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A seismologist at Washington University has provided an unprecedented view of Earth's blazing core-mantle boundary through analysis of seismic waves from a unique array of seismometers in the eastern United States.
Michael E. Wysession, Ph.D., associate professor of earth and planetary sciences in Arts and Sciences, has found that the bottom of the mantle contains two types of rocks that are distinctly separated, much like the continental and oceanic crust at Earth's surface.
Wysession and colleagues from Brown and Northwestern universities and the New Mexico Institute of Mining and Technology made up a seismological team that installed the Missouri-to-Massachusetts (MOMA) network of 18 sophisticated seismometers in 1995 and recorded data until 1996. The National Science Foundation (NSF) funded the network.
MOMA is the first network of seismometers ever deployed across the eastern United States, and it is the first seismographic array used primarily to study the core-mantle boundary, the geologically fascinating division between the rocky mantle and liquid iron outer core that is 2,000 miles beneath our feet.
Writing in the journal Science, Wysession reported that the two distinct types of rock at the base of the mantle are cold slabs of recycled oceanic floor that are spreading horizontally at the core-mantle boundary and a dense layer of mantle dregs that gets pushed around by these descending slabs.
His conclusions are based on the ratio of the two different kinds of seismic waves that emanate from an earthquake, P waves and S waves. Seismic P waves travel in a domino effect, with each bit of rock pushing the next one, right across the Earth. S or shear waves have lateral movements, the way a sideways twist will send a wave traveling down the length of a rope.
The speeds of P and S waves change in different ways as they travel though different materials, and their ratios have previously been used to map out different types of rock at Earth's surface. Wysession is the first to use this approach reliably to investigate the core-mantle boundary.
"We observed a very strange behavior," Wysession said. "The P and S waves usually vary in tandem, especially if variations are due to changes in temperature. We know that slabs of ancient sea floor sink to the base of the mantle, and we expected to see a gradual change as the slabs spread across the top of the core and heat up. Instead, we saw a very sudden change. Two thousand miles beneath Alaska, the S waves travel fast and the P waves are slow. Then as you travel south, they suddenly switch: The P waves are fast, and the S waves are slow. It is like standing on a shoreline with the continent on one side and the ocean on the other."
Wysession explained that the rock at the base of the mantle beneath Alaska used to be part of the Pacific Ocean sea floor, but sank into the mantle more than one hundred million years ago, descending all the way to the top of the core. As this cold rock reaches the bottom of the mantle, it pushes aside what is known as a chemical boundary layer into two large lumps, one beneath the central Pacific and one beneath western Africa, that serve as the birthplace for most of the Earth's hot spot plumes.
"The division between the ancient slab and the chemical boundary layer is quite distinct, meaning that the slabs don't spend much time at the top of the core," Wysession said. "As soon as the slab rock heats up, it probably rises, and the chemical layer can be pushed aside a bit again, but not off the core."
Wysession said that this is very similar to the surface, where mantle convection laterally pushes around the relatively buoyant continents, which are too light to sink.
The discovery of two distinct rock types at the base of the mantle and the evidence that they are moving laterally have historical significance as well. Plate tectonics, which describes how Earth's surface has evolved, got its start 30 years ago from the older theory of continental drift. Two of the most important continental drift features were the geological distinctions between the continents and oceans and the movement of the continents. Wysession believes that we may be on the verge of similar discoveries about how the deep Earth has evolved.
"The other half of plate tectonics is going to be a distinct sort of mantle dynamics, different from the surface," he said. "We are just now piecing together the evidence that will give a full theory for how our planet works."
Wysession is continuing to analyze the MOMA data and expects to find further clues to the function of the core-mantle boundary as the repository of ancient sea floor slabs and source of hot spot plumes. He has just received NSF funding to launch a similar study in 2001 that will analyze seismic data from an array of seismometers stretching from Florida to Edmonton, Alberta.
"We're getting much better glimpses of processes that shape the deep Earth and also an understanding of the circulation of rock from the surface to the core and back up again and how that shapes the evolution of our continents, " Wysession said.
