The Earth's Inner Core

ALL seismic work indicates that at least part of the core of the earth is liquid, since no transverse waves have ever been observed to pass through it. It has been suggested, however, that the core contains an inner core beginning at a depth of approximately 5,000 km. which is solid, although no definite proof of this has ever been given. Bullen¹, however, has shown that the rise in velocity of longitudinal waves at this depth can be explained by assuming it to be solid and of the same composition as the rest of the core. In a recent and detailed analysis of the constitution of the earth's interior, Birch² comes to the conclusion that the inner core is most probably crystalline iron, and the outer part liquid iron, perhaps alloyed with a small fraction of lighter elements.