Types of Volcanoes - Term Paper Example

Types of Volcanoes When people think of volcanoes, they typically think of a mountain in the process of blowing its top. In this situation, they envision red hot material gushing out from the center of a cone-like mountain heading straight for the sky as well as boiling up and over the sides to come rushing down the slopes at any unfortunate people who live down below. They generally consider volcanoes to be unpredictable beasts that can spit ash so high up in the sky that it can cloud an area for days after the event and the lava to be so hot and fast that it will quickly overtake anyone attempting to run from it and bury them in concrete. Part of the reason why people have these impressions are because some volcanoes do act in this way to some extent and Hollywood has done a good job of dramatizing these kinds of events without really explaining why the volcanoes do what they do. By retaining some of the mystery, the planet is given a mysterious quality that also helps to build the suspense of a film. At the same time, these are not necessarily true portrayals of what volcanoes do. There are several different types of volcanoes on our planet; the three major categories they fall into are the composite volcanoes (also called stratovolcanoes), shield volcanoes and cinder cone volcanoes. Each one of these has its own distinct shape, its own form of operation and its own type of output. It is the output and the type of material it releases that designates the type of volcano it is and that dictates the shape of the mountain it forms. While volcanoes still remain largely unpredictable even in the modern age, new technology has been developed that allow scientists to predict with greater and greater accuracy when a mountain is about to erupt and thus save potentially thousands of lives. However, since the science is new and not specific to the minute, it is still often mistrusted. The purpose of this paper is to explain what each of these types of volcanoes are by providing discussion of real-world examples and comparing them to the other types in hopes of developing a more realistic sense of the planet’s major dynamic forces. The stratovolcano is perhaps the most popular type of volcano in the typical person’s imagination because this is the most dramatic and usually presents itself in the classical cone shape. “A stratovolcano has steep sides with a distinguishing cone shape and is frequently composed of several different vents that erupt lava, sometimes in different ways” (Keller, 2010). A live example of a stratovolcano is the Mayon Volcano in the Philippines. This volcano is a part of the so-called Ring of Fire which surrounds the Pacific Ocean and marks areas of subduction of the oceanic plates. The Phillippine Islands lie at a point where at least three different plates are colliding. These are the Eurasian continental plate, the South China Sea plate and the Phillippine plate (American Museum, 2006). The plate that the mountain sits on is actually a fourth plate, a microplate, that is being squeezed out of existence. What makes the Mayon Volcano active is the pressure from the Phillippine Plate as it sinks beneath the microplate and the Eurasian plate in what is called the Bicol Arc (American Museum, 2006). As the heavier Phillippine Plate sinks beneath the other plates, water gets pressed out of it into the mantle above. The energy produced heats up the rock of the upper mantle, causing it to melt creating magma. Magma, or melted rock, is lighter than the rock around it and so it also rises (American Museum, 2006). The Mayon Volcano is one of the places where this magma breaks through to the Earth’s surface in what is typically a pyroclastic flow type of eruption, meaning the lava is fairly liquid and hot. Stratovolcanoes such as Mayon can erupt in several different ways. Pyroclastic flows are common. In this kind of eruption, billowing clouds full of ash and dust come pouring out of the mountain in what appears to be a giant explosion. “Enormous volumes of extremely hot gases, ash and rocks rush down the side of a volcano like an avalanche of sorts” (Keller, 2010). Other forms of eruptions common with this type of volcano but not necessarily present on Mayon are lava domes, in which very thick magma bubbles to the surface like the surface of a raindrop, leaving a large dome-shaped area on the side of a cone. Plinian eruptions occur when a tremendous amount of pressure builds up inside of a volcano until it bursts in a violent explosion that can be so severe it takes most of the mountain with it. The Mayon Volcano rises in a very graceful and symmetrical cone that stands about 2,460 meters high and whose upper slopes are approximately 35-40 degrees with a small summit crater at the top (Global Volcanism Network, 1993). The volcano has erupted at least 49 times since 1616 and has been threatening to erupt again. In December of 2009, lava was seen oozing down the sides of the mountain and more than 80,000 people were evacuated, but the mountain’s last major eruption was in 1993 (CNN, 2009). Pyroclastic flows and mudslides have been common since the last eruption and seimometers used to measure the movement of the earth have indicated an increased level of activity inside the volcano in recent months, indicating another large eruption is approaching. Unlike stratovolcanoes which can exhibit all three different types of lava flows, shield volcanoes typically only exhibit one type of behavior which results in a large, smooth, dome-like mountain – like the shield of a warrior lying on the ground. These volcanoes are different from lava domes such as were discussed above in that they are formed by very fluid lava flows that seep out of the ground from a central vent or from a series of vents to create the flattened mound that characterizes this type of volcano. The mountain doesn’t form quickly in a single eruption or group of eruptions and it isn’t as likely to erupt in a violent explosion. Instead, these kinds of volcanoes are built up over a long period of time. “They are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone” (Watson, 1997). There are numerous examples of shield volcanoes in the world, one of the most famous and largest being Mauna Loa on the Big Island of Hawaii. Unlike the Mayon Volcano, Mauna Loa does not lie along the Ring of Fire or any subduction zone. Instead, this volcano, and all of the active volcanoes in the Hawaiian Island chain, lies along what is known as a hot spot. Hot spots are located in areas where “a plume of magma rising from deep in the Earth’s mantle” weakens the crust and breaks through fissures in the rocks. “The upwelling of the hot magma creates volcanoes, and each individual volcano erupts for a few million years before the movement of the plate carries it away from the rising magma” (Cambridge Encyclopedia, 2010). It is because this magma comes from deep within the Earth that it is so hot and fluid and it is because the lava is so hot and fluid that it creates gentle-seeming mountains with rounded tops and relatively non-violent eruptions. Mauna Loa is long and rounded, but it is classified as one of the largest mountains in the world when one takes into account its base stands on the ocean floor which has itself been pushed down because of the weight of the mountain. The visible mountain rises more than 4 km above sea level, just short of its neighbor Mauna Kea, but it extends down into the sea further. “Its long submarine flanks descend to the sea floor an additional 5 km, and the sea floor in turn is depressed by Mauna Loa’s great mass another 8 km. This makes the volcano’s summit about 17 km (56,000 ft) above its base!” (Griggs, 2006). The volcano provides a base for most of the other Hawaiian Islands and their associated volcanoes, each of which are classified as different volcanoes because they have their own associated magma chamber in which the lava collects before being pushed out above the surface. Mauna Loa is at least as active as the Mayon Volcano, though, having erupted 33 times since documentation began on the mountain in 1843. Like Mayon, the volcano’s last significant eruption occurred decades ago, in 1984. Unlike Mayon, though, the volcano seems to be in a period of dormancy. There has been no increased seismic activity in the past few years, but the volcano’s history keeps it closely monitored and eruptions are always considered possible. Fortunately, Mauna Loa’s eruptions typically result in slow-flowing lava that can be outpaced at a normal human walking rate so, while damages can be great, human casualties from an eruption are generally small. The third major type of volcano is the cinder cone, which can also occur on the slopes of a stratovolcano or even on the smoother sides of a shield volcano. “Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone” (Tilling, 1985). Regardless of whether they are found on the slopes of a larger, different type of volcano or standing on their own in the middle of a field, cinder cone volcanoes almost always form a neat, compact cone-shaped hill around the central vent and rarely pose much risk to human population. An example of an active cinder cone volcano is the Cerro Negro cinder cone found in Nicaragua. Like the Mayon Volcano, the Cerro Negro was formed as the result of a subduction zone but not within the Ring of Fire. Rather than having an oceanic plate simply slipping under a continental plate, this region is characterized as more of a strike and slip movement, in which two plates are grinding against each other more than they are overlapping (Nicaraguan Plate, 2007). It can be surmised that this type of movement does not cause the same kind of heat and does not involve the same amount of water to help conduct the heat, thus creating volcanoes that are characterized more by the relatively cool temperature of the lava. Although these volcanoes can have the same tremendous energy as the stratovolcanoes, sending materials high into the air, the temperature of these materials means they begin to solidify before they hit the ground, creating a mound similar to mounds constructed by ants as they pile grains of sand on top of each other. The Cerro Negro has had lava flows at its base, but these are not the same as those seen in association with the Mayon Volcano or as plentiful in material addition as Mauna Loa’s eruptions. While Mauna Loa is thousands of years old and Mayon is at least hundreds, the Cerro Negro is a relatively new volcano which is common for cinder cone volcanoes. The birthday of this particular mountain is actually recorded as having occurred in 1850. Because it is almost always triggered by nearby earthquakes, the volcano is relatively easy to predict. There have been 23 eruptions of the volcano since its birth with the most recent being in 1999 (Seach, 2008). In spite of this extensive activity, the mountain rises only 675 feet from its base. Although this particular cone does produce lava, sometimes exhibiting beautiful yet unpredictable lava-fountains, it is not expected to have a long life nor is it considered particularly dangerous. All three of the volcanoes discussed here are currently classified as active, yet only one is considered active enough to necessitate recent evacuation. Although all three volcanoes are closely monitored to keep the public safe, again, it is only Mayon that is considered to pose a real threat to human life. Each volcano is formed of lava that leaks up to the surface of the Earth from below, but the type of magma that feeds the volcano and the forces at work underneath it are what determine the shape and the ferocity of the mountain. In both cases where subduction zones have led to the formation of volcanoes, the Mayon and the Cerro Negro, eruptions are characterized by explosive energy resulting in a cone-shaped mountain that varies in height depending on the type of volcano. The drier, cooler temperatures of the cinder cone keep it small while the wetter, hotter material of the stratovolcano quickly build to a massive structure. The hot spot volcano, by contrast, with its fully molten material, erupts in a much more gentle stream, providing those with mobility an opportunity to get away and resulting in a smoother, calmer mountainous structure. References American Museum of Natural History. (2006). “Why is a Volcano Here?: Mayon Volcano Explained.” Mayon Volcano Natural Park. Republic of the Phillippines Department of Environment and Natural Resources, Region 5. Available February 27, 2010 from Cambridge Encyclopedia. (2010). “Mauna Loa – Structure, Eruption History, Relationship with Kilauea, Hazards, Monitoring, Observatories.” Vol. 49. State University. Available February 27, 2010 from CNN World. (December 21, 2009). “Phillippines Volcano Eruption Likely Soon.” CNN. Available February 27, 2010 from Global Volcanism Network. (1993). “Summary of Recent Volcanic Activity.” Bulletin of Volcanology. Vol. 55, N. 4. Griggs, J.D. (2006). “Mauna Loa: Earth’s Largest Volcano.” USGS Hawaiian Volcano Observatory. Available February 27, 2010 from Keller, E. (2010). “What is a Stratovolcano?” Extreme Science. Available February 27, 2010 from “Nicaraguan Plate Movement Not Typical of Earthquake-Prone Areas.” Newswise. (February 14, 2007). Available February 27, 2010 from Seach, John. “Cerro Negro Volcano.” Volcano Live. (2008). Available February 27, 2010 from < http://www.volcanolive.com/cerronegro.html> Tilling. (1985). Volcanoes. USGS General Interest Publication. Watson, John. (1997). “Principal Types of Volcanoes.” USGS. Available February 27, 2010 from Read More