Plate Tectonics (3.4)

Question 1

What were the implications of the seafloor spreading discovery?

Answer 1

• Subduction recognised in 1967.
• Necessary geometric counterpart to seafloor spreading.
• High number of earthquakes associated with deep trenches at margins of ocean basins.
• Earthquakes form inclined planes descending deep into mantle.
• Recognised as tracer of oceanic layer descending into mantle.

Question 2

What is a hotspot?

Answer 2

Zone of upwelling of heat and thus magma formation in an upwelling ‘mantle plume’. Perforate over-riding ‘plates’

Question 3

What did Tuzo Wilson recognise?

Answer 3

• Regular progression of ages of volcanoes in Hawaiian island chain, getting older northwest. Recognised Pacific ocean floor moving as rigid ‘plate’ over fixed volcanic hotspot.
• Recognised significance of fracture zones that offset mid-ocean ridges = (oceanic) transform faults.

Question 4

What are conservative boundaries?

Answer 4

Crust neither created nor destroyed.
Active only in between mid-ocean ridges.
Sense of motion opposite to offset of ridges.
May have been active throughout entire history of ocean basin.

Question 5

What is a transform fault?

Answer 5

Active in between ridge crests only.

Question 6

What is a fracture zone?

Answer 6

Inactive (inactive trace of fossil transform fault).

Question 7

What are transform faults as plate boundaries?

Answer 7

• Strike-slip transform faults may occur within continents/boundary between ocean and continents.
• Transform faults link mid-ocean ridges and subduction zones = continuous boundary of weakness between rigid blocks.
• Lead to recognition of concept of plate tectonics
• Active mobile belts on surface of Earth not isolated but continuous.

Question 8

What are some basic ‘plate tectonic’ concepts?

Answer 8

• Earth’s lithosphere broken up into several tectonic plates that move relative to each other, deform only at their boundaries.
• Boundaries: convergent, divergent, transform.
• 7 major plates, several minor ones.
• Continent-continent convergent margins = mountain belts e.g. Alps.
• Continent-continent divergent margin = rift valleys e.g. East African Rift.

Question 9

What is the rift to drift transition?

Answer 9

Crust thinned, may eventually split to form new ocean basin. Leaves inactive continental margin.

Question 10

What is the ‘Wilson’ cycle?

Answer 10

• Recognised processes of continental rifting and collision, supercontinent formation and breakup, and opening and closing ocean basins must occur many times during history.
• History recorded in buoyant continents, ocean crust always destroyed in subduction zones.