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No plume under Iceland


There is no plume of buoyant material issuing from the earth's lower mantle beneath Iceland, according to scientists - 19 September 2000

The results are announced by an international team from the U.S. Geological Survey, Princeton University, the University of Durham, U.K., and the Icelandic Meteorological Office.

Writing in the September issue of the Geophysical Journal International, the scientists say after a five-year study of the deep structure beneath Iceland, that the hot material rising in the island nation's volcanoes originates in the earth's upper mantle, rather than the deeper lower mantle or core-mantle boundary.

Furthermore, according to lead author, Dr Gillian Foulger, the 30-year-old theory that mantle plumes are responsible for other geologic "hot spots," such as Hawaii and Yellowstone, needs to be reexamined, using modern scientific methods.

One of those methods is seismic tomography, in which scientists analyze seismogram records of earthquakes throughout the world to image the structure of deep, inaccessible parts of the earth. Images that were obtained in the new study show an upper-mantle root beneath Iceland, but this root does not extend below the 650 kilometers (400 miles). Previously it had been thought that Iceland and other "hot spots" sit atop plumes rising from depths of 2,900 kilometers (1,800 miles).

Foulger also suggests a reexamination of helium isotopes for determining the origin of materials erupted from volcanoes. "It has been presumed that if rocks contain high helium-isotope ratios, as many rocks in Iceland do, they must come from the lower mantle," Foulger said, "but those rocks may also come from the upper mantle."

Foulger and her colleagues say that their questioning of the "plume theory" will expand and improve scientists' fundamental understanding of the planet. She said it is also an excellent example of international collaboration to better understand Earth processes.

The complete paper, "The Seismic Anomaly Beneath Iceland Extends Down to the Mantle Transition Zone and No Deeper," may be found in Volume 142, Issue 3 of the Geophysical Journal International, available at academic bookstores.