Based on the Random House Dictionary, © Random House, Inc. 2012.
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| outer layer of the earth, about 22 miles deep under the continents and 6 miles deep under the oceans |
| central portion of the earth believed to be composed mainly of iron and nickel in a molten state |
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astrobleme
(from Greek astron, blema, "star wound"), remains of an ancient meteorite-impact structure on the Earth's surface, generally in the form of a circular scar of crushed and deformed bedrock. Because such telltale features as crater walls, fused silica glass, and meteorite fragments are heavily modified over time by erosion and weathering, the identification of astroblemes is based chiefly on the presence of subsurface shock structures known as shatter cones. These are conically shaped structures that form in the bedrock directly under the point of impact. They radiate in a distinctive pattern from the point of impact and are identifiable even in drill-core samples. The suddenness and intensity of the shattering cannot be produced by other natural means, so it provides a useful criterion for recognizing astroblemes. Using this evidence, the Ashanti Crater in Ghana and the Vredefort Ring structure in South Africa have been identified as probable astroblemes.
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