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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.
It is difficult to predict exactly when an earthquake
might happen and how bad it will be, even in places
known for having earthquakes.
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Alfred Wegener (1880 - 1930)
The theory of plate tectonics and continental drift
were proposed at the beginning of the last century by a
German 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