When people think of volcanoes in the United States, they tend to think of Kiluaea in Hawaii
or Mount Saint Helens in Washington.
Both of these volcanoes have caused great destruction of property and loss of life.
This video will cover the latter of these two well known volcanoes, and discuss the
hazard that it poses in the future.
Despite popular belief, Mount Saint Helens did not go dormant after its catastrophic
1980 eruption, and it will erupt again.
For this video, we are defining an active volcano as any volcano or volcanic field which
has erupted in the last 10,000 years.
Since Mount Saint Helens last erupted in 2008, it falls under this definiton.
Mount Saint Helens can be found in the southwest portion of Washington state.
The volcano is only 33 miles west of another active volcano, Mount Adams.
Like the majority of active volcanoes in Washington, Mount Saint helens is a large stratovolcano.
Stratovolcanoes contain alternating layers of both cooled lava, and ash and tend to have
relatively steep slopes.
Mount Saint Helens is part of the cascade mountain range, and exists due to the juan
de fuca plate subducting underneath the north american plate.
In the cascade mountain range, Mount Saint Helens is a relatively new stratovolcano,
if we are talking on a geologic timescale.
It began forming in a series of eruptions about 40,000 years ago.
Since this period, the volcano has undergone four different long term episodes of volcanic
activity.
These eruptive episodes last for several thousand years and are divided by periods of 5 to 15000
years of little or no activity.
The current eruptive episode began 4500 years ago and continues to the present day.
Since 2400 BC, mount saint helens has erupted at least 44 times.
However, none of these eruptions was as famous as the explosive eruption in 1980.
After over 100 years of dormancy, mount saint helens erupted in march of 1980.
Beginning on March 16th a series of thousands of earthquakes and hundreds of steam explosions
(known as phreatic explosions) began at Mount St. Helens, causing its north side to bulge
outward over 260 feet.
These small phreatic eruptions continued until the end of April when a brief pause occured.
However, the eruption was not over.
Then, on May 18th, a magnitude 5.1 earthquake hit the area.
In an instant, the north side of the mountain collapsed in what was the largest landslide
in recorded history.
The explosion which occured removed 1300 feet of elevation from the mountain's summit in
what is termed a lateral eruption.
From this landslide emerged a massive explosion of hot gasses, magma, and pyroclastic flows.
These pyroclastic flows moved up to 670 miles per hour, which is just slightly slower than
the speed of sound.
These pyroclastic flows and lahars completely took out trees, homes, bridges, and everything
else in their path.
Some of the lahars reached more than 50 miles away.
This eruption was so large, that measurable amounts of ash fell as far away as Oklahoma
and Minnesota.
My father who lived on a farm in Oklahoma, remembered seeing the ash on the ground from
the eruption.
On the 0 to 8 volcanic explosivity index, where each increase in number represents an
eruption 10x larger, the 1980 eruption was only rated as a 5.
The vast majority of the eruptions on Mount Saint Helens were not catastrophic in nature,
but added to the height of the mountain through lava flows and dome building activity.
The last lava dome building eruption occured from 2004 to 2008.
This eruption was about 1000 times smaller than the famous 1980 eruption.
Speaking of catastrophic eruptions, the eruption in 1980 was not truly an isolated incident.
Similarly sized eruptions have occurred 8 times in the last 4500 years.
One of these eruptions which occured in 1860 BC, was 15 times larger than the 1980 eruption.
On screen is a list of these eruptions, which are rated on the volcanic explosivity index
as either a 5 or 6.
The truly distinctive feature of the 1980 eruption was that it caused a lateral blast
and a partial volcanic collapse.
Previous large scale eruptions did not collapse a part of the mountain, but rather added to
it.
As a result of the frequency of eruptions, history of massive explosive eruptions, and
danger of any eruption causing lahars, Mount Saint Helens is rated as a very high threat
volcano.
Among the 161 actively monitored volcanoes and volcanic fields in the United States,
Mount Saint Helens is rated as the second highest risk volcano.
In terms of danger to aviation traffic, it is at the top of the list.
As of 2020, current information points towards the next eruption being another dome building
episode.
The lava composition from the most recent eruptions doesn't contain enough silica to
suggest an immediate future large scale eruption similar to that in 1980 or 1800.
Such large eruptions will occur in the future, but in the short term, the risk is low.
I hope that you enjoyed this video.
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