We get the word "volcano" from the name of an island near Sicily in the Mediterranean Sea. People at the time thought that the steam, smoke and lava coming from the island was coming from the forge of Vulcan, the blacksmith of the Roman gods, who made the thunderbolts for Jupiter and weapons for Mars, the god of war. The term volcano now refers to an opening (vent) on the Earth's surface where lava and hot gases are released.
Sometimes an inactive volcano fills up with water as in Crater Lake in southern Oregon at left. Mount Mazama, a composite volcano, collapsed into itself forming a caldera. A cinder cone from a later eruption has become an island.
Erupting volcanoes can be very violent because of the presence of extremely hot steam and gases or more stately with lava flowing slowly out of the vent. Some volcanoes erupt under the ocean as vents open in the ocean floor. Explosive events sometimes occur because of hot lava mixing with sea water.
The map at right shows some of the active and potentially active volcanoes along the west coast of the US. In Washington State, Mount Baker, Glacier Peak, Mount Rainier, and Mount St. Helens have been active within the last 2,000 years. As the population around each volcanic area grows the possibility of a disastrous eruption increases. The United States Geological Survey (USGS) has a Cascades Volcano Observatory (CVO) that studies these and other volcanoes. Geologists at the CVO monitor for signs of activity, assess hazards and gather scientific information to warn the public of impending eruptions.
Cascade volcanoes are the composite type and build very steep sides. These volcanoes also have very thick magmas that tend to hold in expanding volcanic gases and steam. Tremendous pressure builds up inside the volcano and often leads to explosive and dangerous eruptions.
Scientists have developed a theory called "Plate Tectonics" to explain why volcanoes are usually found along the edges of continents and in mountains beneath the oceans. In this theory the continents float on the surface of the Earth on a continental plate and slide, collide or push other continental plates. The heat and pressure from this movement causes rock deep within the Earth to melt (magma) and force its way to the surface to create volcanoes. Plate movements are also believed to cause earthquakes. It is believed that at one time in the distant past all of the plates formed one huge continent called Pangea. In the diagram the yellow lines indicate plate boundaries and the red lines mark areas of volcanic action. Click image for a larger view of Main Continental Plates diagram.
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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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