Friday, December 26, 2008

Where is the Moho - or Mohorovicic discontinuity?

The Mohorovicic Discontinuity is to be found between the Earth's crust and the earth's mantle. Mohorovicic was a famous Croatian scientific, a seismologist. He specialized in the composition of the Earth, and particularly in earthquakes and in faults in the structure of the earth, which are the cause of earthquakes.

Mohorovicic made a most important discovery. Scientists knew that the earth is made up of a series of layers, rather like an onion. There are many of these layers, all of different materials and all in a different state of development. The main layers are called the Crust, the Mantle, the Liquid Core, and the Solid Core.

The Great Pressures inside the earth force the weaker areas of the rocky layers out of alignment and this pushing, twisting movement is experienced as an earthquake on the surface. These great forces set up a series of waves throughout the various layers of the earth, known as seismic waves.

Mohorovicic discovered a curious fact about the behaviour of these earth waves or tremors. He noticed that the shock waves travel comparatively slowly through the actual crust of the earth. But when they reach the lowest level of the crust - the layer called basaltic rock which rests on the next layer called the mantle - the waves increase dramatically in speed. Sometimes they also change direction.

This curious fact was proved by Mohorovicic's experiments. The importance of his discovery was that it proved the Earth's crust is different from the mantle beneath.

So the Mohorovicic discontinuity is the scientific term used to describe this strange behavior of the earth's shock waves.

Usually this level of the Earth's composition is called the Moho, since even scientists found Mohorovicic's name difficult to pronounce. By plotting these earth waves on sensitive shock recording machines known as seismographs we have found that most of the outer crust of the earth is between 20 and 25 miles thick in continental areas, but only three miles thick under some oceans.

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