Earth's spinning innards

A recent finding about the earth's core could hold a clue to the forces that shape the planet

Published: Monday 30 September 1996

in spite of tremendous advancements in the technology of seismological tools, the deepest interior of the earth has remained inaccessible to geophysicists. So, when a team of seismologists led by Xiao Dong Song and Paul Richards of the Lamont Doherty Earth Observatory at Columbia University, New York, us , announced that the solid iron core of the earth spins faster than the rest of the planet, gaining a lead of almost a tenth of a rotation in the last three decades, it generated lot of excitement among geophysicists ( Science , Vol 273, No 5274).

Theoreticians had predicted that the innermost core rotates faster than the rest of the planet, but they too are taken aback by the high speed of rotation. This, according to David Stevenson, a planetary physicist at the California Institute of Technology, us , will pose additional constraints while making models of earth's magnetic field, which is generated in the molten-iron outer core. The new rotation rate can lead to the measurement of the strength of the magnetic field inside the core, which till date has been a matter of conjecture.

Dong and Richards set out to measure the rotation speed armed with two well known facts about the earth's interior. Firstly, for several years now seismologists have been showing how the crystalline iron of the inner core has a 'grain' much like a piece of wood. This grain, presumably caused by an alignment of the iron crystals, is revealed by the fact that seismic waves travelling along the grain move a little faster than those travelling across it, parallel to the equatorial plane. Secondly, researchers recently showed that this grain, or anisotropy, is not exactly aligned with the axis of rotation. This means that if the inner core rotates at a faster speed, the alignment of the anisotropy and of the fast-moving seismic waves would change over time.

Dong and Richards compared the time taken by the waves that passed through the inner core and the ones that did not. For example, waves passing just outside the inner core from earthquakes in the south Sandwich Islands, off the southern tip of South America, reached Alaska just as fast in 1967 as they did in 1995. But waves passing through the inner core made the trip 0.3 seconds faster in 1995 than in 1967.

The new finding may go a long way in explaining hitherto unknown facts about the magnetic field of the earth, specially the phenomenon of change in the polarity of geomagnetism once in a very long while. Models based on the new finding predict that the inner core is being dragged eastward ahead of the rest of the solid planet by powerful magnetic jets. As Gary Glatzmeier of the Los Alamos National Laboratory, us , puts it, "...the inner core is a little more complicated than we give it credit for".

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