Friday, August 11, 2017

After the Disasters that Formed the Crater Lake and Crooked River Calderas, St. Helens was Hardly a Blip...Right? Uh, Right?

Yup, perspective is everything. I've been going on for several posts about prehistoric volcanic eruptions that were pretty much unimaginable in their violence and destructiveness. The Crater Lake eruption took place 7,700 years ago, and put 15 cubic miles of ash into the atmosphere, covering much of the western North America. The Crooked River Caldera dwarfed the Crater Lake event, with something like 200 cubic miles of ash. A repeat of either event would result in a huge death toll, and a serious threat to civilization. Luckily, such events are relatively rare in the time frame of human history.

Which brings us to the next destination from our field studies journey through the Pacific Northwest last June: Mt. St. Helens in southern Washington. Every school child for the last 37 years knows the familiar profile of the mountain as it exists today, and vaguely knows that it looked much different prior to 1980. It appears in pretty much every science textbook in the United States, being the last volcano to erupt in the lower forty-eight states.
Mt. St. Helens prior to the May 18, 1980 eruption. Source: U.S. Geological Survey

The problem these days is that for many people, St. Helens is ancient history. It is a geological event that took place years before they were born, and as such there is a disconnect regarding the reality and intensity of the events that took place in 1980 and the years following. I'm even guilty of belittling the event by comparing it to the prehistoric eruptions that happened in the region thousands and millions of years ago
Mt. St. Helens in 2002. Note the lava dome in the crater interior
The eruption of Mt. St. Helens may not have been on the scale of Crater Lake or the Crooked River events, but it was a huge event, almost beyond imagination, by any kind of human standard. What had once been a mountain nearly 10,000 feet high was reduced to a smoldering crater 1,300 feet shorter. The trigger that led to the disastrous explosion was the largest mass wasting (landsliding) event ever witnessed by human beings. And many people lost their lives, despite the evacuation and relative remoteness of the mountain.

The eruption began in March of 1980 when a moderate 4.1 magnitude earthquake shook the mountain, the largest ever recorded. Geologists were concerned and wired the mountain with whatever sensors they could think of. The rising mass of magma began to interact with groundwater and ice within the mountain, and a series of ash eruptions tore away at the summit of the volcano. The magma began to push outwards on the north flank of the mountain producing a 600 foot high bulge. Geologists were concerned about slope stability, but what happened was far beyond what they expected, or could even imagine. On May 18, 1980 at 8:32 in the morning, a magnitude 5.1 earthquake shook the bulge loose in a titanic debris avalanched that dwarfed any ever seen by humans. It thundered down the north flank, partly into Spirit Lake, with the remainder shooting down the Toutle River valley for 12 miles. Twelve miles.

The loss of the bulge meant that there was no longer any pressure holding back the gas-rich magma chamber, and it exploded with the power of hundreds of atomic bombs. The main blast was directed north and west, again towards Spirit Lake, and down the Toutle River valley. The ash was moving so fast (around 300 mph) it actually passed the fast-moving debris avalanche, so that in places the ash layer is actually covered by the avalanche deposits.
Mt. St. Helens in 2006, during the eruption that began in 2004. Note the second dome in the crater, behind the first.
The ash traveled north at hurricane speeds, up and over the intervening ridges, ultimately traveling around fifteen miles. The heavy dust-laden air downed nearly every tree in its path, stripping away branches and bark. Unless they were already underground in burrows, no animals survived in the main blast zone. The total area of devastation was around 150 square miles.

USGS geologist David Johnston, monitoring the volcano from an observation station on the ridge that now bears his name was one of the first people killed by the blast. Despite the evacuation orders (which were not far enough away from the volcano anyway), 57 people died.

These are just numbers, and numbers can't always describe the totality of the destruction of this volcanic eruption. You pretty much have to stand in the middle of it, and walk it. My first exposure to the devastation came about dozen years after the main blast (smaller-scale eruptions continued through 1986). I drove to the Windy Ridge Observation Point on the east side, which at the time was the only real viewpoint. The mountain was socked in by clouds (I had one brief glimpse of the outline of the volcano), but mile after mile of downed forest drove home the magnitude of the devastation.
I began bringing my students to the volcano in the 1990s once the road to the Johnston Ridge Observatory was opened. It provides an almost terrifying view into the maw of the gigantic crater, as well as a 360 degree view of the devastation. Tree trunks still litter the landscape, and only a few spindly conifers have begun to replace those that were blown away in 1980. The mountain ecosystem is well underway towards recovery however. Areas that were gray and dusty during my first visit are now green and covered by shrubs and wildflowers.
The mountain erupted again in 2004, and of course all the networks and cable new outlets converged on the mountain to report on the death and devastation. They hung around for a week or so and no one died, so they got bored and went back to their breathless coverage of missing white women (yes, that was a thing for months, and both stories involved Modesto in one way or another). The eruption continued for four more years, and a new 900 foot high dome appeared behind the 1986 dome.
The last time we tried to explore St. Helens with students was in 2011. The day was rainy and fog-bound, and we saw literally nothing of the volcano or the devastation, wasting hours of valuable time in the attempt. That was on my mind last June as we sized up the weather reports for the day of our expected visit... they called for rain. Mrs. Geotripper suggested we might skip a few "minor" stops the day prior to our scheduled visit, and catch the mountain in the late afternoon. It turned out to be a great idea, and all we missed was a view of the crater rim, due to the advance clouds from the coming storm.
The hummocky surface of the debris flow is still very evident in the valley of the Toutle River. The brush has had a harder time covering the small hillocks because soils can't readily develop on the steep slopes. Hundreds of new trees can be seen on the slopes beyond the valley floor however. Two new lakes formed in 1980 because the debris flow blocked some of the side canyons. Ecosystems are developing in these new environments as well.
The forest belt of Washington is not necessarily know for wildflower shows, but the slopes around the Johnston Ridge Observatory are for the time being vast meadows. Until the forest recovers, wildflowers will be part of the visitor experience.
As we traveled down the highway towards our camp, we noted a striking difference in the forest as we left the national monument and entered onto private lands owned by lumber companies. The forest was as uniform as a cornfield. The companies lost a great deal of timber during the eruptions, and it made sense to replant trees as quickly as possible. It's a bit disconcerting to see what amounts to monoculture going on. One hopes that these trees aren't vulnerable to the boring tree beetles that have destroyed forests across the western United States.
We headed down the hill to our camp at Seaquest State Park at Silver Lake. We had been privileged to see most of the volcano, and gained a perspective of the devastation from the ground. I wished I could have shown my students even more, and was reminded of a flight I took to Seattle a number of years ago. It happened to be one of those rare perfectly clear days, and the flight was only half full. The stewards gave me some dirty looks as I gleefully jumped from one side of the plane to the other snapping pictures of Cascades volcanoes (I got great shots of every volcano north of Crater Lake to Mt. Rainier). And I got pictures of Mt. St. Helens from the air for the first time.
We flew on the west side of Mt. St. Helens which provided me a view of Mt. Adams in the distance, and the Toutle River valley in the foreground. The gray strip of flood plain reveals the location of the debris avalanche, providing a perspective on the size of the flow.

In the big picture of Earth history, the eruption of Mt. St. Helens will barely register as a blip. Unless future researchers find the debris avalanche deposits, the record of the eruption in most places will be a thin layer of ash, at most a few inches thick. It would be an unremarkable eruption. But the eruption happened in modern times. We had accurate records of the volcano as it existed before, and excellent documentation of the events of May 18, 1980 (had the day been cloudy, we would be confused by some of the deposits). And we know what the volcano looks like today. The changes by any human standard were huge, and the effects on society very large. There was nothing small about the eruption of Mt. St. Helens.

If you are interested in the St. Helens story, may I recommend an excellent series on the eruptions by fellow geoblogger Dana Hunter at Rosetta Stones (click here for the index). She did a stellar job of bringing the volcano to life, along with the stories of those who were affected by the disaster.
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