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Remembering the Major Eruption of Mount Saint Helens

The Cascade Mountains run from Canada, through Washington, Oregon, California, and they end up in Mexico. These mountains are volcanic and notable example of past volcanic activity include Mount Rainier and Crater Lake. By 1980, scientists had concluded that another volcanic eruption would happen sometime soon. However, the belief was that the most likely spot that would erupt would either be Mount Baker in Washington state or Mount Shasta in California. Few people considered that Mount Saint Helens in Washington might erupt.

This was understandable. The last time that Mount St. Helens erupted was in the 1840’s to 1850’s. Mount Baker isn’t a great distance from Mount St. Helens, and scientists had noticed low-level earthquakes in the area of Mount Baker.

In early 1980, we were living in Albany, Oregon, about 15 miles south of Salem, Oregon, the state capitol. This is north-central Oregon. I remember that there wasn’t a huge amount of concern about the pending eruption of Mount Baker.

Things started to change in April when Mount St. Helens began venting steam from near its summit. This didn’t necessarily mean that Baker wouldn’t blow since the two mountains were believed to share a common source of magma.

On May 15, a swarm of small earthquakes was detected near Mount St. Helens, signaling the movement of magma under the mountain. In hindsight, we know that this was an obvious sign of where the eruption would take place and that it wasn’t Mount Baker that was going to erupt. However, at the time, most geologists still maintained that an eruption of Mount Baker was imminent.

On May 20, at 3:45 pm, there was a stronger earthquake jolt of over 5.0 on the Richter Scale that caused a landslide and avalanche. Almost instantly, Mount St. Helens erupted. The sequence was rather like shaking a bottle of soda pop, then loosening the cap. The rockslide and avalanche moved enough material that the magma was able to blast through what remained.

The magma had been under tremendous pressure and the landslide suddenly released it. The eruption was enormous, instantly pulverizing a huge amount of rock and just as quickly turning snow, ice, and water to superheated steam. The blast moved at up to 670 miles per hour, which means that air and particles broke the sound barrier before heavier ash and particles blasted out.

          The top picture was taken a day before the eruption. The above picture was taken four months after the eruption. 

This meant that the eruption was not only powerful, it was noisy. The eruption was heard in British Columbia, Washington, Idaho, Montana, Oregon, and Northern California. Interestingly, some areas relatively near to the blast didn’t hear it, for reasons I’ll not go into. In Albany, we didn’t hear it, but we certainly felt it. It wasn’t the sort of shaking one might normally experience during an earthquake. Rather, there were a series of jolts.

The ash cloud also reached an altitude of about 15 miles and was about 40 miles wide. It took less than 15 minutes for the ash cloud to reach 80,000 feet and in only about 15 days, the ash had circled the earth.

In the direction of the blast, a huge amount of the forest, 230 square miles in all, was laid down, as well, as if a giant had stepped on it. The forest was literally flattened from the force of the blast.

Earth tremors, venting of gas and ash, and other eruptions continued for quite some time, but it was the first blast that many people mostly remember. It was the most destructive and deadly volcanic eruption in the history of the US, killing 59-65 people, destroying 230 square miles of prime timber (about 30 million cubic feet or 4 billion board feet of timber, enough bo build about 300,000 houses), it destroyed 200 houses, destroyed 15 miles of railways, destroyed 185 miles of highway, destroyed 47 bridges, and the resultant ash destroyed a huge number of crops, including apples, alfalfa, wheat, and potatoes. It is estimated that over 10 million salmon fingerlings, 5,000 deer, 1,500 elk, and countless other wild animals perished. Ash in the water moving through hydro-electric turbines destroyed numerous power generators. The amount of debris eventually flowing into the Columbia River reduced the depth of the channel from nearly 50 feet to only 15, stranding a number of ships.

The energy of the eruption was roughly equivalent of the amount of energy that would be released if 1,600 Hiroshima-sized bombs were detonated all at the same time. About 1 cubic-mile of rock was vaporized in the eruption. About a quarter of a cubic mile of ash was spewed out and the ash fell in an area of about 22,000 square miles. Ash falls happened in 11 states and 5 Canadian provinces.

The cost of cleanup was millions of dollars and the total cost of the eruption was the 2018 equivalent of $3.1 billion.

For all the destruction and the impact the eruption of Mount St. Helens had on so many people, it needs to be remembered that this was a relatively small volcanic eruption. It was quite explosive, but that is because the magma had a silicon level of 65%.

I definitely remember the eruption of Mount St. Helens.

What do you think?

3 points

Written by Rex Trulove

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