Magma heats up as it crystallizes.

Molten rock moving up through a volcano's plumbing prior to an eruption can heat up substantially, an unexpected finding that could affect scientists' models of the eruption process.

Magma crystallizes as it slowly loses heat to the environment, a process in which minerals with the highest melting points are the first to solidify. However, magma can also crystallize when volatile substances such as water and carbon dioxide bubble out suddenly, causing pressure within the lava to drop, says Kathy Cashman, a voleanologist at the University of Oregon in Eugene. When pressure drops slowly, the first minerals to solidify give up large amounts of heat that warms the remaining molten rock, Cashman and her colleagues report in the Sept. 7 Nature.

For their study, the researchers chemically analyzed crystals that had formed within lava that erupted from Mount St. Helens between 1980 and 1982 and from Shiveluch, a Russian volcano, in 2001 and 2002.

Molten rock that had risen to Earth's surface in those volcanoes over the course of weeks or months heated up by as much as 100[degrees]C during its journey, the team's analysis suggests. That degree of warming can alter several physical properties of molten rock, especially its viscosity. Understanding such changes may enable scientists to better predict the timing and violence of future volcanic eruptions, says Cashman.