Understanding the Basics of Stratovolcano, Composite, Cinder Cone, Shield and Super Volcanoes

Understanding the Basics of Stratovolcano, Composite, Cinder Cone, Shield and Super Volcanoes

Thesis Statement: There are several types of volcanoes, some of these are; stratovolcano / composite volcanoes, cinder cone volcanoes, shield volcanoes and super volcanoes.

I. A look at stratovolcano / composite volcanoes.
A. Stratovolcanoes, also known as composite volcanoes, are tall cone shaped volcanoes with steep sides made up of many layers of lava and ash.
1. How stratovolcanoes are formed.
2. Why stratovolcanoes have their shape.
3. An example of a stratovolcano eruption.
4. A look at the Mount St. Helens eruption.

II. An example of cinder cones.
A. Cinder cone volcanoes are the most common type of volcano.
1. How cinder cone volcanoes get their shape.
2. How cinder cone volcanoes are formed.
3. An example of a cinder cone volcano.
4. How erosion effects cinder cone volcanoes.

III. An example of shield volcanoes.
A. Shield volcanoes are the largest type of volcano with gently curved slopes.
1. The way some shield volcanoes are created.
2. An example of shield volcanoes.
3. How the Hawaiian Island shield volcanoes get their shape.

IV. An example of super volcanoes.
A. Super volcanoes are the rarest type of volcano.
1. How super volcanoes are rated.
2. An example of a super volcano.
3. How the Yellowstone super volcanoes was created.
4. The geography around the Yellowstone super volcano.

Understanding the basics of Volcanoes

The surface of the Earth is a dynamic place. Changes to the surface of the Earth can happen in many ways. Some of these ways are plate tectonics, earthquakes, floods and volcanoes. Volcanoes can be found all over the surface of the Earth as well as below all forms of water. There are several types of volcanoes, some of these are; stratovolcano / composite volcanoes, cinder cone volcanoes, shield volcanoes and super volcanoes.

Stratovolcanoes, also known as composite volcanoes, are tall cone shaped volcanoes with steep sides made up of many layers of lava and ash. Harper (2005) states, “In the first case, some volcanoes, like Mount St. Helens, result from the collision of two tectonic plates: an oceanic and a continental plate” (p.2). When two tectonic plates collide, one will rise over the top of the other, forcing the lower plate to melt in the mantel of the Earth. The lava formed by the collision of two tectonic plates is usually not as fluid as that of basaltic lava. This is why stratovolcanoes are tall with steep sides. Typically where the two tectonic plates collide there will be a chain of stratovolcanoes. Weisel & Johnson (1994) state, “Stratovolcanoes are found where shallow mantle currents descend” (p.32). The Cascade’s is an example of a mountain range having stratovolcanoes; Mount Adams, Mount Rainer and Mount St. Helens are just a few. Mount St. Helens showed that stratovolcanoes typically have a more violent eruption then shield volcanoes. Before the 1980 eruption of Mount St. Helens, Erickson (2001) says in his book, Quakes, Eruptions, and Other Geologic Cataclysms, “Mount St. Helens, in Washington state, was an old, dormant volcano with splendid, almost perfectly symmetrical cone” (p.67). There had not been a volcanic eruption in the continental United States for almost sixty years until May 18, 1980 when Mount St. Helens blew. Mount St. Helens was awakened by a series of earth quakes in mid March of 1980. After the series of earth quakes, steam and ash started coming out of the top of the volcano. Erickson (2001) states, “The eruption began when the north slope bulged out as much as four hundred feet, creating avalanches on the mountain’s upper flanks” (p.67). The bulge that formed by the middle of April was the most dangerous part of the volcano. The crater at the top of Mount St. Helens grew larger as the bulge on the north face also grew. This kept happening until late April when the ash and steam stopped coming out of the top of the volcano. With the top of Mount St. Helens not venting the steam and ash, the pressure in the volcano increased. The weight of the dirt and ice on the side of the volcano was enough to hold in the explosion. Typically a stratovolcano erupts through the vent at the top, due to the fact that this is the path of least resistance. On May 18, 1980 an earth quake of more then 5.0 struck Mount St. Helens causing a land slide releasing the pressure. Erickson (2001) states, “The explosion blew off the top third of the mountain and lofted a cubic mile of debris into the atmosphere” (p.67). Once the land slide hit, the vent was able to release a lateral blast of steam, ash and rock which also melted the ice on the top of the volcano. Than sliding down the side of the mountain gaining speeds in excess of seven hundred miles per hour. With the initial explosion over; the hot gases, ash and rock joined with water that was pressed out of the rock due to the explosion and created a pyroclastic flow. Pyroclastic flows are created in two different ways; the first is the collapse of the eruption column that is created when a stratovolcano erupts through the vent at the top, the second is what happened at Mount St. Helens, with the gravitational collapse of the north face of the volcano. The pyroclastic flow also picked up water from lakes and rivers it went through. Pyroclastic flows move extremely fast due to gravity and the gas and water in them that reduces the friction of the flow as it move across the ground. Though the collapse and pyroclastic flows happened rather quickly, the estimated death toll of the Mount St. Helens eruption was less then sixty. Weisel & Johnson (1994) state that, “Stratovolcanoes often provide ample warning before an eruption” (p.28). With the warning stratovolcanoes provide, the people in the danger zone have time to evacuate. This was true of the Mount St. Helens eruption.

Cinder cone volcanoes are the most common type of volcano. They are typically small with steep sides and are cone shaped. Cinder cone volcanoes do not have a specific type of lava associated with them. Erickson (2001) states, “They form by explosive eruptions that deposit layer upon layer of large amounts of pumice and ash” (p.77). This type of volcano can form just about anywhere, even on the outer sides or in the caldera of larger volcanoes. This type of volcano is typically formed by pyroclastic flows that build up over time. According to Luhr & Simkin (1993) in their book, Paricutin: The volcano born in a Mexican cornfield, “The cone has formed by successive layers of pyroclastic material and has an outer slope of 31-33 degrees” (p.287). Even though cinder volcanoes are built over time, they are usually short lived volcanoes compared to shield volcanoes and stratovolcanoes. Paricutin is an example of a cinder cone volcano, which formed over approximately ten years and then went dormant. A volcano is considered dormant when there is no activity present over a period of time. Paricutin is located in southern Mexico along with thousands of other volcanoes. Luhr & Simkin (1993) state, “This zone is called the Mexican Volcanic Belt (MVB), and its frequent eruptions and earthquakes result from northeastward subduction of the Cocos and Rivera plates beneath the southern edge of North America” (p.243). The birth of Paricutin has a unique story behind it. Paricutin began erupting in February of 1943 in a farmer’s field. Unfortunately Luhr & Simkin (1993) state, “The story of the birth of Paricutin volcano has been told somewhat differently by several people who claimed to have witnessed the beginning of the eruption, and it is now difficult to evaluate the truth of these different versions, although none differ fundamentally, except perhaps in small details and in the exact hour of the beginning of the phenomenon” (p.64). Though the birth of Paricutin is in question, its formation is not. This cinder cone volcano had two prominent vents; a north east vent and a south west vent. The eruptions of these vents were studied by many different people from 1943 through 1954. Depending on the study looked at, it depends on which vent was more active at the time. During the life of Paricutin the rim height of the volcano changed due to the collapse by eruptions and erosion. Once cinder cone volcanoes become dormant, they fall pray to erosion. This is not to say that erosion does not happen during its eruption cycle. Types of erosion that can happen to volcanoes are landslides or mud flows. Landslides occur when the ash on the steep side of the volcano loses its ability to adhere to the rest of the volcano. Mud flows occur when water and ash combine on a steep slope and slide down the surface until the friction increases enough to stop the flow or it reaches the bottom of the slope. Since this area of Mexico has so many cinder cone volcanoes of different ages it makes this area a great place for scientists to study them.

Shield volcanoes are the largest type of volcano with gently curved slopes. According to Weisel & Johnson (1994) in their book, Fire on the Mountain: The nature of volcanoes, “For many basaltic shield volcanoes, the story of their growth begins at the bottom of the sea” (p.48). This is how the Hawaiian Islands formed. The Hawaiian Islands are made up over one hundred islands, reefs, and shoals located in the Pacific Ocean not just the eight islands that we can see above the water. This chain extends from the island of Hawaii west ward for over one thousand five hundred miles. The reason the Hawaiian Islands are a chain is that the Pacific plate they are sitting on is move across a hot spot in the mantel of the Earth. Harper (2005) says in her book, The Mount St. Helens Volcanic Eruption, “The third way that volcanoes are formed is by hot spots-this is how the Hawaiian Islands were created. Hot spots are locations under the crust that remain hotter than the areas around them” (p.3). In the case of the Hawaiian islands the hot spot creates a lava dome of basaltic lava that pushed a vent through the crust which lets the lava escape. These hot spots create a bulge on the ocean floor. Until the vent is above the surface of the ocean, the formation of pillow lava will occur. Pillow lava happens when hot lava is cooled rapidly under water. The lava forms a crust that rolls itself into the shape of a pillow. This type of formation is not strong and will collapse under its own weight, creating sand like material. As the formation and collapse of the pillow lava reoccurs the base of the volcano will keep increasing. Once the vent reaches a point high enough above the ocean the basaltic lava will cool slower in the air giving the fluid lava more time to travel away from the vent. This is why shield volcanoes have gently curved slopes and are very wide. Another thing that helps carry lava away from the vent is rivers of lava that over time create a roof and become lava tubes. Lava tubes keep the lava heated and fluid so it can flow further down the slope to create new land. Since the bases of shield volcanoes are made up of sand like material they have a tendency to have land slides which decrease the size of the volcano. Also, as the shield volcano moves off the hot spot, or the bulge, the island created will sink slowly into the ocean. This will happen over a period of thousands of years.

Super volcanoes are the rarest type of volcano. Breining (2007) says in his book, Super Volcano: The ticking time bomb beneath Yellowstone National Park, “Defining super volcanoes begins with knowing how to categorize the size of an eruption” (p.140). Using the Volcanic Explosivity Index chart, a magnitude eight eruption or greater would be classified as a super volcano. A magnitude eight eruption would produce a minimum one thousand cubic kilometers of magma. Two such super volcanoes are Toba, on the Indonesian island of Sumatra, and Yellowstone, in Yellowstone National Park in Wyoming. Breining (2007) states, “Toba’s big blast occurred just seventy four thousand years ago, making it one of only two super volcanoes that may have been experienced by modern man’s immediate predecessors-the biggest bang our kind has ever heard” (p.158). After this eruption Toba created a caldera that filled with water and is now known as Lake Toba. Yellowstone also created calderas after its eruptions though they did not fill with water to form a lake. Breining (2007) says, “Since the cataclysmic eruption of two point one million years age, two other large eruptions have rocked Yellowstone, buried it in volcanic rock, and left behind huge calderas” (p.17). Yellowstone has erupted one point three million years ago and six hundred and forty thousand years ago. Even though Toba has erupted more recently, Yellowstone is the more active super volcano. The Yellowstone volcano is a hot spot volcano that is moving over a vent in the North American plate. The magma in Yellowstone is pooling under the North American crust which is causing Yellowstone’s unique geology. The magma that is pooling under the North American plate is made up of rhylite magma. Breining (2007) states, “Continental plates are made mostly of rhyolite. (Rhyolite is volcanic rock made of basically the same stuff as granite, but granite cools slowly underground to form large crystals)” (p.30). Rhyolite magma is not fluid like basalt lava. Since rhyolite magma is not fluid it will keep building up pressure and heat until the ground above can not contain it and it will then erupt. The ground above the magma shows the pooling process by rising and lowering depending on the amount of magma that has pooled. Geologists estimate that the Yellowstone plateau has risen seventeen hundred feet since the last eruption. The heat from the magma is what powers Yellowstone’s hot springs, mud pits and geysers. Breining (2007) wrote, “The most famously regular of Yellowstone’s geysers is, of course, Old Faithful, named by the Washburn expedition in 1870 for its predictable eruptions” (p.52). Most of the thousands of visitors to Yellowstone do not realize that it is a volcano even though there is hydrothermal activity all around them.

Some dormant and active volcanoes can be very picturesque though can become killers at any time. Over the years all of these volcanoes will become a shadow of what they are now due to erosion. People need to realize that where there is volcanic activity there is also the chance of a volcanic eruption.

References
Weisel, D., & Johnson, C. (1994). Fire on the Mountain: The nature of volcanoes, 25-83. San Francisco, CA: Chronicle Books.

Erickson, J. (2001). Quakes, Eruptions, and Other Geologic Cataclysms: Revealing the earth’s hazards, 57-84. New York, NY: Facts On File, Inc.

Breining, G. (2007). Super Volcano: The ticking time bomb beneath Yellowstone National Park. St. Paul, MN: Voyageur Press.

Harper, K. C. (2005). The Mount St. Helens Volcanic Eruption. New York, NY: Facts On File, Inc.

Luhr, J. F., & Simkin, T. (1993). Paricutin: The volcano born in a Mexican cornfield. Phoenix, AZ: Geoscience Press.