The blob, which drips like honey, is between 15 and 20 million years old
May 27, 2009
Hidden beneath the U.S. West's Great Basin, scientists have spied a giant blob of rocky material dripping like honey.
The Great Basin consists of small mountain ranges separated by valleys and includes most of Nevada, the western half of Utah and portions of other nearby states.
While studying the area, John West of Arizona State University and his colleagues found evidence of a large cylindrical blob of cold material far below the surface of central Nevada.
Comparison of the results with CAT scans of the inside of Earth taken by ASU's Jeff Roth suggested they had found a so-called lithospheric drip. (Earth's lithosphere comprises the crust or outer layer of Earth and the uppermost mantle.)
Here's how it works: "The Earth's mantle, which lies below the thin outer crust we live on, consists of rock which deforms plastically on very long time scales due to the heat and pressure at depth," West said. "In any material which can flow (including the mantle), a heavy object will tend to sink through lighter material."
And this is what the scientists think is happening with the lithospheric drip. A region of heavier material trapped in the lithosphere gets warmed up and begins to sink into the lighter, less dense mantle beneath, pulling a long tail of material after it.
"Honey dripping off of a spoon is a visual aid to what we think the drip looks like," West told LiveScience. "Dripping honey tends to lead with a large blob of honey, with a long tail of material following the initial blob."
He said the blob is between about 30 miles and 60 miles in diameter (between 50 km and 100 km) and extends from a depth of about 47 miles to at least 310 miles (75 km to 500 km) beneath Earth's surface...
The team thinks this drip started some 15 million to 20 million years ago and probably detached from the overlying plate only recently. At first, it was hard for the team to reconcile their discovery with what scientists knew about the region. Over the past tens of millions of years, the Earth's crust in the Great Basin has undergone extension, or stretching.
"We wondered how you could have something like a drip that is drawing material into its center when the surface of the whole area is stretching apart," said ASU researcher Matthew Fouch. "But it turns out that there is an area right above the drip, in fact the only area in the Great Basin, that is currently undergoing contraction."
Last year, Arizona State University Allen McNamara explained how Earth is not neatly divided into a crust, mantle and core. Rather, several large blobs of highly compressed rock — which he described as behaving like honey or peanut butter — exist.
Space tomato packs nutritional super-punch Can we get seeds?
The researchers' analyses suggest the newfound drip won't cause the area to sink down or pop up quickly; nor will it cause earthquakes. In fact, they say there would probably be little or no impact on people living above the drip.
The research, funded by the National Science Foundation, is detailed in the May 24 issue of the journal Nature Geoscience.
Posts
No comments:
Post a Comment