Explosive Volcanoes Due to... Water?

When we picture volcanoes erupting, molten hot magma comes to mind. Surprisingly though, new research says that explosive volcanoes are actually fueled in large part by water.

"Water is a key player," researcher Paul J. Wallace, from the University of Oregon (UO), said in a statement. "It's important not just for understanding how you make magma and volcanoes, but also because the big volcanoes that we have in the Cascades - like Mount Lassen and Mount St. Helens - tend to erupt explosively, in part because they have lots of water."

University of Oregon geologists tapped water in surface rocks to show how magma forms deep underground and produces explosive volcanoes in the Cascade Range. They also examined other elements contained in olivine-rich basalt samples that were gathered from core volcanoes around Lassen Peak in Northern California, at the southern edge of the Cascade chain.

The findings, published in the journal Nature Geoscience, help provide insight into plate tectonics as well as Earth's deep water cycle beneath the Pacific Ring of Fire, which scientists have been studying since the 1960s. The ring, which stretches from New Zealand and along the eastern edge of Asia, contains more than 75 percent of the planet's volcanoes.

To understand how water affects subduction of the oceanic plate, in which layers of different rock types sink into the mantle, the UO team studied hydrogen isotopes in water contained in tiny blobs of glass trapped in olivine crystals in basalt.

Next, the team fed data gained from the rocks into a complex computer model, which showed how rising temperatures during subduction drive water out of different parts of the subducted oceanic crust. Water migrates upwards and causes the top of the subducted oceanic crust to melt, producing magma beneath the Cascade volcanoes.

It turns out hydrogen isotopes are a key part of this process.

"Most of the hydrogen in water contains a single proton," Wallace explained. "But there's also a heavy isotope, deuterium, which has a neutron in addition to the proton. It is important to measure the ratio of the two isotopes. We use this ratio as a thermometer, or probe, to study what's happening deep inside the earth."

"Melting of the subducting oceanic crust and the mantle rock above it would not be possible without the addition of water," added UO doctoral student Kristina J. Walowski, who led the study. "Once the melts reach the surface, the water can directly affect the explosiveness of magma. However, evidence for this information is lost to the atmosphere during violent eruptions."

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