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Volcano plumes found
to well up from unsuspected depths
Jan. 20,
2006
Courtesy Imperial College London
and World Science staff
New research on volcano evolution sheds light on what lies deep beneath our feet, scientists say, and on
how the Earth’s surface can be gradually drawn underground and spewed back up at us through volcanoes.
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Kilauea, one of Hawaii's still-active volcanoes, erupts in a spectacular lava show in 1998.
(Courtesy Lawrence Berkeley National Laboratory)
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The research, published in the research journal Nature, suggests the columns of molten rock that
continuously supply Hawaii’s volcanoes with fresh lava originate almost 3,000 km (1,900 miles) below ground.
This is far deeper than many scientists had thought possible, the researchers said. These “plumes” of hot material would thus come from somewhere at the border between the Earth’s core and its rocky mantle, the vast zone between the Earth’s crust and its hot metal core.
Plumes are hot, narrow currents that well up in the mantle and are thought to be responsible for forming long chains of volcanoes such as those of the Hawaiian Islands.
Eruptions occur when plumes find their way through the ground or a
mountaintop, pushing up volcanoes in the process.
Geologists have intensely debated whether plumes rise from the boundary between the Earth’s core and its surrounding mantle, or from a much shallower boundary layer within the mantle.
The new research demonstrated the presence of material from the Earth’s core, geologists said, by using a new instrument to analyse the element thallium in Hawaiian volcanic rocks.
The device analyzes isotopes, the proportions of different variants of an element.
Researchers say this sort of analysis can reveal the physical, chemical
and biological processes an element has undergone. The instrument is a type of device known as a mass spectrometer.
“It is only recently that scientists have developed the ability to analyse these volcanic rocks in enough detail to reveal exactly where in the Earth’s interior they came from. The previous evidence has unfortunately been quite ambiguous,” said Mark Rehkamper of Imperial College London, one of the researchers.
But the new findings, he added, provide “clear evidence of interaction between the Earth’s core and mantle.”
It also suggests the mantle is a huge “convective system,”
the researchers said—something like a soup being continuously stirred.
The research, they added, furthermore reveals that material from the Earth’s surface is drawn into the mantle to make its way back to the surface in the plumes, over time periods of one or two billion years.
Scientists believe this process, known as
subduction, occurs when two continental plates clash, and one of them
gets pushed underneath the other in the process.
Previous analysis of Hawaiian volcanic rocks looked at the isotope signature of the element osmium in them and appeared to show that
core material was present, the researchers explained.
But some scientists had argued that the seeming presence of core material could have resulted from certain types of contamination by surface materials. The new research demonstrates that the amounts of
these surface substances were much too low for this to be the case, the geologists said.
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