Mount Saint Helens in Washington, United States is classified as stratovolcano.
This is due to its great cone of ruble consisting of lava rock interlayered
with pyroclastic and other deposits. It includes layers of basalt and andesite.
Its present thermal activity is classified as strong steaming. It has exhibited
continuous intermittent activity since 1980, with occasional eruptions of
steam and ash. Prior to May 18, 1980, its height was 9677 feet. It is currently
8364 feet.
On May 18, 1980, after 2 months of local earthquakes and steam eruptions, Mt. St. Helens began a major explosive eruption. The initial eruption was considered a lateral blast which lasted for a few minutes. The volcano continued to erupt for 9 hours, devastating more than 150 square miles of forest, killing countless animals and left over 60 people dead or missing. The eruption caused pyroclastic flows as well as mudflows which reached all the way to the Columbia River, 70 miles away.
The eruption blew volcanic ash consisting of
pulverized old rock from the mountain's core as well as solidified new lava,
more than 15 miles into the air. Winds carried the ash generally eastward
across the United States, and, in trace amounts, around the world. The ash,
which fell in high amounts as far east as Montana, severely disrupted travel
and caused widespread economic loss.
An earthquake, measured 5.1 on the Richter scale,
set off this devastating eruption. The volcano's north flank slid away in
the largest landslide in recorded history. Temperatures within the blast
reached as high as 300 degrees Celsius.
Following the 1980 eruption, more than a dozen
extrusions of thick, pasty lava built a mound shaped lava dome in the new
crater. The dome is about 1100 meters in diameter and 250 meters tall.
