After 123 years of inactivity,
Mount St. Helens awoke on March 20, 1980. It awoke when a magnitude 4.2
earthquake rumbled beneath it.
After 123 years of inactivity,
Mount St. Helens awoke on March 20, 1980. It awoke when a magnitude 4.2
earthquake rumbled beneath it.
Hundreds of smaller earthquakes
followed. Seven days later, the first steam-explosion blasted a 250 foot
wide crater through the volcano's ice cap.
By early April, dozens of
explosions had expanded the crater. It grew to more than 1,000
feet in diameter.
In two short months, an area 1 mile
long and half a mile wide was pushed outward 450 feet. This area became
known as "the bulge."
The bulge was caused by the rise of
molten rock into the volcano. The molten rock pushed aside older rocks to
make room for itself inside the volcano.
By May 18th, scientists estimated
that the volume of new magma in the volcano was enough to
fill a balloon 1,800 feet across.
But the history of Mount St.
Helens extends beyond its recent past. The tapestry of Mount St. Helens'
history is woven from evidence gathered by geologists. This started with
Lieutenant Charles Wilkes' U.S. Exploring Expedition in 1841. Until that time,
the eruptions at Mount St. Helens had been documented only in the oral
tradition of the Klickitat tribes.
For most of its 40,000 years, this
energetic volcano is thought to have been a squat, rough collection of
domes and lava flows.
But beginning about 2500 years ago,
large eruptions of pasty lava created most of Mount St. Helens' smooth,
familiar cone.
Two thousand years ago, large lava
flows filled in the valleys on the south side of the mountain. They
formed many, long, symmetrical lava tubes, like this one in
Hawaii, and Ape Cave.
About the time the Spanish landed on
the Eastern shores of North America, Mount St. Helens was erupting and
depositing ash for miles around.
In the 1700's and early 1800's,
large eruptions covered portions of the Northwest with ash deposits.
These deposits were twice as thick as those from the 1980 eruption. Here
you see a geologist studying different layers of debris deposited during
previous eruptions of Mount St. Helens.
In the 1950's, geologists began an
extensive study of the deposits around Mount St. Helens. In 1975, they
published a report predicting that Mount St. Helens was the volcano in
the lower 48 states most likely to erupt by the end of the century.
The 1980 eruptions of Mount St.
Helens proved just how accurate their predictions were!
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Mount St. Helens & Other
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Geological History
Before the Eruption
Mount St. Helens
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