Before volcanoes erupt, they must âdefrostâ. The magma beneath some of the worldâs most dangerous volcanoes might be relatively cool and solid for more than 99 per cent of the time. That means evidence that it has warmed up and melted could be a sign of an imminent eruption.
Deep beneath each volcano there is a vast magma chamber filled with bubbling, molten rock. Or, at least, so the thinking goes. Magma chambers sit in Earthâs crust, which is relatively cold â between about 200°C and 400°C â so geologists have long wondered whether they are really hot or not.
To find out, of the University of California, Davis, took samples from the lavas around Mount Hood, a potentially active volcano in Oregon that last erupted 220 years ago.
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With of Oregon State University in Corvallis, she isolated crystals from the solidified lava. Using the radioactive decay of uranium in the crystals as a ticking âclockâ that begins when the crystals form in the magma chamber revealed that they are at least 21,000 years old.
Telltale strontium
But when Cooper and Kent looked at the level of strontium within the crystals, they noticed something unusual. There was far more strontium in the cores of the crystals than near their margins. That is strange, because igneous rock begins to melt at around 750ÂșC. At this temperature, the strontium is mobile and diffuses throughout crystals within a few thousand years.
Since the strontium is not evenly mixed, the Mount Hood crystals can have spent no more than 2800 years above 750°C, and perhaps as little as 140 years. That is 13 per cent â or potentially even less than 1 per cent â of the 21,000 years since they formed.
Magma has to be in a mostly liquid state above 750ÂșC for a volcano to erupt, says Cooper. Yet the evidence from Mount Hood suggests that it is rarely in this hot state.
âIf we do detect a mostly liquid magma body [under a volcano], the probability of an eruption would be higher,â she says. âThis process of âdefrostingâ a magma body could happen before we get any other evidence that an eruption is imminent.â
Melt that magma
âItâs very elegant work,â says of the Georgia Institute of Technology in Atlanta. It also raises an obvious question: what triggers melting? There are a few ideas, says Huber, mostly built around the idea that an injection of magma from a greater depth heats up the chamber.
However, a study last year suggested that the magma chambers beneath some volcanoes remain molten for long periods, rather than melting just before they erupt (). That means looking for melting would only help predict some eruptions.
Not all volcanoes melt in this way, agrees of the University of Savoy in ChambĂ©ry, France. âBut the ones that do happen to be the volcanoes that produce the most infrequent and most powerful eruptions.â
âAlainâs point is true,â says Huber. âDefrosting applies in general to crystal-rich eruptions â Pinatubo in 1991 is a good example.â
The 1991 eruption of Mount Pinatubo in the Philippines was the 20th centuryâs second largest eruption from a land-based volcano.
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