---- — I was living in eastern Washington State in May 1980 when Mount St. Helens erupted after a massive landslide triggered by a magnitude 5.1 quake. Vast amounts of molten rock were violently released to the surface of the Earth, erupting not only as sizeable rocks but as fine-grained volcanic ash that floated on the breeze. Us “down-winders” were enveloped in conditions that were dark as night until the ash finally fell to the ground.
Volcanic ash isn’t the same material as the ash in your fireplace — it’s fine-grained volcanic rock. The St. Helens’ ash was a mess all summer long for those of us in eastern Washington State. It blew around and clogged the air filters of cars, as well as making us cough and hack. But thanks to early warnings from the U.S. Geological Survey and actions taken by Washington’s governor to exclude people from the area immediately around the volcano, only 57 people died in the massive eruption St. Helens experienced. Their loss was a tragedy, but the death toll could have been much higher — and would have been without the good work of geologists giving timely warnings to government leaders.
A number of other volcanoes in the mountains of western Washington, western Oregon, and northern California pose similar potential hazards. One of the most beautiful volcanic peaks is Oregon’s Mount Hood. Two researchers have recently published results from their study of Mount Hood in the journal Nature.
Molten rock underground is called magma (it’s termed lava once it gets to the surface). The Nature article by Kari Cooper and Adam Kent explains that there are components of magma under volcanoes that are stored stably deep within the crust for long periods of time — meaning, in this case, tens of thousands to hundreds of thousands of years. An important question is how and when this stable magma can become mobilized and move upwards to start the events that lead to an eruption.
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.”
At the end of the day, what we all want is to be able to better predict when a volcano will blow its top. This recent work is another step toward that goal.
Dr. E. Kirsten Peters is the author of “Planet Rock Doc.” This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University. Peters can be reached at rockdoc.wsu.edu and on Twitter @RockDocWSU.