When magma is within the Earth, it can cool a bit. It’s still very hot by human standards, but this cooler magma is stiff and resists movement — geologists say it has high “viscosity.” (Similarly, honey stored in your fridge has high viscosity and resists pouring out of the bottle, while honey kept on the counter at room temperature has much lower viscosity and will pour much more easily.)
The study by Cooper and Kent shows that magma underneath Mount Hood spends most of its lifetime in a stable, viscous state. However, viscosity can also change in a surprisingly short period of time — perhaps as little as a couple of months — when hotter magma from below is injected into the cooler material.
The researchers argue that’s exactly what happened in Mount Hood’s last two eruptions, those occurring 220 and 1500 years ago.
The good news for Oregonians is that Mount Hood’s eruptions tend not to be as dramatic as that of Mount St. Helens in 1980. At Mount Hood, magma tends to ooze out of the volcano rather than blasting its way up and generating tons of volcanic ash.
The researchers were able to do their work at Mount Hood by looking at the rocks formed by past eruptions. They could date the age of the crystals within those rocks using radioactive decay. But the growth of mineral crystals in magma is partially determined by the temperature of the magma (cooler magma leads to slower crystal growth).
Looking at both the mineral crystals’ age and their growth rates gave the researchers what they needed to estimate the temperature threshold at which magma becomes mobile enough to cause an eruption. That was an important result of the research.
“And what’s encouraging is that modern technology might be able to detect when the magma is beginning to liquefy or mobilize,” Kent emailed me, “and give us warning of a potential eruption.”