what is the composition of Earth’s inner core?
This became the problem to solve and ushered in the mistake which would confuse geophysics for more than half a century. Figure 2. Inge Lehmann’s original diagram of the discovery of Earth’s inner core. The added red arrow points to the ray reflected into the shadow zone, which is the region between her numbers 2 and 3. Francis Birch thought that nickel and iron were always alloyed together in meteorites. He also knew that all elements heavier than nickel and iron, even if taken together, were insufficiently abundant to comprise a mass as great as the inner core. So, Birch made the assumption that the inner core is nickel-containing iron metal which has started to freeze from the liquid iron alloy core, although there is no reason from that standpoint for the inner core to be the size observed; it might be any size. Beginning in 1940, the idea of the Earth’s inner core being partially crystallized iron metal became firmly entrenched and much of geophysics was subsequently built around
