The Theory of Continental Drift (Now Plate Tectonics)
Was devised by Alfred Wegner in 1912
The theory states that the earth is made up of about 20 giant rock plates about 80km’s thick
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These plates float on the earth’s semi-molten asthenosphere (the top layer of the earth’s mantle) and are driven by convection (heat) cycles in the mantle.
Ocean plates move about 10 cm/year
Continental plates about 2 cm/year
It is believed that the plate movements have carried continents across the earth.
It is believed that about 250 million years ago there was one large continent called Pangea.
Gradually due to tectonic forces, Pangea broke into northern Laurasia and southern Gondwanaland.
It is believed that as the plates continue to move, in about 50 million years Africa will break apart and some of California will break apart and drift north.
Wegner’s Evidence to Support Continental Drift
1) Jigsaw fit - Many continents appear to fit together like a puzzle.
2) Fossils - He found similar fossils on different continents which are currently far apart. (Africa & S. America)
3) Geological Evidence – rock types and mountain ranges are similar on
different continents which are far apart.
(North America & Europe)
4) Ice Sheets – Ice covered Africa, India, Australia and S. America about 250 million years ago. Only possible if they
moved.
Additional Evidence to Support Continental Drift
1) Plate Movements – plate movements along plate boundaries
(Earthquakes, Volcanoes)