Mount St. Helen's
May 18th, 1980
Active composite volcano in Washington, United States.
Well known for ash explosions and pyroclastic flows.
Deadliest and most economically destructive
volcanic event in the history of the United
57 people killed.
Over 7, 000 big game animals killed, including
deer, elk and bear.
250 homes, 47 bridges, 15miles of railways and
185miles of highway destroyed.
The eruption caused a massive debris
Formed within the last 40 years.
Three main river systems.
Three dams for hydroelectric power
After months of activity leading up to the explosion, including the growth of a large bulge on the
north part of the mountain, a M5.1 earthquake caused the entire north flank of the mountain to
slide away in the largest landslide in recorded history. The magma inside of St. Helens burst forth
into a large-scale pyroclastic flow that
flattened vegetation and buildings. The
collapse of the northern flank of St.
Helens mixed with ice, snow, and water to
create lahars (volcanic mudflows). For
more than nine hours, a vigorous plume of
ash erupted, eventually reaching 12 to 16
miles (20 to 27 km) above sea level. The
plume moved eastward at an average
speed of 60 miles per hour. Less severe
outbursts continued through the night
and for the next several days. The
removal of the north side of the mountain
reduced St. Helens' height by about 1,300
feet (400 m).