Mount St Helens
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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. Mount St. Helens is one of the Cascade Volcanoes that reach from Washington to California.
The mountain has erupted lava a number of times. Note the size of the rock wall compared to the tourist in this roadcut near Mt. St. Helens. Some layers have been bent due to stress. Click images or here for larger detail image or larger full image.
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Several layers of lava. Click for larger image.
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Geology Terms
Here are some basic terms used on this page. Find more geology terms in the Glossary.
- Ash
- Fragments of less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.
- Basalt
- Volcanic rock caused by partial melting of the Earth's crust.
- Caldera
- A large volcanic depression, commonly circular or elliptical when seen from above.
- 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).
- Dome
- A steep-sided mound that forms when viscous (thick like honey) lava piles up near a volcanic vent (opening at the surface).
- Fumarole
- A vent that releases volcanic gases and steam.
- Lahar
- 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.
- Pumice
- 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.
- Vent
- An opening at the surface where magma, gas and steam erupt.
- Volcano
- A vent at the surface where magma, gas and steam erupt. Also, the landform constructed by volcanic material.
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