Danish seismologist, Inge Lehmann sends a letter on March 29, 1936, to Harold Jeffreys, a leading authority in geophysics, describing her hypothesis that that Earth’s center consisted of two parts: a solid inner core surrounded by a liquid outer core, separated by what has come to be called the Lehmann Discontinuity.
At the time, geophysicists believed Earth to be made up of a liquid core surrounded by a solid mantle, itself surrounded by a crust, all separated by abrupt density changes in the Earth called “discontinuities.” She had arrived at het hypothesis by analyzing multiple records of earthquake shock wave data results which consisted of P and S waves, one of them the large, June,1929, New Zealand earthquake. She went on to publish a paper in September 1936. succinctly called “P′,” arguing for what now is taken for granted: That buried inside the Earth’s molten layer is a solid core. Lehmann’s hypothesis was confirmed in 1970 when more advanced seismographs detected waves deflecting off this solid core. More
