The Rock Doc: Studying magma may shed light on eruptions

I was living in Eastern Washington in May 1980 when Mount St. Helens erupted after a massive landslide triggered by a magnitude-5.1 earthquake.

Vast amounts of molten rock were violently released to the surface, erupting as rocks and fine-grained volcanic ash that floated on the breeze. Those of us downwind were enveloped in conditions as dark as night until the ash finally fell to the ground.

A number of other volcanoes in the Cascade mountains pose similar potential hazards. One of the most beautiful is Oregon’s Mount Hood. Two researchers recently published results from their study of Mount Hood in the journal Nature.

Molten rock underground is called magma. 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 – perhaps hundreds of thousands of years. An important question is how and when this stable magma can become mobilized and move upward 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.” (Honey stored in your fridge has high viscosity, while honey kept on the counter at room temperature has much lower viscosity.)

The study by Cooper and Kent shows that magma underneath Mount Hood spends most of its lifetime in this stable, viscous state. However, viscosity can also change in a surprisingly short period of time – perhaps in 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 1,500 years ago.

The good news for Oregonians is that Mount Hood’s eruptions tend not to be as dramatic as the 1980 event at Mount St. Helens. 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’ ages and their growth rates gave the researchers what they needed to estimate the temperature threshold needed for making the magma mobile enough to cause an eruption.

“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.”

We all want to be able to better predict when a volcano will blow its top. This recent work is another step toward that goal.