Friday, April 22, 2005
If rocks could talk...
It's also a preview of how the latest 3D seismic tomographic imaging will very soon be revealing hidden structures in the ground under the whole of continental America, says Yellowstone researcher Professor Bob Smith of the University of Utah. Smith made the announcement recently in the keynote address at the first national meeting of the EarthScope program. "I call this the ancestral Earthscope experiment," says Smith, of the extensive Yellowstone seismic and global positioning system (GPS) network.
Using Yellowstone's unusually dense network of seismic stations in and around the national park, Smith and other researchers of the Yellowstone Volcano Observatory have gathered data of how seismic waves from earthquakes move through the ground to create a 3D image of the structures deep under Yellowstone.
They found a giant column, where seismic waves are slowed by extra hot and partially melted rocks, which extends down about 660 kilometres then stops. The column is tilted to the northwest at depth, another strange feature, Smith says.What's behind the hot rocks?Among the possible explanations for the tilted column of hot rock is that it is a plume of material that started way down at the edge of the Earth's core, buoyed upwards and then got caught up in the conveyer-belt-like flow in the Earth's mantle, the region beneath the crust.
As the plume got stretched to the east by the easterly mantle flow, it was stretched so thin that it broke apart or was "beheaded", Smith explains. What is seen under Yellowstone is just the head of the plume, with the rest of it detached and still down in the mantle somewhere to the west or northwest. "This is what really drives the Yellowstone system today," says Smith of the much smaller, but still huge, magma body that lies directly atop the plume and directly under Yellowstone's geysers.
More 3D views like this into the interior of North America can be expected in a few years as Earthscope's USArray program marches a 70-kilometre grid of 400 seismographs west to east over the contiguous US states and Alaska in the next five years, says Smith.The USArray has already scores of seismographs in California. "Unlike the 40 years Smith has had to collect data, Earthscope reports all the data for everybody without waiting for years and years," says Dr Kaye Shedlock, of Earthscope. "At the end of five years we'll have a marvellous new set of data," agrees Smith. Other aspects of Earthscope include a continental network of GPS stations to watch in real time as the continent changes shape, called the Plate Boundary Observatory, and an experiment that is drilling directly into California's San Andreas Fault.