Plate Boundaries: From Pangaea to Now

270-250 mya: not much change

~180 mya: Laurasia/Gondwana

India still attached to Gondwana

~130 mya: North America split from Laurasia

Atlantic opening

~70 mya:India on the move across Tethys Sea

Mediterranean wide open to east

Italy moving north

~30 mya: India not quite ‘docked’

Mediterranean open east/west

Panama open thus no ‘Gulf Stream’

Italy ‘docked’ thus Alps grow

Plate Boundary Types

Divergent

70,000 km length globally

Ridges approx. 3000-4000 km wide

Shallow, small earthquakes

Vertical faulting

Mid-Atlantic spreading center: westward ho! Hot spot left behind?

Examples

Red Sea

East Africa Rift Valley: aulacogen

Heavy concentration of population in rift valley

Iceland

Two hot spots

Convergent

Oceanic-Continental boundary

“Benioff zone” = seismic zone

Dips: 30-90 degrees

Brittle region to ~ 10-20 km depth

Continental-Continental

Greatest earthquakes on Earth

Since India docked, pushed another 2000 km north; still 5 cm/y

Transform

Conservation of lithosphere

Oceanic: Commonly leaky (volcanic)

Continental: Tend to ‘lock’, thus huge quakes upon release

San Andreas, California

Anatolian, Turkey

Next

Earthquakes

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