Volcanoes

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Effects of Volcanoes
The plates, which are about 20 miles thick, make up the Earth's crust and are a chief cause of volcanic activity. These plates are always in motion. They move very slowly; however, at times, they bump into each other. These movements put a lot of pressure on the surface rock. Volcanoes obtain their energy from such movement and pressure. Volcanoes form at the boundaries of these plates where two types of movement occur: two plates will collide with each other, or the plates will move apart from each other. Some of these plate layers are cooled and are made up of rigid rocks. The effects on the landscape are lava that releases onto the Earth's surface. When that lava comes to the Earth's surface, it is red hot, and sometimes the temperature is more than 2012 degrees Fahrenheit. Fluid lava flows swiftly down a volcano's slopes. Sticky lava flows more slowly. As the lava cools, it hardens into many different formations on the landscape. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe. Stickier lava cools into rough, jagged sheets of rock called aa. Pahoehoe and aa cover large areas of Hawaii, where the terms originated. The stickiest lava forms flows of boulders and rubble called block flows. It may also form mounds of lava called domes. Volcanoes, when exploded, can send ash, millions of rock particles, and volcanic gas tens of miles into the air. The resulting ash fallout can affect large areas hundreds of miles downwind. Gas pours out of volcanoes in large quantities during almost all eruptions. The gas is made up particularly of steam but may also include carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano's magma, but some steam may also be produced when rising magma heats water in the ground. Effects on the living population of the area: eruptions pose direct and indirect volcano hazards to people and property, both on the ground and in the air. Direct hazards are pyroclastic flows, lava flows, falling ash, and debris flows. Pyroclastic flows have hot ash, rock fragments, and gas in them. These flows of hot ash, rock fragments, and gas are deadly because of their high temperatures of 850° C. Also, they can travel down the sides of a volcano at speeds of up to 200 miles per hour. Indirect hazards are famine, disease, and economic loss. The ash and dust from a volcanic eruption can cause crops to fail, and volcanic ash can clog machinery, damage roofs, and short-circuit electrical equipment. The economic loss can be enormous.
Resources:
USGS "Types of Volcanic Eruptions" http://pubs.usgs.gov/gip/volc/eruptions.html
USGS "MSHNVM and CVO" http://vulcan.wr.usgs.gov/Volcanoes/MSH/ImageMaps/SWWash/swwash_map.html
"MSH Map, Cross-sections, and Time-depth Plot" http://spike.geophys.washington.edu/SEIS/PNSN/HELENS/mshfigs.html
"Simplified Eruptive History of Mount St. Helens" http://vulcan.wr.usgs.gov/Imgs/Gif/MSH/Graphics/EruptiveHistory/msh_eruptive_stages.gif
Edward W. Wolfe and Thomas C. Pierson, 1995, "Volcanic-Hazard Zonation for Mount St. Helens Washington, 1995": USGS Open-File Report 95-497 http://vulcan.wr.usgs.gov/Volcanoes/MSH/Hazards/OFR95-497/OFR95-497.html
"Encart Reference, Volcano, Types of Volcano" © 1993-2000 Microsoft Corporation http://encarta.msn.com/find/concise.asp?ti=761570122&sid=20#s20
"Volcano," Microsoft® Encarta® Online Encyclopedia 2000 © 1997-2000 Microsoft Corporation http://encarta.msn.com/find/concise.asp?mod=1&ti=761570122&page=3#s22vb
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