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Mysteries of the Inner Earth

March 1997

Every day, the world turns, bathing the continents in light and then darkness. But just as your own world may feel more complicated than an easy day-night rotation, researchers recently discovered that even for Earth, nothing is that simple.

Deep within our planet, the Earth's inner core is also spinning -- only it's moving faster than the surface. Every 400 years or so, it will overtake those of us riding on the outside.

This startling discovery was made when two NSF-funded seismologists working at the Lamont-Doherty Earth Observatory at Columbia University took on the challenge of investigating an unproven theory. The theory states that the inner core rotates separately from the rest of the planet, as predicted by an unproven model of the Earth's magnetic field.

Researchers Xiaodong Song and Paul Richards used seismic wave readings from 38 earthquakes between 1967 and 1995. They tracked waves that moved from the south Atlantic, through the inner core, to College, Alaska. They found that the waves in the 1990s were 0.3 seconds faster than those in the 1960s.

In human terms, this change may seem negligible, but by geological standards the inner core is a fast mover. Every year it rotates one longitudinal degree more than the rest of Earth.

Earth's inner core is made of crystalline iron and is a main player in Earth's magnetic field. Song expects the discovery will help explain how the magnetic field works. In addition, the data will be helpful in explaining the planet's temperature budget -- that is, how Earth gains and loses heat.

Working concurrently, and writing in the same issue of the journal Nature, a recent recipient of the Presidential Early Career Award for Scientists and Engineers, Michael Wysession, developed a new way of looking at the underworld. He created a map of Earth's core-mantle boundary.

The core-mantle boundary is the place where the outer core meets the lower mantle. It's a place of transition, where the solid rock of the lower mantle sits atop the liquid iron of the outer core. And where continent-sized variations, reminiscent of the surface continents, stretch across the boundary.

"We're at the really early stages of mapping," says NSF-funded Wysession of Washington University in St. Louis. "Think of the early maps of the New World. The continents all look funny, the shapes and details are wrong, but their general outlines are correct."

Wysession analyzed eight years worth of earthquake data, focusing on diffracted P-waves, seismic waves that travel around the core-mantle boundary.

Besides providing information about the structure of the boundary, Wysession says his research helps prove that the area is a graveyard for the sea-floors of ancient oceans.

When the super-continent Pangaea began to break up 200 million years ago -- leading to the present-day continents -- there began to be a pulse of increased plate subduction. This is the process where plates containing oceanic crust sink beneath the edges of continents.

"That rock had to go someplace, and we see now where it went -- all the way to top of the core," Wysession says. "The map shows us the other half of plate-tectonics: subducting plates can sink all the way to the core-mantle boundary that is Earth's great landfill."


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