Sudden Destruction in May
On May 18, 1980, Mount St. Helens had a massive explosion that forever changed the picturesque alpine landscape, killed almost 60 people and sent ash for hundreds of miles. USGS Photo 5/18/1980.
The force of the eruption coated eastern Washington with a thick layer of light gray ash. When wet the ash became as dense as cement making it hard to remove from lawns, roofs and roads. The ash can still be seen along I-90 and elsewhere in the area. Parts of Idaho and Montana had deposits as the ash was caught up in the jetstream winds.
The blast removed 1000 feet off the top of the mountain, leveled 200 square miles of forest to the north, moved Spirit Lake and formed new lakes. The sound of the explosion could be heard as far away as Canada. Giant mudflows raced down the mountain into local rivers destroying bridges, vehicles and houses. The sound of the explosion could be heard as far away as Canada.
Mount St. Helens is one of the Cascade Volcanoes that reach from Washington to California.
Before and After the Eruption
Earthquakes caused by magma forcing its way to the surface, called harmonic tremors, preceded the blast. As magma, super-heated steam and other hot gases pushed their way to the surface this bulge (left) formed on the north side of the mountain. The eruption was triggered when the lower half of the bulge slid down the mountain and allowed the steam and gases to escape. The hot blast set trees on fire and boiled lake water on contact. Although no lava erupted, mud from melted snow, ash and dirt poured down the mountain and into nearby rivers destroying bridges, roads and houses.USGS Photo.
As you can see from the before and after photos (below) the mountain really blew its top. The lush green forests and crystal clear mountain lakes were turned into gray treeless areas and a new lake. The photos were taken from the same place. Note that the trees in the foreground have disappeared and that there is no longer a forest at the base of the mountain. A new lava dome has risen in the crater since the eruption. USGS Photos.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Fragments of less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.
- Volcanic rock caused by partial melting of the Earth's crust.
- A large volcanic depression, commonly circular or elliptical when seen from above, caused by a volcano collapsing into itself.
- Cinder Cone
- A circular or oval cone made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent.
- Composite Volcano
- A steep-sided volcano composed of many layers of volcanic rocks, usually made from high-viscosity (thick like honey) lava, ash and rock debris (broken pieces).
- A steep-sided mound that forms when viscous (thick like honey) lava piles up near a volcanic vent (opening at the surface).
- A vent that releases volcanic gases and steam.
- A mixture of water and rock debris that forms on the slopes of a volcano. Also known as a mudflow or debris flow. The term comes from Indonesia.
- A light-colored volcanic rock containing lots of bubbles from trapped gases. This rock can sometimes float on water.
- Pyroclastic Flow
- A hot, fast moving and high-density (thick like a dust storm) mixture of ash, pumice, rock fragments and gas formed during explosive eruptions.
- Shield Volcano
- A volcano shaped like a bowl in the middle with long gentle slopes made by basaltic lava flows.
- An opening at the surface where magma, gas and steam erupt.
- A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
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