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Earth's mantle and core.

The waves set up by earth-quakes apparently did not reach all parts of the Earth's surface even when they were strong enough to be detected everywhere.

There was a shadow zone within which no waves were detectable.

The German-born American geologist Beno Gutenberg (1889-1960), studying this phenomenon, suggested in 1914 that there was a core at the center of the Earth, about 2100 miles in radius, that was markedly different in density and chemical composition from the material outside that radius. Earthquake waves entering the core would be refracted in such a way as to be sent beyond the shadow zone.

Based on the fact that transverse waves did not enter the core at all, Gutenberg suggested that the core was liquid.

Thus the Earth is dividend into two parts, an inner core, which (from its high density, and the fact that iron meteorites are common) seems likely to consist of liquid nickel-iron in a 1-to-9 ratio, and an outer mantle made up of rocky material. These two are in about the proportion of yolk to white in an egg, and the outermost crust is about the proportion of the eggshell.

The sharp dividing line between the mantle and the core is called the Gutenberg discontinuity.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
Words:208
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