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Inner core
The Earth's inner core is the Earth's innermost part and according to seismological studies, it is primarily a solid ball with a radius of about 1220 kilometers, or 760 miles (about 70% of the Moon's radius). It is believed to consist primarily of an iron–nickel alloy and to be approximately the same temperature as the surface of the Sun: approximately 5700 K (5400 °C).〔
==Discovery==
The Earth was discovered to have a solid inner core distinct from its liquid outer core in 1936, by the seismologist Inge Lehmann, who deduced its presence by studying seismographs of earthquakes in New Zealand; she observed that the seismic waves reflect off the boundary of the inner core and can be detected by sensitive seismographs on the Earth's surface. This boundary is known as the Bullen discontinuity, or sometimes as the Lehmann discontinuity.〔Although another discontinuity is named after Lehmann, this usage still can be found: see for example: ,and (''From here to "hell ," or the D'' layer'' ), About.com〕 A few years later, in 1940, it was hypothesized that this inner core was made of solid iron; its rigidity was confirmed in 1971.
The outer core was determined to be liquid from observations showing that compressional waves pass through it, but elastic shear waves do not – or do so only very weakly. The solidity of the inner core had been difficult to establish because the elastic shear waves that are expected to pass through a solid mass are very weak and difficult for seismographs on the Earth's surface to detect, since they become so attenuated on their way from the inner core to the surface by their passage through the liquid outer core. Dziewonski and Gilbert established that measurements of normal modes of vibration of Earth caused by large earthquakes were consistent with a liquid outer core. Recent claims that shear waves have been detected passing through the inner core were initially controversial, but are now gaining acceptance.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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