Geologists are a step closer to solving a puzzle about how magma that forms deep underground produces explosive volcanoes in the Cascade Range.
“Water is a key player,” says Paul J. Wallace, a professor of geological sciences at University of Oregon.
“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.”
Ring of Fire
For a new study published in Nature Geoscience, researchers examined water and other elements contained in olivine-rich basalt samples that were gathered from cinder cone volcanoes surrounding Lassen Peak in Northern California, at the southern edge of the Cascade chain.
The discovery helps explain plate tectonics and the Earth’s deep water cycle beneath the Pacific Ring of Fire.
Scientists have been studying the region’s propensity for big earthquakes and explosive volcanoes since the 1960s. The ring contains more than 75 percent of the planet’s volcanoes and stretches from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America.
To understand how water affects subduction of the oceanic plate, in which layers of different rock types sink into the mantle, researchers used equipment at four laboratories to examine hydrogen isotopes in water contained in tiny blobs of glass trapped in olivine crystals in basalt.
Next, the data was fed into a complex computer model developed by coauthor Ikudo Wada, then of Japan’s Tohoku University. She has since joined the University of Minnesota.
That combination opened a window on how rising temperatures during subduction drive water out of different parts of the subducted oceanic crust, Walowski says. Water migrates upwards and causes the top of the subducted oceanic crust to melt, producing magma beneath the Cascade volcanoes.
The key part of the study, Wallace says, involved hydrogen isotopes.
“Most of the hydrogen in water contains a single proton. 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,” Walowski says.
“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.”
Researchers from the Carnegie Institution and the US Geological Survey are coauthors of the study. The National Science Foundation and Carnegie Institution supported the work.
Source: University of Oregon