Friday, March 02, 2007

Underground Ocean Discovered Under Asia

Two seismologists have announced the discovery of an underground ocean beneath Asia. Live Science excerpt:

Scientists scanning the deep interior of Earth have found evidence of a vast water reservoir beneath eastern Asia that is at least the volume of the Arctic Ocean. The discovery marks the first time such a large body of water been has found in the planet’s deep mantle.

The finding, made by Michael Wysession, a seismologist at Washington State University in St. Louis, and his former graduate student Jesse Lawrence, now at the University of California, San Diego, will be detailed in a forthcoming monograph to be published by the American Geophysical Union.

The pair analyzed more than 600,000 seismograms—records of waves generated by earthquakes
traveling through the Earth—collected from instruments scattered around the planet. They noticed a region beneath Asia where seismic waves appeared to dampen, or “attenuate,” and also slow down slightly. “Water slows the speed of waves a little,” Wysession explained. “Lots of damping and a little slowing match the predictions for water very well.”

Previous predictions calculated that if a cold slab of the ocean
floor were to sink thousands of miles into the Earth’s mantle, the hot temperatures would cause water stored inside the rock to evaporate out. “That is exactly what we show here,” Wysession said. “Water inside the rock goes down with the sinking slab and it’s quite cold, but it heats up the deeper it goes, and the rock eventually becomes unstable and loses its water.”

The water then rises up into the overlying region, which becomes saturated with water
. “It would still look like solid rock to you,” Wysession told LiveScience. “You would have to put it in the lab to find the water in it.” Although they appear solid, the composition of some ocean floor rocks is up to 15 percent water. “The water molecules are actually stuck in the mineral structure of the rock,” Wysession explained. “As you heat this up, it eventually dehydrates. It’s like taking clay and firing it to get all the water out.”

The researchers estimate that up to 0.1 percent of the rock sinking down into the Earth’s mantle in that part of the world is water, which works out to about an Arctic Ocean’s worth of water. Wysession has dubbed the new underground feature the “Beijing anomaly,” because seismic wave attenuation was found to be highest beneath the Chinese capital city. Wysession first used the moniker during a presentation of his work at the University of Beijing.

The nature of plate tectonics points to something like this occurring. With the amount of movement the continental shelves experience, it makes sense that some of the water from our oceans would seep into the inner layers of the planet. I suppose a logical question to ask would be, are there other underground bodies of water underneath the other continents, or even under the ocean floors? Another question would be, how long has this underground ocean existed? That would seem to be harder to pin down, due to the constant movement of the plates, combined with the evaporation detailed in the excerpt. It now seems that we can begin to find out, thanks to Wysession's and Lawrence's work.

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