New Scientist
Image: dherrera_96
Before volcanoes erupt, they must “defrost”. The magma beneath some of the world’s 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 Earth’s crust, which is relatively cold – between about 200°C and 400°C – so geologists have long wondered whether they are really hot or not.
To find out, Kari Cooper 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.
With Adam Kent 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 “clock” that begins when the crystals form in the magma chamber revealed that they are at least 21,000 years old.
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 750ºC. 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 – or potentially even less than 1 per cent – of the 21,000 years since they formed. Read more on newscientist.com…
