A. Construction of the Earth..
  Core, Mantle, Crust

=> lithosphere = rigid
=> asthenosphere = plastic
- rigid and less dense lithosphere  floats and glides across plastic and more dense asthenosphere

2. Mechanism for movement.....

=> mantle convection

=> differential crustal density: Push & Pull
 => oceanic crust

3. Formation of the Oceans........

a. sea floor spreading = plates move away from each other…
=> new oceanic crust forms here = mid-ocean ridges
=> rate of spreading = 2 - 8 cm/yr

Why doesn’t the Earth expand?

b. subduction => oceanic crust destroyed

=> increased P & T + H2O => causes melting

=> occurs at trenches

Plate Boundaries

1. Divergent Boundaries => tensional forces

a. ocean-ocean divergence => mid-ocean ridges form....

=> ex.: Mid-Atlantic Ridge and Iceland

b. continent-continent divergence
=> continental rifting -> most oceans began this way - proto MOR’s
  => ex.: Arabian Shield....…

*tectonism at divergent boundaries = volcanoes, EQ’s, new crust.

2. Transform Boundaries -> shear forces....

=> plates slide past one another     => transform faults
 - form perpendicular to MOR’s
=> ex.: San Andreas Fault

**tectonism at transform boundaries = EQ’s
3. Convergent Boundaries =  compressional forces....
 
 a. ocean - continent convergent boundaries (subduction zones)
 
=> Andean Arcs
 
=> subduction of denser crust at trench

=> causes melting and volcanism

=> ex.: Cascades and Andes
 
b. ocean - ocean convergent boundaries (subduction zones)

- Island Arcs

Subduction of more dense (older) oceanic crust

Ex: Japan

c. continent -continent => continental crust clogs a subduction zone

=> continental collision

=> ex.: Himalayas, Alps, Appalachians

** tectonism at convergent boundaries = volcanoes, EQ’s, mountain building, crust destroyed