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. The blast removed 1000 feet off the top of the mountain, leveled 200 square miles of forest to the north and boiled the water out of Spirit Lake. The sound of the explosion could be heard as far away as Canada.
Earthquakes caused by magma forcing its way to the surface, called harmonic tremors, preceded the blast. The magma, underground steam and hot gases caused a bulge on the northern side of the mountain. When the bulge became too high it caused a landslide which released the steam, hot gases and ash. Although no lava erupted, mud from melted snow, ash and dirt poured down the mountain and into nearby rivers destroying bridges, roads and houses. The sudden event drastically changed the area around the mountain including forming new lakes.
The area is now recovering as wildlife, grass and trees return. Over a 18 million trees were planted to reforest the area.
|"The afternoon of the eruption some friends and I were returning to Spokane from a trip into British Columbia, Canada. Arriving from the north into the city around 4 PM it was as dark as at midnight. I noticed that the street lights were on yet visibility was less than a block ahead as if we were in a severe winter snowstorm. The fine, light gray ash was several inches thick on the roads and still falling." [Draggoo]|
Mount St. Helens
The extremely hot interior of Earth causes the volcanic activity that shapes and re-shapes the planet. The Cascade volcanoes are some of the most active volcanoes in the world and can from time to time erupt. In the case of Mount St. Helens the force of the volcanic explosion affected hundreds of square miles of land and the plants, animals and people in the area. Click on the image or here to find out more about Mount St. Helens. USGS Photo.
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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.
- 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 wet concrete) 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.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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