Distribution of Oceans and Continents

Continental drift and its evidence, sea floor spreading, plate tectonics, the three kinds of plate boundary and the journey of the Indian plate.

Key ideas

• Wegener said all continents were one super continent, Pangaea, ringed by the ocean Panthalassa; it began to split about 200 million years ago.
• Evidence: the jig-saw fit of the Atlantic coasts, rocks of the same age across oceans, matching tillite, placer gold in Ghana from Brazil, and fossils like Mesosaurus.
• Wegener’s forces were too weak; Arthur Holmes suggested convection currents in the mantle as the driving force.
• Hess’s sea floor spreading: lava at mid-ocean ridges pushes the floor apart, and old floor sinks at trenches, so ocean crust is young.
• A plate is a rigid slab of lithosphere moving over the asthenosphere; there are seven major plates and several minor ones.
• Divergent boundaries make new crust, convergent boundaries destroy it in subduction zones, and transform boundaries neither make nor destroy it.
• India moved north from near Australia, poured out the Deccan Traps and hit Asia 40–50 million years ago, raising the Himalayas.

Remember

Watch out

• Swapping Pangaea (the land) and Panthalassa (the ocean).
• Getting the unit change wrong: cm a year times a million years is cm, so divide by 100,000 for km.
• Saying crust is made at convergent boundaries (it is destroyed there).

Worked example