| 单词 | earth |
| 释义 | earth The planet that orbits the sun between the planets Venus and Mars at a mean distance from the sun of 149 600 000 km. It has a mass of about 5.976 × 1024 kg and an equatorial diameter of 12 756 km. The earth consists of three layers: the gaseous atmosphere (see earth's atmosphere), the liquid hydrosphere, and the solid lithosphere. The solid part of the earth also consists of three layers: the crust with a mean thickness of about 32 km under the land and 10 km under the seas; the mantle , which extends some 2900 km below the crust; and the core , part of which is believed to be liquid. The crust has a relative density of about 3 and consists largely of sedimentary rocks overlaying igneous rocks. The composition of the crust is: oxygen 47%, silicon 28%, aluminium 8%, iron 4.5%, calcium 3.5%, sodium and potassium 2.5% each, and magnesium 2.2%. Hydrogen, carbon, phosphorus, and sulphur are all present to an extent of less than 1%. The mantle reaches a relative density of about 5.5 at its maximum depth and is believed to consist mainly of silicate rocks. The core is believed to have a maximum relative density of 13 and a maximum temperature of 6400 K. See also geomagnetism |
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