The word Volcano comes from the little island of Vulcano in the Mediterranean Sea off the coast of Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan - the blacksmith of the Roman gods. Volcanoes are mountains, but they are very different from other mountains. They are not created by folding or faulting or erosion. Instead volcanoes are built by the accumulations of their own eruptive products - lava, gas, ashflows and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the earth. Driven by buoyancy and gas pressure, the molten rock (called Magma), which is lighter than the surrounding solid rock, forces its way upward and may ultimately break through zones of weaknesses in the Earth’s crust.
As the rising magma nears the Earth’s surface, pressure decreases, which causes the gases in the magma to expand. Once magma is erupted, it is called Lava. If the ground breaks open an eruption occurs, and the molten rock may pour from the vent as non-explosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments. Larger fragments fall back around the vent, and accumulations begin and the mountain takes its shape. Some of the finer ash particles may be injected miles into the atmosphere and are carried many times around the world by stratospheric winds before settling down.
TYPES OF VOLCANOES Calderas - The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth’s surface. When such a large volume of magma is removed for beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a Caldera. Usually found at the top of a volcanoe. Cinder Cones - Cinder cones (Scoria 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. These are the most explosive.
Shield Volcanoes - Shield volcanoes are the largest on Earth. The Hawaiian volcanoes are shield volcanoes. They are made up of the build up of success lava flows. Because of this they are not steep, their slopes are very gradual. There eruptions are not explosive. Composite Volcanoes - Composite (Strato-volcanoes) volcanoes comprise the largest percent (60) of the Earth’s volcanoes. They are typically steep sided and are built from alternating layers of lava and cinders. These volcanoes can also be very explosive. Some of the world’s most majestic and beautiful mountains are this type of volcano. Mount Fuji in Japan and Mount St. Helens are composite volcanoes. Continental Volcanoes or Volcanic Arcs - A range of Volcanoes created when an oceanic plate subsides under a continental plate. The old volcanoes on the west coast of North America are of this type.
Island Arc Volcanoes - These volcanoes form a range of islands. They occur when two oceanic plates collide and one subsides under the other. Many Pacific islands are Island Arc volcanoes. Hot Spot Volcanoes - Like in Hawaii, when a series of volcanoes are created due to the constant eruption of a Hot Spot. Lava Plateaus - Not all eruptions have to form mountains. If the flow of lava is very slow and quiet the lava will literally flood the surrounding area and create broad flat plateaus. Iceland is a good example. There are many more kinds, for example if a volcano is formed under water it is called a Submarine Volcano or Seamounts. Volcanoes that are made under a ice or a glacier are called Tuyas. If the lava is very sticky and does not flow far from its vent, it creates steep mounds called Lava Domes.
Products of volcanic eruption http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/products_page.html
In some instances the magma never reaches the surface or breaks the ground above it. Huge reservoirs of magma are created to form ‘Batholiths’ or smaller reservoirs called ‘Laccoliths’. Both these are horizontal in structure. Vertical creations are called ‘Sills’. When these cool, they form intrusive igneous rock and are only discovered through erosion or mining.
There is a great range in the explosivity of volcanic eruptions. Many eruptions are relatively quiet and are characterized by the calm, nonviolent extrusion of lava flows on the earth's surface. If the material is fluid in nature (solid and semi-solid) this type of flow is called Pryoclastic Flow (the typical hot lava that flows down a volcano). Other eruptions, however, are highly explosive and are characterized by the violent ejection of fragmented volcanic debris, called tephra, which can extend tens of kilometers into the atmosphere above the volcano. The type of volcanic eruption depends on a variety of factors, which are ultimately linked to the composition of the magma (molten rock) underlying the volcano.
The controlling factors on explosivity are viscosity, temperature, and the amount of dissolved gases in the magma. Nonexplosive eruption Eruption of the Pu'u O'o volcano on the east zone of the larger Kilauea volcano, Hawaii. http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/Puuoo84_page.html
The amount of lava that is generated over a period of time is called the effusion rate.
Explosive Eruption: Eruption of a voluminous plume of tephra is typical of explosive eruptions, as demonstrated in the 1980 eruption of Mt. St. Helens. These eruptive tephra plumes can travel tens of kilometers into the stratosphere. http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/msh5_page.html
Gas How does the amount of dissolved gases affect the explosivity of volcanoes? The dissolved gases in the magma provides the force for explosive eruptions. As magma rises toward the surface, dissolved gases in the liquid rock begin to come out of solution (called exsolution), and bubbles begin to form in the magma. The magma gets “frothy”. Not all magma has a lot of dissolved gases. The more the gas the more explosive the volcano.
Explosive eruptions, which get started by exsolution of gases, can be made more dramatic by sudden decompression, which lowers the confining pressure on the magma. Decompression is when the volcano blows its lid so to speak. Decompression is similar to what can happen to a bottle of pop when the cap is removed: the sudden release of pressure can cause the CO2 to come out of solution explosively and rapidly, resulting in the pop to spray out of the bottle. This happens when a volcanic mountain suddenly breaks open or the magma plug from a previous eruption gets pushed out of the way. Mt St. Helen’s is an example. An earthquake or tremour can open up the ground releasing this pressure and a violent eruption occurs.
Viscosity How does viscosity affect explosivity? Viscosity is defined as the ability of a substance to resist flow. In a sense, viscosity is the inverse of fluidity. Magma may also contain a material that is crystal in structure called silca. Generally speaking, magma with a higher silica concentration has a higher viscosity. It resists flow up through the vent. The lava here is thick and sticky and gases cannot escape. Granitic rock is rich in silica. Click here to read an excellent article on the study of why some volcanoes erupt violently.
Temperature The higher the temperature, the more fluid a substance becomes, thus lowering its viscosity. The lava in this case is thin and runny and gases escape easily. Think of engine oil!
Every year about 60 volcanoes erupt, but most of the activity is pretty weak. How do volcanologists measure how big an eruption is? There is not any single feature that determines the "bigness", but the following eruption magnitude scale - called the Volcanic Explosivity Index or VEI - is based on a number of things that can be observed during an eruption. According to this scale, really huge eruptions don't happen very often, luckily!
What is the relationship of volcanic eruptions to plate tectonics? Volcanos tend to be highly concentrated near convergent plate boundaries (also called subduction zones). As one oceanic crustal plate descends under the other plate, it gets very hot as it enters the asthenosphere in the upper mantle and the rock melts.
Island arc formed by oceanic-oceanic subduction. Volcanic arc formed by oceanic-continental subduction http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html
Volcanoes can in one of the following three states: Active, extinct or dormant. You can figure these out! Needless to say Volcanoes can be dangerous to human population. They also bring excellent soil for farming. The advantageous and disadvantages of living near a volcano are beyond this slide show. Sounds like a good essay. THE END!