 INTERLUDE 7-1 Earth's Rapidly Spinning Core |
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Seismic waves racing out from the site of earthquakes allow geologists to map the interior makeup of planet Earth.  (Sec. 7.3) Recent analyses of old earthquake data suggest that Earth's inner core is spinning slightly faster than the rest of our planet. The whole planet rotates in the same direction (west to east), but the inner core takes about 2/3 of a second less to complete its daily rotation than the bulk of the matter surrounding it—which means that it gains a quarter turn each century.
Geologists arrived at this rather surprising result by carefully timing seismic P-waves as they moved through Earth's interior. Actually, the scientists used historical data acquired over the past few decades, mainly from 38 earthquakes that occurred in the South Atlantic Ocean and whose seismic vibrations had to pierce the inner core on their way to monitoring stations in Alaska. As discussed in the text, only P-waves can make it through the liquid outer core, to be detected on the opposite side of Earth. For many years, geologists have known that P-waves travel through the inner core about 3 or 4 percent faster along north—south pathways than along those close to the plane of the equator. This difference probably results from the orientation of the iron atoms in the solid inner core; the atoms there must be aligned, as in a crystal lattice. Where the atomic alignment parallels the motion of the P-waves, those P-waves travel just a little faster—a little like going "with the grain" rather than against it. |
The new finding is that the route of the fastest seismic waves through Earth's deep interior is gradually shifting eastward, implying that the inner core has some motion slightly different from the rest of the planet. In the accompanying figure, the red region is the solid inner core, the orange region the liquid outer core. The dashed lines passing through the core depict the paths of the fastest seismic waves from 1900 to 1996. The data indicate that the axis of the "fast track" for the P-waves is shifting by about 1° per year relative to the crust above. That axis now intersects Earth's surface in the Arctic Ocean northeast of Siberia; some 30 years ago it was 33° farther west. Gradual as it may be, this shift amounts to about 20 km at the edge of the inner core—around a million times faster than the typical rates (centimeters per year) at which the continents creep across Earth's surface.
These discoveries may help us understand how the inner core is continuing to grow by condensing from the molten outer core as Earth cools, and how these two regions interact to produce Earth's magnetic field. As noted elsewhere in this chapter, the dynamo theory of magnetism requires rotating, electrically conducting matter within Earth's interior. |
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