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Inge Lehmann: the Danish scientist who discovered Earth has a solid inner core

Published: 29th May, 2020 at 00:00
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Between 1929 and 1939, seismologist Inge Lehmann compared various data sets from earthquakes to conclusively prove what was at the Earth's core.

On 17 June 1929, at around 10:17 local time, a 7.3-magnitude earthquake struck the South Island of New Zealand. Waves from the quake were recorded on seismometers around the world, notably in Frankfurt, Copenhagen, Baku, Sverdlovsk and Irkutsk.

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These devices consisted of a heavy weight, suspended from a frame.

When the Earth and the frame vibrated, the inertia of the weight prevented it from moving with them, creating a difference in motion that could be captured by a pen on a rolling sheet of paper.

The first accurate seismometers responded to up and down movements in a horizontal arm, but shortly before the New Zealand earthquake, a new kind of seismometer using a vertically suspended weight came into play, and these proved crucial in the discovery.

Danish seismologist Inge Lehmann, shown in 1932 © The Royal Library, National Library of Denmark, and University of Copenhagen University Library
Danish seismologist Inge Lehmann, shown in 1932 © The Royal Library, National Library of Denmark, and University of Copenhagen University Library

Danish seismologist Inge Lehmann had been working for a couple of years comparing the output of seismic stations.

Initially working with published data, and then going to the original records as “published readings were not always satisfactory”, Lehmann discovered oddities in the wave patterns.

She realised that seismic waves arriving between around 104° and 140° from the epicentre had interacted with a solid inner core, disproving the previously accepted belief that the Earth’s core was entirely liquid.

Her discovery was celebrated with a Google Doodle on her 127th birthday.

Read more about the Earth's core:

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Authors

Brian is a writer of popular science books, with a background in experimental physics. The topics he writes on range from infinity to how to build a time machine. He has also written regular columns, features and reviews for numerous magazines and newspapers, and given lectures at the Royal Institution in London, Oxford and Cambridge Universities, and Cheltenham Festival of Science.

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