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 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.|
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.
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.
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.