Plate TectonicsAlfred Wegener 1915 Continental Drift

Pangaea

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Sonar

Atlantic Ocean

World Ocean Floor

Magnetic Field

Magnetic north is several degrees off geographic north, and it alsowanders around.

Rocks and magnetism

Fe-rich minerals recordthe direction of ‘north’ aswell as their latitude asthey crystallize.

Basalt, the kind of rock thatmakes up oceanic crust, hasFe-rich minerals.

Magnetic Reversals

The Earth’s magnetic fieldreverses every so often. Whenthis happens, magnetic northbecomes located near the SouthPole.

Magnetometer readings show that basaltic rocks have the same readings on either sideof the MOR (mid-oceanic ridge).

Magma generation at MOR

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Basalts are youngest along the MOR, where they are generated, then getprogressively older away from, and on either side of, the MOR.

Seafloor Spreading

Generation of basalt at mid-oceanic ridge

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Tectonic Plates

3 major types

Pillow Basalt

Lava extruded in water takes the form of ‘pillows.’

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Divergent Plate Boundaries

MOR is 42,000miles long

Triple Junction

Initial formation of a divergent plate boundary takes the form of severaltriple junctions.

As spreading continues, one of the rift valleys has to fail: Aulacogen

East African Rift

Aulacogen

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Mature Divergent Boundary

3 types of…

3 types of convergent plateboundaries:

1) Oceanic-Continental

2) Oceanic-Oceanic

3) Continental-Continental

Oceanic-Continental Convergence

Accretionary Wedge

Volcanic arc appears approx. 200 miles inland from the trench.Magma pulses separated by about 400 years.Partial melting creates continental crust (we’ll discuss later).

Oceanic-Oceanic Convergence

Older, and thus colder & denser, crust subducts.Similar to oceanic-continental convergence, but forms

volcanic island arc.

Earthquake Distribution Map

Distribution of earthquakes mirrors the tectonic boundaries.

Benioff Zone

Deeper earthquakes arealways found far inlandof the trench.

Continental-Continental Convergence

Collision happens after oceanic crust is completelyconsumed during continental-oceanic subduction.

Continental crust is not dense enough to be forceddown into the mantle.

Ocean-bottom sedimentsforced upward.

Collision

Himalaya

India-Asia Collision

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Trenches

Trenches: Where oceanic crust is subducting

Transform Plate Boundary

San Andreas Fault

SegmentsEach segment moves at a different rate, some not at all

Earthquake Hazards

Mantle Plumes

Subducting oceanic crust melts at the core-mantle boundary, releasing lots of magma.Magma rises to the surface over a 600,000 year period.

Hawaii

Hawaiian hot spot

Hot spot is fixedPlate moves over hot spot

Hawaiian Hot Spot

Hot Spots

Yellowstone

Yellowstone

Fallout

Earth