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 some at times bump in to each other. These movements put a lot of pressure on the surface rock. Volcanoes obtain their energies from such movement and pressure. Volcanoes form at the boundaries of these plate where two types of movement occurs, two plates will collide with each other or the plates will move apart from each other. Some of these plates layers are cooled and are made up of rigid rocks.


The affects on the landscape is lava that releases onto the Earth\'s surface. When that lava comes to the Earths 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, the lava 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.
Affects 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 are fast up to 250km/h or sometimes even greater. Lava flow by the other hand moves much slower than pyroclastic flows. Also lava flow is more life threatening but can produce a lot of property damage as well as economic loss. Volcanic ash can collapse roofs and damage crops, especially if they become wet from rainfall. Debris flows called lahars that are mixtures of volcanic debris and water made from melted snow or ice or heavy rainfall. If anything comes in the path of lahar can destroy and also travel quickly through valleys.


Indirect hazards are usually non volcanic effects that accompany or follow eruptions. Examples are rainfall-caused debris flow, and post eruption disease and famine. Tsunamis are large seismic sea waves generated by sudden movement of the seafloor. This sudden seafloor movement can be caused by a large earthquake or by the collapse of an island volcano during or after an eruption. Tsunamis can devastate low-lying coastal areas and can be deadly if people living in such areas are not evacuated. Indirect hazards also include volcanic deposits from large eruptions. These deposits can blanket farm fields and grazing lands, leading to the loss of crops and livestock and ultimately to the starvation of people dependent on them for life. During the period from the 17th century to the 19th century, tsunamis and posteruption starvation and disease caused most eruption-related deaths.
Possible future activity can be noticed sometimes by predicting the next eruption of an active or a dormant volcano. Scientists generally expect a volcano active if it has erupted one or more times in historical time. The thought is by the less poor, however, because written history is much more longer for volcanoes in some parts of the world, for instance in Japan and Italy, than in other parts, such as the United States and New Zealand. Dormant volcanoes are currently inactive but considered by scientists to have potential for future eruption. Long dormant are defined as extinct because volcanoes believed to lack potential for renewed activity. Other possible future activity can be predicted by scientists taking measurements of events leading up to possible activity due to eruptions. Some examples are earthquakes, ground movement, and the releases of gases.

Table
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Distance (via river channels) Estimated travel time, hr:min Estimated travel time, hr:min
from Mount St. Helens, km (mi)
---------------------------------------------------------------------------------------------
NFT SFT,P,M,K
---------------------------------------------------------------------------------------------
10 (6.2) 0:37 0:11
20 (12.4) 1:08 0:30
30 (18.6) 1:37 0:54
40 (24.9) 2:16 1:21
50 (31.1) 2:53 1:49
60 (37.3) 3:27 2:20
70 (43.5) 3:48 2:53
80 (49.7) 4:43 3:31
90 (55.9) 6:36 4:18
100 (62.1) 8:50 5:12
---------------------------------------------------------------------------------------------

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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