Molten rock under the American Southwest

New measurement method reveals existence of melt layer at 410 kilometers depth

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Scientists have discovered a kilometer-thick layer of molten rock just 410 kilometers below the surface of the earth beneath the southwestern United States. As reported in the journal Nature, this layer could explain a previously recorded seismic anomaly at that depth and possibly confirm a hypothesis in 2003 on rising cladding rocks in the subsurface.

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The interior of the earth is not uniform, but consists of rocks of varying density and composition. The reflections and refractions of earthquake waves at different depths prove this, among other things. One of these seismic border zones lies at a depth of 410 kilometers. So far, however, was not exactly known why right there. As early as 2003, however, geoscientists at the American Yale University had published a hypothesis on the composition and state of the rocks in the mantle, in which this depth played an important role: they assumed that ascending mantle rocks would lose and melt the water bound in its crystal structure,

But there was no evidence for this hypothesis. "The idea is a kilometer-thick layer of molten rock just 410 kilometers below the surface of the earth, scientists have discovered under the southwestern United States. Interesting and controversial among geophysicists, " said James Tyburczy of Arizona State University. "That's why Dan and I thought we'd give it a try." But it was not that easy, as previous methods provided some information on density and, above all, differences in density, but did not provide more accurate data on the composition of the rock in question.

Conductivity measurement with "cosmic" help

A classical geophysical sampling technique, the magnetotelluric, can measure changes in the conductivity of rocks even at depths that can not be achieved by direct sampling. "Rocks are semiconductors and rocks that contain more hydrogen in their crystal structure, " explains Tyburczy. "They conduct better than rocks that are partially melted." The main source of stored hydrogen is mostly the water that is bound in the crystals. But this method has its limits, because for several hundred kilometers of depth it is not designed. display

Tyburczy and his colleague Daniel Toffelmier therefore fell for another idea: They got "cosmic" help - from the sun. From it emanates a continuous, but in strength and intensity varying stream of charged particles, the solar wind. It influences the magnetic field of the earth and this in turn generates weak but measurable electrical currents in the underground. Long-term measurements of these variable currents allow conclusions about the composition of the rocks - similar to the magnetotelluric, but with a greater range in terms of depth.

Field data indicate melt layer

Specifically, the researchers started with a model of rock composition at different depths and compared the calculated data with the electromagnetic measurement data. They corrected the seed parameters of their model until the values ​​matched. It showed that the data for the southwestern United States fit only if the model considered the existence of a melt layer at a depth of 410 kilometers.

"Without such a melting zone at this depth, we can not explain the field observations, " says Toffelmier. "But when we inserted a 5-30 kilometer thick conductive melting zone, we got a much better match." The horizontal extent of this layer, however, is unknown so far, as the data does not disclose it. Although the seismic measurements indicate that the 410-kilometer fault occurs globally, the two researchers do not believe that the melt layer is also permeable. They think they are rather spotty. Therefore, the hypothesis of the Yale researchers can only partially confirm the two scientists. "Our model has only one dimension; we now have to start looking in two and three dimensions, " explains Toffelmier. We just saw the tip of the iceberg.

(Arizona State University, 22.06.2007 - NPO)