Seismologists See Earth's Dynamic Interior as Interplay of Temperature, Pressure, Chemistry

Thursday, October 25, 2007

The research, conducted by scientists Nicholas Schmerr and Edward Garnero of Arizona State University in Tempe, is published in the October 26 issue of the journal Science.

This view of Earth's inner workings depicts the inner planet as a living organism where events that happen deep within can affect what happens at its surface, like the rub and slip of tectonic plates and the rumble of volcanoes.

The new research into these inner workings shows that in Earth's upper mantle (an area that extends down to 660 kilometers), more than temperature and pressure play a role.

"It has long been a goal of seismologists to distinguish between thermal and chemical effects on seismic velocities in Earth's deep interior," said Robin Reichlin, program director in the National Science Foundation (NSF)'s division of earth sciences, which funded the research. "This work is a tantalizing step toward solving this important problem, necessary for understanding the internal structure and dynamics of our planet."

The simplest model of the mantle--the layer of the Earth's interior just beneath the crust--is that of a convective heat engine. Like a pot of boiling water, the mantle has parts that are hot and welling up, as in the mid-Atlantic rift, and parts that are cooler and sinking, as in subduction zones. There, crust sinks into the Earth, mixing and transforming into different material "phases," like graphite turning into diamond.

"A great deal of past research on mantle structure has interpreted anomalous seismic observations as due to thermal variations within the mantle," Schmerr said. "We're trying to get people to think about how the interior of the Earth can be not just thermally different but also chemically different."

Schmerr's and Garnero's work shows that Earth's interior possesses an exotic brew of material that goes beyond simply hot and cold convection currents.

To make their observations, Schmerr and Garnero used data from the USArray, which is part of the NSF-funded EarthScope project.

"The USArray is 500 seismometers deployed in a movable grid across the United States," Schmerr said. "It's an unheard of density of seismometers."

Schmerr and Garnero used seismic waves from earthquakes to measure where phase transitions occur in the interior of Earth by looking for where waves reflect off these boundaries.

They studied seismic waves that reflect off the underside of phase transitions halfway between an earthquake and a seismometer. The density and other characteristics of the material they travel through affect how the waves move, and give geologists an idea of the structure of the inner Earth.

Beneath South America, Schmerr's research found the 410-kilometer phase boundary bending the wrong way. The mantle beneath South America is predicted to be relatively cold due to cold and dense former oceanic crust and the underlying tectonic plate sinking into the planet from the subduction zone along the west coast. In such a region, the 410-kilometer boundary would normally be upwarped, but using energy from far away earthquakes that reflect off the deep boundaries in this study area, Schmerr and Garnero found that the boundary significantly deepened.

They postulate that either hydrogen or iron concentrations are responsible for the observed deflection of the 410 discontinuity.

"This study lets us constrain the temperature and composition to a certain degree, imaging this structure inside the Earth and saying: 'These are not just thermal effects -- there's also some sort of chemical aspect to it as well,'" Schmerr said.

Source: NSF