A new study published recently in the journal Nature says that before an eruption, the volcanic magma – or the molten/semi molten rock under the volcano – must first be in a state where it is fluid enough to erupt.
“People think about there being this big reservoir of liquid magma under a volcano, but we don’t think it’s in that state all the time,” said Kari Cooper, lead author of the study and an associate professor at the University of California-Davis.
They’ve also found that the time it takes for magma to liquefy takes less time than previously thought, making a dormant volcano an active one in as little as a couple of months.
The researchers made their findings after studying Mount Hood, a dormant volcano near the Oregon/Washington border.
The magma that would supply an eruption of Mt. Hood lies between four to five kilometers beneath the volcano.
“The question is, ‘what percentage of time is the magma in an eruptible state?”’ Cooper asked.
The California/Oregon research team found that the magma tucked beneath Mt. Hood has been has been stored for at least 20,000 to 100,000 years. It has been in a cold or immobile state for between 88 percent and 99 percent of those years.
For the magma to liquefy to eruption levels, the researchers said that its temperature will need be more than 750 degrees centigrade.
Kari Cooper discusses research conducted with colleague Adam Kent at Mount Hood (UC Davis)
“If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator,” said the co-author of the study, Adam Kent, from Oregon State University. “It just isn’t very mobile. For Mount Hood, the threshold seems to be about 750 degrees centigrade. 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.”
This boost in temperature is caused when hot magma located much deeper beneath the Earth’s crust pushes its way to the surface and mixes with the cooler more solid volcanic rock.
Kent said that the mixing of the hot and colder types of magma is what set off Mount Hood’s last two eruptions about 220- and 1,500-years-ago.
Fortunately, according to the researchers, when Mt. Hood did erupt, they weren’t very violent in nature. The magma oozed out of the top of the volcano, instead of exploding like other volcanic eruptions.
“What happens when they mix is what happens when you squeeze a tube of toothpaste in the middle,” Kent said. “A big glob kind of plops out the top, but in the case of Mount Hood – it doesn’t blow the mountain to pieces.”
Kent and his colleague Alison Koleszar found in a previously conducted study, that mixing magma from two different sources, that may also have different compositions, not only could trigger an eruption, but also adds a “constraining factor” that determines just how violent the eruption could be.
The researchers said that crystals form within the magma as it cools. The ability for the magma to be mobile depends on the amount of crystallization. When the volcanic rock is more than 50 percent crystalline it’s pretty much immobile and not really in a state for eruption.
As the magma grows colder, the scientists added, the crystallization process itself also slows down.
Studying volcanic rock from previous Mount Hood eruptions the researchers were able to determine the age of the crystals by observing the rate of decay of naturally occurring radioactive elements.
According to the study, calculating a combination of the magma crystal’s age along with its rate of growth can provide scientists with a geologic fingerprint to help them determine just when the magma becomes heated enough to cause an eruption.
“What is encouraging from another standpoint is that modern technology should be able to detect when magma is beginning to liquefy, or mobilize,” Kent said, “and that may give us warning of a potential eruption. Monitoring gases, utilizing seismic waves and studying ground deformation through GPS are a few of the techniques that could tell us that things are warming.”