It has to do with the complicated and varying recipe of liquid, hot magma and cooling water that combine beneath the Earth's crust to cause a volcanic eruption.A new study has uncovered part of the answer for why the Ring of Fire, and other volcanic arcs around the world, occur in the narrow spaces that they do.
Researchers have known for decades that curving chains of volcanoes, or volcanic arcs — such as the Aleutian Islands off the coast of Alaska — form where tectonic plates interact.
"It has been recognized for almost 50 years that the volcanic arcs form where one oceanic plate sinks beneath another," said Phillip England of Oxford University in England.
"But while many models of this process have been put forward, none have been able to explain the location, and narrowness, of the volcanic arcs," England said.
Scientists do know why eruptions of peaks along these volcanic arcs are extremely violent. The molten rock contains a high proportion of water which, as superheated gas, provides the power for the explosive eruptions. In addition, the presence of the water lowers the melting temperature of the rock, which aids the transformation from solid rock to hot, liquid magma — a necessary ingredient for volcanoes.
"Most previous explanations for the origins of volcanoes suggested that this kind of 'wet' melting is responsible for getting a volcano started," said Richard Katz, also of Oxford.
However, it turns out that water mixes in with the molten rock in fairly broad regions of the Earth's mantle, so the mere presence of water doesn't explain why the volcanic chains are so narrow.
"We noticed that there is a very simple geometrical pattern in the distribution of the volcanoes which provides a powerful clue as to what is going on," Katz said.
The crater of Hawaii's Pu'u 'O'o volcano, another of the volcanoes of that comprise the Ring of Fire. Credit: USGS.
Using a mathematical model of heat transport in the regions where two plates collide, the Oxford team showed that the observed geometrical pattern can only be explained if the volcanoes are localized above the few narrow regions in which mantle melts, seemingly paradoxically, in the absence of water.
These limited spots of water-free rock melt then blaze a trail through the Earth's mantle up to the surface, and explosive water-laden magma follows, allowing for the violent eruptions.
In addition to hosting devastating eruptions, the volcanic chains hold valuable clues to the evolution of the earth, because they are the surface expressions of a gigantic chemical factory in which molten rock separates from the mantle to solidify as the crust we live on, and from which significant volumes of gas are emitted into the atmosphere.
Katz, England and their colleagues now intend to investigate the implications of their results for the chemical processes happening deep beneath the volcanic chains.
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