The duration for dormant volcanoes to become active is a lot shorter than the time taken for the magma in them to turn to its near-solid state, a new study finds.
Researchers from Oregon State University suggest that the magma sitting 4-5 kilometers beneath the surface of Oregon's Mount Hood has been stored in near-solid conditions for thousands of years, but the time it takes to liquefy and potentially erupt can be as little as a couple of months.
An eruption takes place when temperature of the rock is elevated to more than 750 degrees Celsius. This can only happen when the hot magnum from the earth's crust rises to the surface.
"If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator," Adam Kent, an Oregon State University geologist and co-author of the study, said in a press statement. "It just isn't very mobile. For Mount Hood, the threshold seems to be about 750 degrees (C) -- if it warms up just 50 to 75 degrees above that, it greatly increases the viscosity of the magma and makes it easier to mobilize."
Studying the temperature at which magma resides in the crust of the Earth can help researchers better understand the type and timing of potential eruptions that may occur in the future.
Most often an eruption is caused when the hot magnum from underneath heats the cooler magnum on top, allowing both the magnums to mix. This was also the cause of the last two Mount Hood eruptions on Oregon that took place about 220 and 1,500 years ago. Fortunately, Mount Hood's eruptions are not particularly violent. Instead of the lava exploding out, it calmly oozes out from the top of the volcano. Previous studies have established that the mixing of the hot and cool magnum not only leads to an eruption but also determines how violent the explosion will be.
"Until now, people monitor volcanoes looking for changes in the temperature of gases or water, and the number of earthquakes, to indicate the magma is ready to erupt," ABC News quoted Kent as saying. "Our work gives some extra insight into how to interoperate results from geophysical and seismic studies which identify large bodies of liquid magma."
Mount Hood is a sub-duction zone volcano, sitting atop a collision where one of Earth's tectonic plates slides into the mantle, the hotter layer beneath Earth's crust, underneath another plate. Fluids released from the descending plate melt rocks above it, which ascend to the surface, eventually forming volcanoes.
When Mount Hood's magma first rose up through the crust into its present-day chamber, it cooled and formed crystals. The researchers were able to document the age of the crystals by the rate of decay of naturally occurring radioactive elements. Temperature has a big role to play in the growth of these crystals. If it is too cold, the crystals don't grow quickly.
"What we found was that the magma has been stored beneath Mount Hood for at least 20,000 years -- and probably more like 100,000 years," Kent said, according to Live Science. "And during the time it's been there, it's been in cold storage -- like the peanut butter in the fridge -- a minimum of 88 percent of the time, and likely more than 99 percent of the time."
Even though hot magma from below can quickly mobilize the magma chamber below the surface of a volcano, most of the time magma is held under conditions that make it difficult for it to erupt. This is one of the reasons why even though hot magnum dwells inside a volcano, it doesn't erupt.
The study was funded by the National Science Foundation and published in the journal Nature.