Volcano plumes originate much deeper than was thought, geologists find
and World Science staff
New research on volcano evolution sheds light on what lies deep beneath our feet, scientists say, and on the way material from the Earth’s surface is gradually sucked into the ground and then spewed back up at us through volcanoes.
The research, published in the research journal Nature, suggests the columns of molten rock that provide Hawaii’s volcanoes with a continuous supply of 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. 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 was 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.
“What remains is clear evidence of interaction between the Earth’s core and mantle.”
The evidence, he added, suggests that the mantle constitutes a huge so-called convective system—something like a soup being continuously stirred. It also reveals that sedimentary 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.
Previous analysis of Hawaiian volcanic rocks looked at the isotope signature of the element osmium in them and appeared to show that material from the earth’s core 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 this sedimentary material were much too low for this to be the case, the geologists said.
* * *
Send us a comment
on this story, or send
it to a friend