<P> The Earth's inner core is the Earth's innermost part . It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 70% of the Moon's radius . It is composed of an iron--nickel alloy and some light elements . The temperature at the inner core boundary is approximately 5700 K (5400 ° C). </P> <P> The Earth was discovered to have a solid inner core distinct from its liquid outer core in 1936, by the Danish seismologist Inge Lehmann, who deduced its presence by studying seismograms from 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 . A few years later, in 1940, it was hypothesized that this inner core was made of solid iron; its rigidity was confirmed in 1971 . </P>

What is a solid iron layer that makes up a small amount of earth's mass