March 23, 2012
Scientists Study Deep Rocks Under Pacific Ocean
Scientists interested in the construction of the rock layers immediately under the Earth´s crust, the lithosphere and asthenosphere, have new tools to help analyze these layers and further understand plate tectonics.
The researchers have been using seismic waves to study the lithosphere-asthenosphere boundary, or LAB. This boundary is where the hot, convecting mantle asthenosphere and the overlying cold and rigid lithosphere meet.
It has been found that seismic waves move faster through the lithosphere and decrease through the asthenosphere. Some areas indicate a 5 to 10 percent decrease in wave speed between 35 and 120 km depth. This is where scientists think the Gutenberg discontinuity connects.
Using older technology, scientists had difficulty observing seismic discontinuities shallower than 100 km. Regions below the ocean floor were only accessible via ocean bottom seismometers or where seismic stations were installed on ocean islands.
But a new technique, reports a recent press release, incorporates seismic waves that sample beneath remote regions of the Earth at higher frequencies, and new signal processing techniques enables Nick Schmerr, a Carnegie and NASA Goddard scientist, to hone in on the Gutenberg discontinuity.
Schmerr discovered that the seismic discontinuity is not a Pacific-wide feature, but is detected only where there are areas of recent surface volcanism. And he found the Gutenberg shows deeper under older rocks showing it is indeed part of the LAB.
According to Schmerr, “The really interesting part of this work is that is confirms partially molten rock is not present throughout in the asthenosphere of the Earth. This significantly narrows the range of possible mechanisms that give rise to the low viscosity of the asthenosphere. This means plate tectonics are enabled on Earth because of mantle composition and or grain size, not necessarily the presence of melt.”
Schmerr´s work is published in the journal Science.
Other seismic surveys are being conducted in the Pacific ocean. Scientists are beginning to study the subsurface water cycle which they believe is just as important as the surface water cycle.
They believe that water is carried deep into the Earth by hydration reactions along the subducting plate. Dehydration deep in the crust releases the fluids into the overlying mantle promoting melting and volcanism.
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These seismic surveys are being conducted over the Mariana Trench. Doug Wiens a Washington University St. Louis professor of earth and planetary science says, “We think that much of the water that goes down at the Mariana trench actually comes back out of the Earth into the atmosphere as water vapor when the volcanoes erupt hundreds of miles away.
Scientists believe that studying the subsurface water cycle within subduction zones, like the Mariana trench, will allow them to better understand island-arc volcanism and subduction-zone earthquakes.