Plate tectonic theory of crustal evolution

Question 1

Alfred Wegner Tectonic Plate Theory and Continental Drift

Answer 1

• Continents were once joined together and have gradually moved apart = similar fossils and rock type and geologic structure

Question 2

Harry Hess sea floor spreading

Answer 2

• Constructive plate boundary
• New land formed by the separation of North American and Eurasian plate
  = Iron in the new rock either faced North or South - depending on the magnetism period

North = 'normal' magnetism periods
South = Reversed magnetism periods

Question 3

Convectional theory of why plates move

Answer 3

• Hot spots around core = generate thermal convection currents within the asthenosphere
• Magma rises towards crust, spreads cools and sinks
• Continuous
• New crust is formed along constructive boundaries
• Older crust being destroyed at destructive boundaries

Question 4

Alternative theories

Answer 4

• Forces - act on plates = also be generated by boundaries
• Push from ridge, drag plates down at trenches
  or
• Act along the sides of plates at conservative boundaries

Question 5

Ridge push and gravitational sliding

Answer 5

• Constructive margins
• Upwelling of hot magma at ocean ridges
  = Buoyancy effect
  = 2-3km above the ocean floor
• Gravity - ocean plates experience a force that acts away from the ridge

Question 6

Slab pull

Answer 6

• Destructive plate boundaries
• Cool, thick oceanic lithosphere is gravitationally unstable as it is denser than asthenosphere
• Plate descends under continental - gravity generated force pulls the whole oceanic plate down
  = Negative buoyancy = Slab pull

Question 7

Control of plates

Answer 7

• Combination of forces - moves at different rates.