Apr. 5, 1999 A seismologist at Washington University in St. Louis has provided an unprecedented view of the Earth's blazing core-mantle boundary through analysis of seismic waves from a unique array of eastern U.S. seismometers.
Michael E. Wysession, Ph.D., associate professor of earth and planetary sciences at Washington University, finds that the bottom of the mantle contains two types of rocks that are distinctly separated, much like the continental and oceanic crust at the Earth's surface, Wysession is part of a seismological team that installed the Missouri-to-Massachusetts (MOMA) network of 18 sophisticated seismometers in 1995 and recorded data until 1996. 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 April 2, 1999 issue of Science, Wysession reports that the two distinct types of rock that exist 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 travelling 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 or rock at the Earth's surface. Wysession is the first to reliably use this approach to investigate the core-mantle boundary.
"We observed a very strange behavior," says Wysession. "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. 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 explains 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 birth place for most of the Earth's hotspot 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 says. "As soon as the slab rock heats up, it probably rises, and the chemical layer can be pushed aside a bit, but not off the core." Wysession says that this is very similar to the surface, where mantle convection laterally pushes around the relatively buoyant continents, which are too light to sink.
Wysession and his colleagues at Washington University, Brown University, Northwestern University, and the New Mexico Institute of Mining and Technology, analyzed core-diffracting waves emanating from 50 earthquakes that occurred in Earth's major earthquake belt, stretching from New Zealand to Japan. The waves diffract, or bend, around the core, in much the same way sound waves bend around the corner of a building. As part of their analysis, they attempted to model the P and S wave speeds observed at the core-mantle boundary, and found another fascinating result.
"The only way to match the velocities beneath Alaska is using an anisotropic material, and the best way to make something anisotropic is to shear it," says Wysession.
Anisotropy occurs when the microscopic mineral grains that make up a rock are preferentially aligned in a particular direction. This occurs in the upper mantle for the predominant mineral olivine, which gets aligned in the direction that the rock is convecting, or flowing. The anisotropy is observed as a "splitting" of the S waves, where the S waves oscillating in one direction arrive faster than the S waves oscillating in a perpendicular direction. Wysession and colleagues observed this kind of S wave splitting in their core-diffracted waves beneath Alaska.
"We knew that anisotropy could explain the splitting of the S waves, but it turns out that this also can explain the unusually slow P waves. This means that we are observing the ancient sea floor slabs flowing outward near the top of the core."
This is one of the first times that the actual flow direction of deep rock has been observed, with the shearing associated with this flowing generating seismically observable anisotropy.
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 aspects of Continental Drift were that the continents and oceans were geologically distinct, and that the continents were moving. Wysession feels that we may be on the verge of discovering 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 says. "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.
"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 says.
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