Plate Tectonics

Summer and Will

How did the theory come about?The theory of plate tectonics and continental drift

were proposed at the beginning of the last century by scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed.

It took more than 50 years for Wegener’s theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully.

Evidence for Plate Tectonics

So what was the evidence for Wegener's theory?Plate tectonics explained why earthquakes and volcanoes

were concentrated in specific places - around the boundaries of moving plates.

The match in shape between the east coast of South America and the west coast of Africa suggests both were once part of a single continent. There are similar patterns of rocks and similar fossils on both sides of the Atlantic - including the fossil remains of land animals that would have been unable to swim across an ocean.

• The Earth's crust and upper part of the mantle are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

• The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

• Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.