Volcanoes can go from dormant to active quickly

Magma remains in cold storage beneath volcanoes, but it may take a very short time to liquefy and erupt, a new study suggests.
Magma sitting 4-5km beneath the surface of Oregon's Mount Hood has been stored in near-solid conditions for thousands of years, but that the time it takes to liquefy and potentially erupt is surprisingly short - perhaps as little as a couple of months, researchers said.
The key, scientists said, is to elevate the temperature of the rock to more than 750 degrees Celsius, which can happen when hot magma from deep within the Earth's crust rises to the surface.

It is the mixing of the two types of magma that triggered Mount Hood's last two eruptions - about 220 and 1,500 years ago, said Adam Kent, an Oregon State University geologist and co-author of the study.
"If the temperature of the rock is too cold, the magma is like peanut butter in a refrigerator," Kent said.
"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 mobilise," said Kent.

The scientists are interested in the temperature at which magma resides in the crust, they said, since it is likely to have important influence over the timing and types of eruptions that could occur.

The hotter magma from down deep warms the cooler magma stored at 4-5 kilometres, making it possible for both magmas to mix and to be transported to the surface to eventually produce an eruption.
A previous study by Kent and OSU postdoctoral researcher Alison Koleszar found that the mixing of the two magma sources - which have different compositions - is both a trigger to an eruption and a constraining factor on how violent it can be.

"What happens when they mix is what happens when you squeeze a tube of toothpaste in the middle. A big glob kind of plops out the top, but in the case of Mount Hood - it doesn't blow the mountain to pieces," said Kent.
Little was known about the physical conditions of magma storage and what it takes to mobilise the magma, researchers said.
Researchers set out to find if they could determine how long Mount Hood's magma chamber has been there, and in what condition.
"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.

"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 per cent of the time, and likely more than 99 per cent of the time," said Kent.
The study was published in the journal Nature.