This paper will discuss volcanoes, where they exist, how they are formed, and how eruptions are caused.
For the people who live near them, volcanoes occupy a large part of the history and mythology. The ancient Romans told of Vulcan, the god of fire and metals who kept his blacksmith shop beneath certain mountains. The Hawaiians believed that Pele, the hot-tempered goddess, was responsible for the formation of their islands. When Pele argued with her sister, her foot-stamping would cause earthquakes, then she would use her magic stick to dig out the craters from which the lava flowed (Mayberry 1997).
However, science was slow to catch up to the importance of volcanism in the evolution of the Earth. In the 18th century, there was a major school of thought that held that molten rocks and volcanoes were merely accidents caused by burning coal seams.
Geologists today realize that the process involved in the creation of a volcano is complex and profound, resulting from the thermal evolution of planetary bodies and plate tectonics. It is difficult for heat to escape from large bodies by conduction or radiation. Instead, partial meltdowns and the buoyant rise of magma are major contributors to the process of heat flux from inside the Earth. Volcanoes are the surface manifestation of this thermal process, which starts deep inside the Earth, and which hurls its results in the form of ash and fire, high into the atmosphere (Decker and Decker 1981).
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What is a Volcano?
The term volcano can refer either to the landform created by the solidified lava and fragmented volcanic debris that build up near the vent, or to the vent itself, from which the magma erupts to the surface. For example, one could refer to the large lava flows that erupted from the Kilauea Volcano in Hawaii, with the word volcano meaning a vent. However, one could also say that Kilauea is a gently sloping volcano of modest size as volcanoes go, and in this case the reference would be to the landform.
By a broad definition, all igneous rocks come from volcanoes. If igneous rocks harden from magma that has not gotten to the surface, they are referred to as intrusive igneous rocks, and this process is called plutonism. Igneous rocks that cool and harden at the Earth’s surface are called extrusive igneous rocks, and are unequivocally products of volcanism.
Types of Volcanoes:
Volcanoes can be classified by their eruption habits, and are generally arranged by progressive increases in the explosiveness of their eruptions. The type of eruption also is significant in the evolution of a volcanic landform, which forms an important link between eruptive habit and volcanic structure.
In classifying eruptions, volcanic activity and volcanic areas are commonly divided into six major types by order of increasing degree of explosiveness: 1) Icelandic; 2) Hawaiian; 3) Strombolian; 4) Vulcanian; 5) Pelean; and 6) Plinian (Columbia Encyclopedia 1993).
The Icelandic type is characterized by emissions of molten basaltic lava that flow from long parallel fissures. Outpourings like that frequently build lava plateaus. The Hawaiian type is similar to the Icelandic, except that fluid lava flows from the top and radial fissures to form shield volcanoes. This type of volcano is also referred to as “basaltic” (McCready 1994).
The Strombolian classification involves moderate bursts of expanding gases that eject clouds of incandescent lava in cyclical or nearly continuous small eruptions. Because of these small intermittent eruptions, Stromboli Volcano (off the western coast of Italy) has been called the “lighthouse of the Mediterranean”.
... Volcanic Materials Three different types of materials may erupt from an active volcano. These materials are lava, tephra which are rock fragments, and gases. The type ... by the release of volcanic gas. The sudden escape of high-pressure volcanic gas from magma is the driving force for eruptions. Gases come from the magma ...
The Vulcanian type of volcano, named for the island of Vulcano that is next to Stromboli, generally involves moderate explosions of gas which contain fine volcanic particles (volcanic ash).
This mixture creates dark, turbulent eruption clouds that rapidly rise and expand in convoluted shapes.
The Pelean type is associated with explosive outbursts that produce dense mixtures of hot volcnic fragments and gas. It was named after the destructive eruption of Mt. Pelée in Martinique in 1902. The liquified slurry produced by Pelean-type volcanoes are heavier than air but are low in viscosity and pour down valleys and slopes at speeds of more than 60 miles per hour.
The Plinean type is a very violent type if eruption that is exemplified by the outburst of Mt. Vesuvius in AD 79 that killed the famous Roman scholar Pliny the Elder, which gave it the name. In this type of eruption, gases boiling out of gas-rich magma generate enormous, nearly continuous jetting blasts that rip apart and core out the magma column. The uprushing gases and volcanic fragments appear like a giant rocker blast straight up. Plinean eruption clouds can rise into the stratosphere, where they can hang for hours. Mt. St. Helens was this type of eruption, sometimes referred to as “andesitic” (McCready 1994).
Shapes of Volcanoes:
Geologists often classify volcanoes into four main shapes: cinder cone, composite, shield, and lava domes. Cinder cones are the simplest. They are formed by particles and blobs of lava which erupt from a single vent. Most cinder cones have a bowl-shaped crater at the summit and are seldom more than a thousand or so feet higher than their surroundings.
Composite volcanoes, or stratovolcanoes, are typically steep-sided, symmetrical cones built of alternating layers of lava flows, ash, cinders, and blocks. Some of the best-known volcanoes in the world are composite volcanoes, including Mt. Fuji in Japan, Mt. Shasta in California, Mt. Rainier and Mt. St. Helens in Washington, and Mt. Hood in Oregon. They typically have a crater at the summit that contains a central vent or a cluster of vents. The essential feature of a composite volcano is a conduit system through which magma rises to the surface.
Shield volcanoes are built almost entirely from fluid lava flows, which gradually build up a broad, gently sloping cone of flat, domical shape, with a profile like a warrior’s shield. Kilauea and Muana Loa are shield volcanoes.
... possible future eruptions (Decker 105). The Seven Different Types of Volcanoes The seven different types of volcanoes are: Submarine volcanoes; Ridges and vents; Shield volcanoes; Lava plateaus and ... some lava domes still contain enough gas to cause explosions (see figure 3). Composite volcanoes and strato volcanoes are typically steep sided, symmetrical cones of ...
Lava domes are formed by small masses of lava too viscous to flow far; therefore, the lava piles up around the vent, which hardens, then eventually shatters, spilling fragments down the sides. Mt. Pelée in Martinique and Mt. Lassen in California are lava domes (Information Please Almanac 1997).
Volcanoes affect humans in many ways. Their destructiveness is awesome, but the risk involved can be reduced by assessing volcanic hazards and forecasting volcanic eruptions. Volcanoes provide fertile soil, valuable mineral deposits, and geothermal energy. Over geologic time volcanoes recycle Earth’s hydrosphere and atmosphere and affect climate.
Columbia Encylcopedia (1993) “Volcanoes” pg. 38682
Decker, R. and Decker, B. (1981) Volcanoes
Information Please™ Almanac “Principle types of volcanoes” (1997)
Mayberry, C. (1997) “Volcanoes” Science Weekly V13;n11
McCready, S. (1994) “Volcanoes inside and out” Sea Frontiers V40;n5