Plate Tectonics
Alfred 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 also
wanders around.
Rocks and magnetism
Fe-rich minerals record
the direction of ‘north’ as
well as their latitude as
they crystallize.
Basalt, the kind of rock that
makes up oceanic crust, has
Fe-rich minerals.
Magnetic Reversals
The Earth’s magnetic field
reverses every so often. When
this happens, magnetic north
becomes located near the South
Pole.
Magnetometer readings show that basaltic rocks have the same readings on either side
of the MOR (mid-oceanic ridge).
Magma generation at MOR
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Basalts are youngest along the MOR, where they are generated, then get
progressively 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 42,000
miles long
Triple Junction
Initial formation of a divergent plate boundary takes the form of several
triple 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 convergent plate
boundaries:
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 are
always found far inland
of the trench.
Continental-Continental Convergence
Ocean-bottom sediments
forced upward.
Collision happens after oceanic crust is completely
consumed during continental-oceanic subduction.
Continental crust is not dense enough to be forced
down into the mantle.
Collision
Himalaya
India-Asia Collision
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Trenches
Trenches: Where oceanic crust is subducting
Transform Plate Boundary
San Andreas Fault
Segments
Each 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 fixed
Plate moves over hot spot
Hawaiian Hot Spot
Hot Spots
Earth