Natural Disasters II: Earthquakes

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Much of our understanding of earthquakes is based upon a revolutionary theory of earth dynamics, called PLATE TECTONICS.

Earthquakes are one of the most frightening of natural disasters as they occur without warning and can cause enormous damage - an earthquake in China in 1976 is estimated to have resulted in 750,000 deaths. They are caused by the sudden movement of pieces of the earth's crust. When this movement occurs it causes waves of energy to extend outwards (much like ripples in a pond when a pebble is dropped in) that are felt as ground shaking.

Our understanding of earthquakes has grown immensely over the past decades although prediction is still uncertain. Much of our understanding is based upon a revolutionary theory of earth dynamics, called PLATE TECTONICS. This comprehensive theory offers an explanation for both volcanoes and earthquakes.

1. Continental Drift

Have you ever noticed how the continents of the world seem to fit together? Notice how South America can be rotated and made to fit the west coast of Africa. Is this just chance or is it possible that the continents were once joined together to form one 'supercontinent'. In the early part of the century a German geophysicist Alfred Wegener used map, geological and vegetation evidence to propose a theory that there had once been a supercontinent - he called it Pangaea - that broke apart and the pieces 'drifted' into their present position. His theory of Continental Drift was dismissed as wishful thinking as no one could suggest a method by which continents could move through the rocks on the sea-floor.

By the 1960's evidence collected from the ocean floors were revealing an unknown terrain. Instead of the flat, featureless surface they expected to find there were tall mountain chains and deep valleys that were longer than anything found on land. Moreover global maps of volcanoes and earthquakes showed distinctive patterns. These pieces of evidence were used to formulate a new theory about how the earth 'worked'.

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