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Origin And Occurrence Of Volcanism In The Usa

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Volcanism exists all over the world. From the freezing lands of Alaska to the mountains of South America, volcanism continues to help shape the land in which we live in. Our origins lie in volcanic activity which took place billions of years ago as the Earth was still forming. To this day, it continues in areas where plate tectonics meet in a convergent zone and through subduction, allow magma to reach the surface. In other areas, volcanism occurs after a plate has moved. The United States in noteworthy in that it had significant volcanism in its past that transcends into the present and will continue to do so in its future.

What exactly is volcanism? Volcanism, according to "Physical Geology," is "volcanic activity, including the eruption of lava and rock fragments and gas explosions" (3). The structures most associated with volcanism are volcanoes, which are "a hill or mountain constructed by the extrusion of lava or rock fragments from a vent" (3). In the United States, volcanoes generally fall in one of three regions; the Alaskan Region, the Hawaiian Region and the Pacific-Northwest Region (6). Each region has its own unique characteristics. The theory of plate tectonics states that the Earth's surface is broken down into little patches that continually shift, pass and collide with each other at a very slow rate of only about an inch a year. Volcanoes are created when two plates collide together, and that is called a convergent zone. Since the oceanic crust is thinner than the continental crust, it goes "under" the continental crust and the water seeps up into the holes on surface and helps the magma travel up the surface. Thus, volcanoes are only usually only found near a cost or anywhere two plates meet. Volcanoes in Hawaii are a bit different in that they are formed as the Pacific plate is moving northwest. (2)

Alaskan volcanoes are very plentiful, with over 100 areas exhibiting volcanic behavior (5). "Most of these volcanoes are located along the 2,500 km-long (1,550 mile-long) Aleutian Arc, which extends westward to Kamchatka and forms the northern portion of the Pacific "ring of fire". The ring of fire is a term used to describe the land surround the pacific plate which is where most volcanoes occur. Other volcanoes which have been active within the last few thousand years exist in southeastern Alaska and in the Wrangell Mountains." (5) Eruptions in the Aleution Arc are very commonplace. "Those in the largely unpopulated western arc often go unremarked by all but volcanologists. The remote volcanoes are potentially hazardous, however - jet airplanes which enter eruption clouds often are severely damaged, and sometimes lose all engines temporarily." (5). Also, "at least 20 catastrophic caldera-forming eruptions have occurred in the past 10,000 years; the awesome eruption of 1912 at Novarupta in the Katmai National Monument is the most recent. Scientists are particularly concerned about the volcanoes whose eruptions can affect the Cook Inlet region, where 60 percent of Alaska's population lives." (6)

The Hawaiian Region is unique because the volcanoes there are all shield volcanoes and for the most part, the explosions are safe. Mauna Loa and Kilauea are the two major volcanoes on the island because they are the most recent ones. What also makes Hawaiian volcanoes interesting is that when they erupt, they lava cools up and expands the property of Hawaii. "Hawaii is essential lava" and it just keeps growing and growing. Volcanoes in this region are usually shield volcanoes that have very frequent eruptions. Of an interesting note, eruptions are frequent enough that people can even watch them from a safe distance. Since the magma is mostly composed of basalt, explosions are extremely rare for Hawaiian volcanoes. According to the U.S. Geology Survey, the Hawaiian Islands are critical for helping scientists understand how volcanoes work.

"Hawaiian volcanoes erupt at their summit calderas and from their flanks along linear rift zones that extend from the calderas. Calderas are large steep-walled depressions that form when a volcano's summit region collapses, usually after a large eruption empties or partly empties a reservoir of magma beneath the volcano. Rift zones are areas of weakness within a volcano that extend from the surface to depths of several kilometers" (6)

Of interesting note is that Mauna Loa rises more than a thousand feet and is taller than Mount Everest. The most recent eruption was in 1984. Just like the Energizer bunny keeps on "going and going" Kilauea keeps on growing and growing and "about 90 percent of Kilauea's surface is covered with lava flows that are less than 1,100 years old." (6). Most likely, activity here will continue as long as the pacific plate keeps on moving.

The Pacific-Northwest Region goes all the way from Washington State to California. The volcanoes in this region are created as the result of plates that used to converge with each other. Volcanic activity varies from active to ancient. The example that almost everyone has some understanding of is Mount St. Helens. Mount St. Helens was thought to have been extent before it erupted in 1980. The eruption was so sudden and powerful that as the debris solidified it actually became a taller volcano. "The 1980 eruption of Mount St. Helens in southwest Washington dramatically illustrated the type of volcanic activity and destruction these volcanoes can produce"(1). Thing began with an earthquake with a "magnitude 5.1 earthquake struck beneath the volcano at 08:32 on May 18, setting in motion the devastating eruption" (1). Also, "within



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