Most active volcanoes exist in "plate boundary" regions. As one plate passes underneath another plate, it pushes that plate upwards, which creates mountainous regions. Since the plates are moving above magma, when the magma comes through a crack to the surface, a volcano is created.
One of the most famous volcanoes in the United States is Mt. St. Helens in Washington, which erupted in 1980. This eruption killed 57 people and destroyed 250 homes and 185 miles of roadway. Its eruption caused an avalanche that shortened the elevation by 1300 feet. The volcano was closed to all except researchers for several years, and was re-opened to climbers in 1987 until 2004, when the volcano showed renewed signs of activity. It was again opened in 2006.
The erupation impacted the climate in the 80s due to the debris cloud that spread over much of western United States. Currently, there are earthquakes and occasional steam plumes associated with the volcano, although the risk of a major eruption is deemed low.
Here are several sites with more information:
http://pubs.usgs.gov/gip/volc/tectonics.htmlhttp://sciencespot.net/Pages/kdzethsci2.html
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