February Editorial      

Sixty days to eruption.

How long does it take for a previously dormant volcano to become active? Less time than one might think, according to a new study of conditions at Oregon’s volcanic Mount Hood. This study shows that, under the volcano, the magma within the Earth’s crust (about 4-5 km in depth) is at a relatively cool temperature. This is still very hot at around 750C, but at this temperature the magma is cool enough to be semi-solid. However, this solidity is highly unstable. Changes of only 50C to 75C are sufficient to remobilize the magma to the point of eruption. Furthermore those changes may happen much faster than previously thought - as fast as a couple of months.

Mount Hood is a stratovolcano (see our earlier article on ‘Types of volcano’ here). It is located in the Cascade Volcanic Arc in northern Oregon in the USA. In the last 15,000 years this volcano has had two notable eruptions, the most recent being around just 200 years ago. Studies of the volcano have been newly published in the magazine Nature (ref) by Kari Cooper from the University of California and Adam Kent from Oregon State University. The researchers show that most of the magma 4-5 km below the Mount Hood is in a semisolid state for most of the time - and that time may endure for thousands of years. The authors calculate that there is only a brief period (at most 12% and likely less than 1% of the time) when the magma is actually above the critical temperature at which it is easily mobilized as part of an active volcano.

To reach those conclusions the scientists analyzed the age of crystals within the magma and the speed at which the crystals grew. To determine age, the researchers used uranium-series disequilibria - a process also known as uranium-thorium dating. With this technique, the age of a crystal is calculated from the degree to which secular equilibrium within a sample has been restored between thorium-230 and its radioactive parent, uranium-234. In ‘secular equilibrium’ the quantity of a radioactive isotope remains constant, because the production rate from decay of the parent isotope is equal to its own decay rate.

The size of the crystals tell quite a bit about the temperature of the magma, because the colder the magma, the more slowly the crystals grow. So a comparison between the size of a crystal and the age of that crystal is a good guide to the conditions below the volcano in which that crystal grew.

Kent summarizes the discovery as follows: "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, … 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….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 decreases the viscosity of the magma and makes it easier to mobilise."

This mobilization can happen quickly when much hotter magma from a greater depth pushes up and mixes with the cooler, shallow magma. The authors think that this mixing of hot and cold magma was the trigger which caused the last two eruptions of Mount Hood.

The authors believe that it may only take a couple of months for the shallow magma to reach critical temperature and erupt. However, the good news is that modern technology should be able to detect when magma is beginning to liquefy and so give warning of a pending eruption.

Journal Reference:
Kari M Cooper and Adam J.R Kent. Rapid remobilization of magmatic crystals kept in cold storage. Nature, 2014. Published online February 16.
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12991.html