Mount St Helens 2

Before and After the Eruption

Mount St. Helens bulge before 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.

Mount St. Helens before erupting

Mount St. Helens before erupting (top) and after (below).
Click image or here for a closer view of the crater. All USGS Photos.

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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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