In March 1980, a series of
volcanic explosions and
pyroclastic flows began at
Mount St. Helens in
Skamania County, Washington, United States. A series of
phreatic blasts occurred from the summit and escalated for nearly two months until a major explosive eruption took place on May
18, 1980, at 8:32
a.m. The eruption, which had a
volcanic explosivity index of
5, was the first to occur in the
contiguous United States since the much smaller 1915 eruption of
Lassen Peak in California. It has often been considered the most disastrous volcanic eruption in U.S. history.
The eruption was preceded by a series of earthquakes and steam-venting episodes caused by an injection of
magma at shallow depth below the
volcano that created a large bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:11
am
PDT (
UTC−7) on May 18, 1980, caused the entire weakened north face to slide away, a
sector collapse which was the largest
subaerial landslide in recorded history. This allowed the partly molten rock, rich in high-pressure gas and steam, to suddenly explode northward toward
Spirit Lake in a hot mix of
lava and pulverized older rock, overtaking the landslide. An
eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in eleven U.S. states and various Canadian provinces. At the same time, snow, ice, and several entire
glaciers on the volcano melted, forming a series of large
lahars (volcanic
mudslides) that reached as far as the
Columbia River, nearly 50 miles (80 km; 260,000 ft) to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. The thermal energy released during the eruption was equal to 26 megatons of
TNT. (
Full article...)
.jpg)
.jpg)
.jpg)
.svg)
.svg)
.svg)
.jpeg)
