Earth
The Earth is the only known planet on the solar system where life exists in full form. Most of its area is covered by water. Learn more about the earth layers, crust and other information.
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Deserts
The ocean currents of Atlantic
Continents
Most Viewed in Geography
Conservation
Highest range of mountains in the world
Earth Erosion – causes and prevention
Quakes which strike the Earth's core are sharply deflected because of the sudden change in density of the material through which they travel. Those that just miss the core travel on as normal. This produces a large gap in the waves reaching the surface (a shadow zone) and has helped to prove the existence of a central core of very dense material.
The distribution of the main earthquake regions corresponds closely to that of volcanic regions. Both are associated with recent mountain-building.
Earthquakes
The Earth's crust is subject to large-scale movements, called Earth movements, which cause faulting and folding of the crustal rocks. In many areas of the world these movements are associated with a mountain building process. The vibrations caused by the fracturing and movement of the crustal rocks are transmitted through the Earth, and are called earthquakes. Some earthquakes are caused by violent occurrences such is volcanic explosions and major landslides.
The majority of earthquakes originate from movements within the upper 50 miles of the Earth's crust. The point of origin is called the focus, from which the vibrations radiate in all directions. The speed at which they travel depends upon the density of the rocks through which they pass. It is greatest in dense rocks such as granite and least in loose sands and gravels.
The point on the Earth's surface directly above the focus is called the epicenter. This is the first point to be affected by an earthquake, and suffers the greatest damage. Away from the epicenter the vibrations are less strong.
The study of earthquakes is called seismology. A simple scale is often used to estimate the intensity of an earthquake - the Mercalli Scale. At the lower end of the scale are the very feeble tremors that can be detected only by delicate instruments. At the other end are the catastrophic quakes which produce yawning crevasses in the ground and destroy everything within a large radius of the epicenter.
There are recording stations all over the world where all seismic activity, from major earthquakes to the faintest earth tremor, is continuously recorded. They use sensitive measuring instruments called seismographs.
Although earthquakes can occur anywhere, more than 80 per cent of those experienced in modern times have occurred in two major belts, both associated with mountain-formation. These are: the belt around the Pacific, running northwards from Chile to Alaska, across to Japan and south to New Guinea; and the belt extending from Spain and northern Africa, through the Middle East to the East Indies.
