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Oldham, Richard Dixon

waves core earth’s crust

(1858–1936) British seismologist and geologist: first observed P and S waves and discovered the Earth’s core.

Oldham was educated at the Royal School of Mines, in 1879 joining the Geological Survey of India (of which his father was director). Upon retirement in 1903 he became director of the Indian Museum in Calcutta. Following the violent Assam earthquake of 1897, Oldham was able clearly to distinguish in the seismograph record for the first time between the P( primary, or compressional) and S( secondary, or shear) waves (predicted theoretically ) and the tertiary (surface) waves. In 1906 he discovered that at points on the Earth’s surface opposite to the epicentre of an earthquake the P waves arrive later than expected, when compared with their arrival times at other places on the globe. He correctly recognized this as clear evidence for the existence of a relatively dense Earth’s core, through which P waves travel more slowly than in the mantle.

In 1913 showed that the core is at least partly liquid; and in 1936 deduced that within it there is a solid inner core. Earlier than this, had shown that the Earth’s outer crust must be less dense than the mantle, and had calculated the thickness of the crust. All of these geophysical studies depended on seismological results.

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