Earth's innermost core is solid: Scientists

Earth's innermost core is solid: Scientists

SAN FRANCISCO (Reuters): Inside a liquid core, the center of the Earth is solid, scientists said at a meeting of the American Geophysical Union, providing the first direct evidence backing a long-held theory.

The long-sought finding, which had been hinted at but never proven, came from an analysis of seismic waves generated by a very strong earthquake that occurred in June 1996 about 360 miles (600 kms) below Indonesia.

Emile Okal, a professor of geological sciences at Northwestern University and an author of the study, said on Thursday that scientists have known for some time that the Earth was composed of a rocky mantle floating on a liquid core of molten iron.

But geophysicists also speculated that at some greater depth, the pressure would be so high that the iron would become solid, Okal said.

In the 1930s, seismologists found a discontinuity in the velocity of waves that passed through the center of the Earth, suggesting there was some kind of layering. But the problem was that the waves carried the signature of a liquid, not a solid.

Okal said a solid can be distinguished from a liquid in that it can sustain two different kinds of waves, while a liquid can only sustain one. Only the first type of wave, characteristic of liquids, had ever been observed coming from the Earth's core.

But Okal and his colleague in France, Yves Cansi, used a seismic network spread across France to study the Indonesian earthquake and were able for the first time to detect the seismic signature of a solid.

Okal said the finding could be of value in the field of materials sciences because it indicates iron behaves in a unique way under tremendous pressure.

"Understanding how the qualities of materials are affected under extremely high temperatures -- millions of times the atmospheric pressure -- might be applicable for different materials at not-so-heavy pressures," he said.