Under the Hood

Mount Hood last erupted more than 200 years ago, but at Crater Rock, not far from the summit, the signs of volcanic activity are unmistakable. Gas vents and hot springs emit sulfur fumes. Vapors rising from deep under the mountain carve snow caves, which can seem like sanctuaries for climbers but often hold deadly concentrations of CO2 and other gases. Rocks fall frequently from the steep unstable cliffs of the partially collapsed crater.

Odds are low that Oregon’s tallest mountain will erupt any time soon, but when it does, scientists have a pretty good idea of what will happen. Driven by the grinding of tectonic plates deep in the planet’s crust, hot magma will infuse cooler lava chambers closer to the surface (an event geologists call “recharge”). Pressure will build. Rocks will begin to crack.

In some volcanoes, not much happens after recharge. The lava chambers may be hotter and ready to burst, but the lid stays on, and the molten rock gradually cools. It takes quite a punch to force a mass the consistency of oatmeal up through miles of tortuous fractures.

Hood, however, is impatient. Within weeks of recharge, lava starts moving and gases start bubbling out through the crater. Melting snow and ice generate debris flows down the mountain’s flanks sweeping away forests and filling rivers with sand and rock. (In the distant past, such events have careened down the Hood River valley and across the Columbia River.) Eventually, lava emerges and snakes down the mountainsides, adding to Hood’s bulk and remaking its classic profile.

That’s been the story for more than a half-million years, says Adam Kent, Oregon State University geologist. When he first arrived in Oregon in 2003, the Australia native learned that while scientists from the U.S. Geological Survey’s Cascade Volcano Observatory knew a lot about hazards posed by eruptions, Hood’s underground plumbing remained largely a mystery.

Since then, Kent has analyzed the remnants of old lava flows to learn how the mountain behaves. He and post-doctoral researcher Alison Koleszar have climbed to the crater, brought samples back to their labs and squeezed clues from rocks. The chemical composition of Hood’s lava flows has remained amazingly uniform over the centuries, and they have found that the mountain may represent an extreme end of volcanic systems and may in fact be unique in the Cascades. Unlike Mount St. Helens, Mount Jefferson and others in this spectacular range, Hood doesn’t explode; it oozes.

Crystal Visions

Kent keeps a piece of Mount Hood in his office. This flat gray rock looks like some kind of exotic concrete. It sparkles with crystals. Irregular, coffee-colored spots about the size of a quarter dot its surface. Dark flecks of hornblende (composed of iron, calcium, silicon and magnesium) are scattered across its surface like pepper on a fried egg.

“When this rock came to the surface,” Kent says, “it was partly liquid. It records information about the last stage of the eruption. But if you want to know more about the long-term conditions in the crust where this magma was being stored, you need to look at the crystals.”

Like tree rings, crystals grow from the inside out over time, says Kent. At their heart are the original minerals — formed out of common elements such as calcium, iron, silicon, magnesium and aluminum. As hot rock pulses up from below, crystals go through warming and cooling phases. Mineral layers form on the outside edges and create a record of temperature, pressure and chemistry. Each ring tells a story about a new pulse of melted rock that cycled the crystal through heating and cooling.

Crystals also trap tiny remnants of some of the original parent material, melted rock that is created as tectonic plates grind against each other. Analysis of these trapped particles — what geologists call “melt inclusions” — provides a picture of the minerals and volatile gases (water, carbon dioxide, sulfur, chlorine and fluorine) that emerge from deep in the crust and can give a mountain shape and personality. When concentrations of those gases are high, explosive eruptions are more likely.

To find out what distinguishes Hood from its neighbors, Kent, Koleszar and their team separate crystals from surrounding rock. In the Oregon State geology lab, they subject samples of the mountain to diamond-tipped saws, acids and devices that pound stones into dust or polish them to a fine sheen (Polishing can be pricey. Grinding pastes that contain diamond particles can cost upwards of $300 for half an ounce). They separate crystals further by exposing rock fragments to magnetic fields or dropping them in dense liquids.

“Some samples are crumbly and fall apart easily,” says Koleszar. “Those are harder to work with. Nice clean pure volcanic glass is great. It polishes like butter. It’s so soft compared to some of the minerals we use.”

Once separated, sliced and mounted on slides, crystals undergo analysis by electron beam that reveals fine structural details or laser and mass spectrometry that tell scientists what trace elements are present in each crystal ring. The result is an accumulation of evidence that allows geologists to explain Hood’s eruption process and compare the mountain to other volcanoes.

More Fizz, No Pop

In a paper published in the Journal of Volcanology and Geothermal Research in 2012, Koleszar and co-authors Kent, William Scott of the USGS and Paul Wallace of the University of Oregon described their findings. They reported that Hood’s magma contains the ingredients for explosive eruptions: magma pumped regularly into the mountain from below, a chemical profile similar to that of other explosive volcanoes and high levels of volatile gases. However, at Hood, those gases tend to escape readily like fizz from an open can of soda. That’s because a 100 degree increase in temperature — an increase that happens as hot rock flows into magma chambers under the mountain — makes the flowing rock five to 10 times less thick.

“Imagine you are blowing into a straw in a milkshake,” says Koleszar. “It’s so thick that the bubbles don’t come out right away, but when they do, they burst and throw stuff up in the air. Compare that to blowing into a straw in a glass of milk. Bubbles just come easily to the surface. That’s more like what we see at Mount Hood.”

Mount Hood may be unique in the Cascades, but it joins a select group of volcanoes worldwide (Mount Unzen in Japan, Soufriére Hills Volcano in Montserrat, Mount Dutton in Alaska) that tend to ooze instead of explode. Nevertheless, volcanoes can also demonstrate both types of behavior, and there’s no guarantee that Hood will always operate as it has in the past. Two well-known explosive volcanoes — Mount Pelée in the Caribbean and Mount Pinatubo in the Philippines — have exhibited both types of eruptions. Moreover, geologists know that pulses of hot magma, which occur at Mount Hood, can cause explosions such as the 1980 Mount St. Helens eruption.

“We’re still trying to figure out why Hood only erupts right after a recharge event,” says Koleszar. “It may be that it just doesn’t have the oomph to erupt at other times.

“It seems like such a boring volcano,” she adds. “It erupts the same thing all the time; it doesn’t seem to do anything interesting. It’s an icon, it’s a beautiful volcano and it’s Oregon’s volcano. But when you start to tease things apart a little bit, it does get interesting, exactly because it is so boring.”

_____________________________

See a February 17, 2014 OSU news story about Adam Kent’s research.  The time from an infusion of hot magma into Mount Hood’s magma chamber to an eruption could be as little as a few months.