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Nature

Scientists Explain Why Mount Hood Doesn't Explode Like The Others

At over half a million years old, Mount Hood stands tall over the Oregon landscape, a volcano that has not erupted like so many of its cousins in the Cascade Mountains and throughout the Pacific Ring of Fire. Scientists have finally discovered why, and it has a lot to do with milkshakes.

The research was published online in the Journal of Volcanology and Geothermal Research by lead author Alison Koleszar of Oregon State University.

Volcanic eruptions are usually described as “high-explosivity” or “low-explosivity”

Obviously, the high-explosivity events are the ones we see the most of, and they are often referred to as Plinian eruptions, named for Pliny the Younger who was witness to and described the eruption of Mount Vesuvius on the 24th of August, 79 AD. A high-explosivity event is recognisable by the ejection of magma and fragments into the atmosphere.

However, studying the rock around Mount Hood has shown no such high-explosivity events, despite having a similar chemical magma composition and gas contents as volcanoes that have suffered high-explosivity events.

The reason for this, according to Koleszar, is that any eruption that takes place within Mount Hood are preceded by episodes of intense mixing between magmas of different temperatures deep beneath the surface.

Hot magma from deep below Mount Hood rises to mix with the cooler magma that is closer to the surface, increasing its temperature and its viscosity. So instead of an explosive eruption, Mount Hood simply oozes magma out the top of the volcano and piles it up to form a lava dome.

Where do the milkshakes come into it? I’ll let Koleszar explain.

“If you take a straw and blow bubbles into a glass of milk, it will bubble up and allow the pressure to escape,” Koleszar said. “But if you blow bubbles into a thick milkshake you need more pressure and it essentially ‘erupts’ with more force as bits of milkshake get thrown into the air. Add a little heat to the milkshake, though, and it thins out and bubbles gently when you blow into it, more like the glass of milk.

“That what Mount Hood has been doing – heating things up enough to avoid a major explosion.”

More Hiccup than Eruption

Adam Kent, an OSU volcanologist who was Koleszar’s major professor when she earned her doctorate, describes what happens at Mount Hood as more of a hiccup than an explosive eruption.

The researchers looked at three separate eruptive evens on Mount Hood from the past 30,000 years, the last of which took place only 220 years ago. Each event resulted in the formation of lava domes near Mount Hood’s summit, such as Crater Rock on the south side of the mountain, which is a remnant of one such lava dome.

“Instead of an explosion, it would be more like squeezing a tube of toothpaste,” said Kent, who also is an author on the study. “Lava piles up to form a dome; the dome eventually collapses under its own weight and forms a hot landslide that travels down the side of the volcano. In contrast, during a Plinian event such as the kind seen at other volcanoes, ash and rock are blown high into the air and distributed all over.”

Although Mount Hood lacks an explosive history, it doesn’t mean the 11,240-foot peak is completely docile. Collapses of the lava dome at Crater Rock about 1,500 years ago, and again 220 years ago, sent scalding landslides of hot lava blocks down the south side of the volcano, Kent pointed out.

“These types of events have dominated the eruptive activity at Mount Hood for the past 30,000 years,” Kent said. “The other danger is from lahars, which are major mudflows that stream down the side of the mountain at some 50 miles-an-hour, with the consistency of cement. They result when heat from the magma melts snow and mixes it with the volcanic ash and rock.

“Lahars probably accompany most eruptions of the volcano, and can even occur between eruptions after heavy rains or rapid snowmelt,” Kent added. “And they can go quite a ways – all the way to the Columbia River, for instance.”

Koleszar said few other volcanoes around the world act quite like Mt. Hood. It is, she said, a poster child for low-explosivity eruptions.

“Mount Hood is really cool because it is such a model for one extreme of volcano behavior,” Koleszar pointed out. “It may not have the colorful history of Mount Mazama or St. Helens, but it has its own niche among volcanoes – and now we better understand why it behaves the way it does.”

Source: Oregon State University




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