Magma sitting 2 to 3 miles beneath Mount Hood has been stored in near-solid conditions for thousands of years but the time it takes to liquefy and potentially erupt is surprisingly short -- perhaps as little as a couple of months, geologist Adam Kent said.
Hot magma from deep within the Earth's crust rising toward the surface can elevate the temperature of the stationary magma to more than 1,300 degrees Fahrenheit, and such mixing of the two types of magma triggered Mount Hood's last two eruptions -- about 220 and 1,500 years ago, Kent said.
"If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator," he said. "It just isn't very mobile. For Mount Hood, the threshold seems to be about 750 degrees centigrade (1,300 F) -- 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."
The hotter magma from down deep warms the cooler magma above, making it possible for both to mix and to be transported to the surface to eventually produce an eruption, he said.
However, he noted, Mount Hood's eruptions are not particularly violent, as instead of exploding the magma tends to ooze out the top of the peak.
"What happens when they mix is what happens when you squeeze a tube of toothpaste in the middle," he 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."