Wandering Through Washington, Part VI – Mount St. Helens Devastation

The day after arriving in Castle Rock, we hopped into the car for a drive to Mount St. Helens National Volcanic Monument. We wanted to take a look at the mountain that blew its top in 1980. As we drove along, we kept expecting to see the mountain. We stopped in at a few viewpoints and visitor centers, but still no view of the mountain. It was hiding in a low cloud bank. The devastation was easily visible in the valley below us, but not that mountain. We finally decided to turn around and return another day to take it in.

A few days later, after a wonderful visit to the Fort Vancouver National Historic Site, we had a bright, sunny day for a drive back to Mount St. Helens. There we were, driving along the same road, and as we rounded a bend, there it was. There was no mistaking which mountain was Mt. St. Helens. The rest of the time we were in Castle Rock, we read and studied what we could of the history of the area.

The sheer size and ensuing devastation of the 1980 eruption was awesome. For example, in less than 3 minutes, 230 square miles of forest was destroyed. The Toutle River was buried under a mud/debris field measuring up to 600 feet in places. The mudflows stretched 75 miles away. The numbers were eye-opening.

So what exactly happened? Let’s go over the history first, then the timeline of events.

Mount St. Helens has been an active volcano for a long time (in human terms anyway). The Native Americans called it ‘Fire Mountain’ and had several legends about it being a witch who switched back and forth between being beautiful and a monster. It is part of the chain of volcanoes that make up the Cascade mountain chain. You can draw a pretty straight line connecting all the volcanoes when you look at a map of the area. From Lassen Peak and Mount Shasta in Northern California, to Mount McLoughlin, Crater Lake, Mount Thielsen, Three Sisters,  Mount Jefferson, and Mount Hood in Oregon. Next are Mount Adams, Mount Baker, Glacier Peak, Mount St. Helens, and Mount Rainier in Washington. And finally, Mount Garibaldi in British Columbia. All of these mountains are on the subduction zone between the Juan de Fuca Plate and the North American Plate. This subduction zone is the result of the smaller Juan de Fuca Plate moving ever so slowly under the North American Plate.  All of the volcanoes occur in the area where the subducting plate has reached approximately 61 miles under the surface.

The first European sighting of the mountain was by George Vancouver who named it after his mentor, Lord Fitzherbert, Baron of St. Helens. Lewis and Clark described it in their journals in 1805. No Europeans had seen Mt. St. Helens erupt, but apparently, it had erupted a few times in the early 1800s. It was in 1836 that the first European recorded an eyewitness account of the mountain erupting. Other accounts were recorded in the 1840s and 1850s.This is also the time of the Westward Expansion so a lot more people were arriving in the area. Then it seemed to quiet down, as no new eruptions were observed. Until 1980.

On March 20, 1980, the mountain started waking up. An earthquake registering 4 on the Richter scale was detected in Seattle. It was centered directly below Mount St. Helens. Many more small earthquakes followed that day. Since the USGS believed this might be a precursor to an eruption, they went to the mountain and added more seismometers and scientists to watch and study the mountain. During one 12 hour period, 47 earthquakes were recorded at magnitude of 3.0 or higher.

March 27 brought the first small explosions that formed a new crater at the summit. A few days later, another one opened, this one emitting blue flames. About 93 small eruptions of steam and ash happened between March 27 and April 1. Then the nature of the earthquakes changed to a harmonic tremor indicating that magma was on the move deep underground. During the month of April a large bulge began forming on the north side of the mountain. It was growing at an astonishing rate of 5 feet a day. This flurry of activity meant a large eruption was eminent, but of course, no one could predict when it would happen.

These events were also bringing a large number of sightseers. Eventually governor Dixie Lee Ray closed the area to all but scientists and loggers. When nothing happened for awhile, the government decided to let homeowners go back for a few hours to check on things. The first group went on Saturday, May 17. The next group was ready and waiting on Sunday, but at 8:32am, the real show began.

An earthquake registering 5.1 started it off. This earthquake triggered a massive landslide. The entire summit and north face of the mountain slid down. This allowed the magma and gasses building inside the mountain to release in a huge explosion. About 3 seconds after the landslide, this explosion released an enormous cloud of magma, ash, gases, and debris called a pyroclastic flow. The flow moved faster than the landslide (which was still working its way down the mountain). The pyroclastic flow was moving at 450 miles an hour. It obliterated the forests on and around the mountain. It covered the 5 miles to the Observatory Ridge, where the US Geological Survey was monitoring the mountain, in seconds. Gary Johnston, a volcanologist, was able to yell into his radio “Vancouver, Vancouver! This is it!” They never found him.  The ridge and observatory is named after him now.

Next up were the mud flows, also called lahars. The mud consisted of mountain chunks, dirt, ash, and lots and lots of water (steam, melted snow, river and lake water). The big chunks were left behind and are called hummocks. They still litter the valley floor. The blast reached Spirit Lake and over ran it,  pushing water 700 feet up over the banks and down the river. The trees that had been felled were and washed back into the lake. The wave of water was 860 feet high and raced downstream taking downed trees, ash, and other debris with it. The Toutle River was flooded.

There were three distinct areas of destruction from the lahar and initial pyroclastic flow. The first was the direct blast zone. It extended nearly 8 miles out and pretty much nothing survived in that zone. Trees were obliterated, or carried away. The next zone was called the channeled blast zone and extended 19 mile out from the volcano. This  is where trees were blown down like toothpicks, their trunks lying in the direction of the blast. Areas were scoured down to the bedrock, leaving little soil and seeds for regeneration. The furthest area was the seared zone. The trees were singed and some were stripped of their branches, but the energy was not strong enough to snap the tree trunks.

Additional steam explosions were still happening on the mountain and melted the glaciers in minutes. Water roared down the mountain, flooding 6 rivers (including the Toutle). The mudflows, as mentioned above, extended 75 miles out. The 504 highway which was the main road into the north side of the mountain was covered in 6 feet of mud. The Toutle River was completely buried.

The ash cloud rose into the air as much as 12-15 miles. The ash fell in 12 neighboring states for days, depositing up to 6 inches of ash. Yakima, a small city south east of Mt. St. Helens, went dark from the amount of ash falling. People had to wear face masks to keep from breathing it in. Luckily, the actions of the USGS and government of closing down the area, limited the number of people who were in the area. So while the casualties were low, 57 people still lost their lives that day. They had made the mistake of believing the cordoned off area was sufficient because scientists were expecting an upward explosion, not a lateral one.

One of the most interesting things we learned was the “Zone of Silence.” Because of some funky physics, people within 60 miles of the blast heard nothing. From 60 to 200 miles away, they could hear the explosion quite clearly. So those who needed the sound to know they were in danger had no warning. The physics had to do with how the sound waves bounced up to the atmosphere and were reflected back, but that 60 mile area was silent. Weird.

 

The eruption lasted for nine hours. It was several days before anyone could see the top of Mt. St. Helens. In the end, the mountain was 1,300 feet shorter. The day before, it was 9,677 feet. After the blast, it measured in at 8,364. The crater formed at the summit was a mile wide and 2 miles long. It removed 2/3rds of a cubic mile of the mountain.  When Mother Earth clears her throat, you’d best be listening.

 

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