CogKnoSys

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Physical

Mount St. Helens

Mount St. Helens is an active stratovolcano in Washington whose 1980 eruption blew out its side and lowered its summit.

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The idea

Mount St. Helens is an active stratovolcano in Skamania County, Washington. It lies 52 miles northeast of Portland, Oregon, and 98 miles south of Seattle. A stratovolcano is a steep, cone-shaped volcano built from many alternating layers of hardened lava, ash and rock. The mountain belongs to the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire. The local Cowlitz people call it Lawetlat'la, and the Klickitat call it Loowit or Louwala-Clough. Its English name honors a British diplomat, Baron St Helens, who was a friend of the explorer George Vancouver.

On May 18, 1980, a magnitude 5.1 earthquake triggered a massive debris avalanche that set off a sideways, or lateral, eruption. The blast lowered the summit from 9,677 feet to 8,363 feet and left a horseshoe-shaped crater about a mile wide. The eruption killed 57 people. It destroyed 200 homes, 47 bridges, 15 miles of railways and 185 miles of highway. No volcanic event in U.S. history has caused more economic damage.

The eruption badly disrupted the forests and other land ecosystems near the volcano. Lakes and streams recovered faster, because the ash let water life multiply rapidly. Within six years, most lakes in the area were back to normal.

Check yourself

Question 1 of 2

What kind of eruption did the 1980 debris avalanche set off?

Pick one. The first pick counts.

Composition

what it's made of

  • Stratovolcano cone — A volcano built of layered lava and ash, part of the Cascade Volcanic Arc.
  • Horseshoe crater — The crater, about a mile wide, left by the lateral eruption of 1980.
  • Lava domes — Mounds of lava in the crater; geologists say their shape means future eruptions will need more pressure.
  • Debris avalanche deposit — The rock and debris of the 1980 avalanche, 0.6 cubic miles in volume.
  • National Volcanic Monument — The protected area Congress established around the volcano in 1982.

Connection

how it relates

Under Lower Columbia Mountains and Volcanoes

Cause and effect

  1. Cowlitz Riverstrength 80 of 100

    Volcanic mudflows from the May 1980 eruption overtook the Toutle River, a major tributary of the Cowlitz.

Related

  1. Cowlitz Indian Tribestrength 60 of 100

    The local Cowlitz people call the mountain Lawetlat'la.

  2. Igneous Rockstrength 60 of 100

    The lava and ash a volcano erupts cool and harden into igneous rock.

In place

  1. Portland, Oregonstrength 50 of 100

    Mount St. Helens lies 52 miles northeast of Portland, Oregon.

Change

how and why it evolves

Past
The Goat Rocks eruptive period, which started in 1800, is the first for which both oral and written records exist. After the great blast, small explosions and dome-building eruptions went on until 1986. Magma reached the surface again in October 2004 and built a new lava dome. In July 2008, scientists determined that this eruption had ended.
Now
Since 1982 the mountain has been protected within the Mount St. Helens National Volcanic Monument. It is a popular hiking spot that people climb year-round, and since 1987 climbers have needed a permit.
Next
Geologists predict that future eruptions will be more destructive, because the shape of the lava domes means more pressure must build before an eruption. Exactly when the next eruption will come cannot be known in advance.

Knowledge you can walk. Read the idea, then step up, across or down.

Mount St. Helens

Mount St. Helens is an active stratovolcano in Washington whose 1980 eruption blew out its side and lowered its summit.

See it on the map

Learn, discuss, create, buy or invest

The idea

Mount St. Helens is an active stratovolcano in Skamania County, Washington. It lies 52 miles northeast of Portland, Oregon, and 98 miles south of Seattle. A stratovolcano is a steep, cone-shaped volcano built from many alternating layers of hardened lava, ash and rock. The mountain belongs to the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire. The local Cowlitz people call it Lawetlat'la, and the Klickitat call it Loowit or Louwala-Clough. Its English name honors a British diplomat, Baron St Helens, who was a friend of the explorer George Vancouver.

On May 18, 1980, a magnitude 5.1 earthquake triggered a massive debris avalanche that set off a sideways, or lateral, eruption. The blast lowered the summit from 9,677 feet to 8,363 feet and left a horseshoe-shaped crater about a mile wide. The eruption killed 57 people. It destroyed 200 homes, 47 bridges, 15 miles of railways and 185 miles of highway. No volcanic event in U.S. history has caused more economic damage.

The eruption badly disrupted the forests and other land ecosystems near the volcano. Lakes and streams recovered faster, because the ash let water life multiply rapidly. Within six years, most lakes in the area were back to normal.

Check yourself

2 quick questions, instant feedback.