Volcanism

Today’s post looks at the highly-documented and shockingly violent eruption of Washington state’s Mount St. Helens. This particular event was not only the most comprehensively studied and documented volcanic eruption, but the event resulted in the largest landslide in recorded history.

Also, it’s an interesting story, as many volcanoes are. I’ve visited the volcano several times, the last time this past June. Even with the land and forest finally starting to recover and regrow, the extent of damage is still staggering, especially when you’re standing directly in it. The observation deck (where you can watch films on the event, or look at specimens of trees and other materials caught in the blast), sits in the valley directly below the volcano. I must say, looking up at it takes my breath away every time. If you are ever able to visit Washington, go here. No exceptions. I know I say this about every place I talk about, but I say it for good reason.

On May 18, 1980, the event began with an avalanche of hot water, mud, and debris, and was followed by an explosive vertical eruption that continued for most of that day. The source of the eruption came from a shallow chamber, shaped like a horseshoe, and the hollowing out of this chamber is what produced such violent and long-lasting eruptions. We’ll get in to the causes of violent eruptions in just a bit, so sit tight.

Unlike many volcanoes, Mount St. Helens exploded from its northern side, sending ash, debris, and hot lava deposits to the north, northeast, and northwest. The landslide and eruptions together destroyed over 600 square kilometers of land, most of which included forests surrounding the mountain. Part of this terrain included Spirit Lake, which was not only smothered by the blast, but its waters partially contributed to the high-volume mud slide that proved so destructive and catastrophic to the area. The cloud of ash and pyroclastics emitted during the ensuing eruptions were enormous, and devastating in their own way. They reached as high as 26 kilometers up and out from the mouth, and dropped down to about 14 kilometers. That was enough for high-altitude winds to send the ash and debris to central Washington and, within days, the rest of the United States.

What I found to be relatively fascinating is the sheer size of the whole thing. From the eruption, to the land slide, to the affected area, down to the dome itself. This thing is enormous, compared to some of the domes I’ve seen myself. It reaches 1,600 feet long, 1,000 feet across, and 550 tall. That’s just the resulting mound of lava. This doesn’t include the cavernous hole surrounding it. Look at pictures, or watch documentaries, and you’ll get an idea for how big this beast is.

Needless to say, though the eruption was an incredible sight, and an event fascinating to read about, it was devastating to the surrounding (and not-so-surrounding) land and people. Over $1 billion in damages and loss resulted from this event, much of which came from the timber and lumber industry (not surprising, considering how extensive the forest was destroyed). On top of that, 57 people were killed (46-60).

What makes the eruption of Mount St. Helens so different from other eruptions, say, those you see in Hawaii? A lot of it has to do with magma (not a lot, all of it actually). What makes volcanoes like those in Hawaii so different is the composition of their magmas. Notice how dark and loose their lava flows are? These flows are mafic, or high in heavy and dark metals and minerals, and low in volatiles (evaporated gases, including water) and silica. This makes them less viscous, less resistant, and therefore less violent.

Volcanoes like Mount St. Helens, however, are quite different. When you have a higher percentage of silica, the magma gets sticky, thick, and is lower in temperature than basaltic lava flows. This does a lot to slow down the magma, making it more resistant in an eruption and in a lava flow. Also, because of the stickiness and thickness of these high-silica magmas, trapped gases don’t have any room to be pushed up and out of the magma chamber, like you would find in a mafic lava flow. Instead, they remain trapped in the magma chamber (which sits inside or below the volcano) and expand. This creates a much more violent expulsion of magma, resulting in eruptions like Mount St. Helens (82-84).

I may have mentioned this in a previous post, but the location of a volcano says a lot about the magma its going to produce. Mafic, low-silica magmas are typical of volcanoes resting on hot spots (a “plume” of lava springing from the middle of a plate, rather than at a subduction zone), whereas volcanoes with a greater amount of silica and dissolved gasses come from areas where two plates meet, especially when you have an oceanic plate meeting a continental plate.

Aren’t volcanoes fascinating? So much goes into their composition, their nature, and the consequences of their activity, the deeper you go into studying them, the more complex you find volcanoes to be.

Now go explore, find a volcano, and fall madly in love with it.