GIANT BLOB FOUND UNDER NEVADA (no kidding)

[JT Allen]

(May 27) -- 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.

 

The Great Basin, which covers most of Nevada, western Utah and parts of other states, is characterized by mountain ranges separated by valleys. Scientists have discovered a huge, oozing blob of rock beneath the basin that may have begun forming about 15 million years ago. Above is Nevada's Great Basin National Park.

 

While studying the area, John West of Arizona State University (ASU) 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 and extends from a depth of about 47 miles to at least 310 miles 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.

 

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.

 

Source: LiveScience

[Guest Texas Tech]

Common conversion point stacking of receiver functions has been applied to data from 21 seismic stations located throughout southern California to produce an image of the upper mantle transition zone. This method has gained wide acceptance in the investigation of the upper mantle transition zone (TZ) which is generally believed to be the depth range in the mantle where olivine undergoes a series of phase changes. The olivine to spinel structure phase change is generally believed to be responsible for the 410 km discontinuity. A phase transformation in the olivine mineral system to perovskite and magnesiowüstite is generally accepted to be associated with the 660 km discontinuity. An investigation of depth variations in the 410 and 660 km discontinuities is generally considered to be a means to identify variations in mantle temperatures associated with the respective phase change. That is that the 410 will generally become shallow in cooler regions of the mantle and deeper in warm regions. The 660 will behave oppositely in response to thermal variation in the mantle. As a result we would find a thick transition zone in cooler regions and a thin transition zone in warm regions.

 

The transverse ranges in southern California have been subject to much investigation because they do not appear to have an adequate crustal root to account for observed topography. Humphreys and Hager (1990) have found evidence of very localized subduction or a “drip” of lithospheric material beneath this region, as a result of convergence due to a kink in the San Andreas fault system, which can account for the observed topography. Using seismic tomography they imaged a high velocity anomaly (generally considered to be evidence for cooler mantle temperatures) directly beneath the transverse ranges to a depth of almost 300 km. While they do not suggest that this feature penetrates the upper mantle transition zone, we have found in receiver function images that there is an anomalously thick TZ directly beneath the proposed drip which would indicate a small localized low temperature anomaly. We interpret this feature as evidence that the “drip” feature penetrate the TZ or may be captured within the TZ. Our TZ image also suggests other localized down-welling features beneath southern California.

[Guest Murray]

Do you have images? This is neat stuff!