Where do most volcanoes form?
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It is A, i am doing this same homework
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Volcanoes are the result of the dynamic of Earth.\nEarth's crust is broken in many blocks (tectonic plates) that are in motion respect one another. Mostly of volcanoes are located along boundaries of adjacent plates.\nVolcanic eruptions near tectonic plates' boundaries are driven by the magma that ascend from deeply beneath the crust\n\nScientific predict volcanic eruptions by studying some geophysical and geochemical parameters:\n * increasing of the seismic activity near the volcano\n * changes in the quantity of gases (CO2, SO2, etc) expelled by the volcano\n * observed changes in magnetic or geoelectrical explorations: studying the surface of the volcano with magnetic geoelectrical methods.\n\nYou can read more at links below
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http://www.geology.sdsu.edu/how_volcanoes_work/subducvolc_page.html\n\nstratovolcanoes\n\nbut on land your answer is probably going to be basaltic
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Well, yeah, there is a relationship.  Most of our recent volcanoes occur near the boundaries of plates.  At mid-ocean ridges hot magma is welling up and cooling to make the plates on either side larger.  Iceland, with its many volcanoes, is on the Mid-Atlantic Ridge.  Where oceanic plates are pushed under other plates (subducted), they are forced deeper into earth where it's hotter, and the subducted plate tends to melt.  The lighter parts of the melt try to work their way toward the surface (because they're less dense, and, hence buoyant), and cause volcanoes such as those of the Pacific's Ring of Fire--Indonesian volcanoes, Japanese volcanoes such as Mount Fuji, Alaska's volcanoes, the Cascade Range in Washington State, and so on.\n\nSome volcanoes are associated with "hot spots", where a plume of magma flows toward the surface, but is not associated with a plate boundary.  The Hawaiian Islands are an example of such a hot spot.  Plate tectonics also affect things here.  The plate moves over the hot spot, continually making new islands.  The newest one hasn't reached the surface yet.  Hawai'i is the newest one that's above the surface, and as you move north and west along the Hawai'ian Archipelago, the islands are progressively older and, because they've had more time to be eroded, smaller.  Northwest of the archipelago are the Emperor Seamounts, islands that were eroded until they no longer reach the ocean's surface.
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The largest volcano crater on earth is the Yellowstone crater.\nhttp://search.yahoo.com/search?p=+Yellowstone+Mega+volcano+crater&prssweb=Search&ei=UTF-8&fr=ytff1-&x=wrt
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There are volcanoes because of the interaction between multiple plate boundaries which causes the magma in the upper mantle (from beneath the crustal lithosphere) to rise, erupt and form lava at the surface.  \n\nVolcanoes tend to occur at either subduction zones, rifts or at hot spots.  Subduction zones are located at convergent oceanic/oceanic or oceanic/continental plate boundaries, where the denser oceanic crust sinks into the mantle beneath the other plate.  The minerals in the subducted slab dehydrates through metamorphic processes, which releases fluids to the mantle wedge to produce the melt (magma), which erupts to form volcanoes.  \n\nFor rifts, during the early stages of continental rifting (where continents are pulled apart because of extensional forces), volcanic activity can occur because crustal extension can allow magma to rise and intrude further up the crust.  Therefore, volcanic activity can occur in such situations.  eg: East African Rift. \n\nIn mid oceanic ridges, the divergent motion of oceanic plates allows mantle material to rise from below.  This tends to form a submarine ridge rather than a volcano.\n\nHowever, volcanoes can occur within the plate.  These are usually called hotspots, where a plume (a plume of anomalously hot magma) rise from deep within the mantle (probably as deep as the core-mantle boundary) to the surface.  These are stationary and therefore, it can leave a trail of volcanoes as the lithospheric plates move over the mantle.  Eg: Hawaii-Emperor seamount chain.
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From the web site link below:\n\nObsidian occurs only where geologic processes create volcanoes and where the chemical composition of the magma is rich in silica. Obsidian-bearing volcanoes are typically located in or near areas of crustal instability or mountain building. In North America, obsidian is found only in localized areas of the West, where the processes of plate tectonics have created geologic conditions favorable to volcanism and the formation of obsidian. Obsidian typically forms near the end of a volcanic cycle and is often associated with domes of volcanic rock, such as the hills of Glass Buttes, Oregon.
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This from the USGS has all the answers you are looking for:\nhttp://volcanoes.usgs.gov/Products/Pglossary/ShieldVolcano.html\n"Volcanoes with broad, gentle slopes and built by the eruption of fluid basalt lava are called shield volcanoes. Basalt lava tends to build enormous, low-angle cones because it flows across the ground easily and can form lava tubes that enable lava to flow tens of kilometers from an erupting vent with very little cooling. The largest volcanoes on Earth are shield volcanoes. The name comes from a perceived resemblance to the shape of a warrior's shield."\nExamples of shield volcanoes are the volcanoes in the Hawaiian island chain, including Kilauea, Mauna Loa, Haleakala, etc.\nHope this is helpful :-)
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A volcano is a geological landform usually generated by the eruption through a planet's surface of magma, molten rock welling up from the planet's interior. Other forms of volcano include ice volcanoes (particularly on some moons of Jupiter, Saturn and Neptune) and mud volcanoes. On Earth, volcanoes tend to occur near the boundaries of crustal plates. There are Diff types of volcanoes:\n1)Extinct--lowest possible chance of erupting.Whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.\n\n2)Dormant --those that are not currently active but could become restless or erupt again. Confusion however, can arise because many volcanoes which scientists consider to be active are referred to as dormant by laypersons or in the media.\n3)Active--it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time.\n  \nIt is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years. The Smithsonian Global Volcanism Program's definition of 'active' is having erupted within the last 10,000 years.So A volcano may occur anytime as earthquakes do.
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Volcanoes occur in hotspots where the earths crust is thin, most often around subduction zones, or areas where the tectonic plates are colliding and one plate is sliding under the other. There are other places where volcanos or hotspots occur, such as Hawaii, but in that case the chain of islands known as Hawaii are being pushed north by the movement of the tectonic plate there. In fact right now there is another island being built beneath the ocean just south of the main island. The earth is slowly cooling though events such as these. Beneath the crust the magma is under intense pressure and it has to escape somewhere, so it does escape at these weak places in the earths crust which we call volcanos.\n\nOf course I could have just plagarized my answer like  mikah_nine, but instead I opted to answer from my own knowledge and even include a source.
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1st answerer is good on location.  Difference between the types are due to the composition of the lava and where they occur.\n\nShield volcanoes, such as those found in Hawaii, have magma that is very low in silica, high in basalt (often referred to as "basaltic" volcanoes).  This means that the lava flows easily, instead of exploding.  That is why there is not a steep slope to the volcano and the biggest threat is just to remain out of the lavas way.\n\nStrato-volcanoes (Mt. St Helens) are found in subduction zones and produce a lava that is described as Andesitic (named after the Andes Mountains in South America).  This lava is high in Silica, which makes it much more viscous, and therefore, more explosive.  This explosiviness defines the shape of the cone, with much steeper sides.  With these types of volcanoes you have the explosive lava threat, but you are also threatened by pyro-clastic flows, and lahars.\n\nCinder cones are the same, compositionally as strato-volcanoes, but are not always located in a subduction zone, and are usually quite alot smaller.
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Like most of the interior of the earth, the movements and dynamics of magma are poorly understood. However, it is known that an eruption usually follows movement of magma upwards into the solid layer (the earth's crust) beneath a volcano and occupying a magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet surface as lava, or the rising magma can heat water in the surrounding landform and cause explosive discharges of steam; either this or escaping gases from the magma can produce forceful ejections of rocks, cinders, volcanic glass, and/or volcanic ash also known as tephra. While always displaying powerful forces, eruptions can vary from effusive to extremely explosive.\n\nMost volcanoes on the land are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust. As the oceanic crust 'subducts', it is forced into a higher-temperature region called the asthenosphere, and melts. Friction between these moving plates may also be a cause of melting. The reduced density of the melting rock forces the newly formed magma to rise. As the magma rises through weak areas in the continental crust it may eventually erupt as one or more volcanoes. For example, Mount St. Helens is found inland from the margin between the oceanic Juan de Fuca Plate and the continental North American Plate. The oceanic crust is normally a highly-'mafic' (iron and magnesium rich) rock, but as it rises through granitic (more silicic) continental plates, it may cause 'partial melting' of both mantle rocks and the plate rocks, producing magmas of varying silicity. Volcanic arcs forming near subduction zones, on the edges of continental plates, usually form high-silicity (high-viscosity) stratovolcanoes. Examples include the South American Andes and the Cascade range.\n \nA volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame. The truth is that a volcano seldom emits either smoke or flame, although various combinations of hydrogen, carbon, oxygen, and sulfur do sometimes ignite. What is mistaken for smoke consists of vast volumes of fine dust (called volcanic ash), mingled with steam and other vapors, chiefly sulfurous. Most of what appears to be flames is the glare from the erupting materials, glowing because of their high temperature; this glare reflects off the clouds of dust and steam, resembling fire.\n\nPerhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular shape, formed by a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.\n\nVolcanic cones are usually smaller features composed of loose ash and cinder, with occasional masses of stone which have been tossed violently into the air by the eruptive forces (and are thus called ejecta). Within the crater of a volcano there may be numerous cones from which vapours are continually issuing, with occasional volleys of ashes and stones. In some volcanoes these cones form lower down the mountain, along rift zones or fractures. When the cone is eroded these rifts or lava filled fractures remain as radial near vertical dikes of volcanic rock. For example the radiating dikes at Shiprock in NW New Mexico.
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The opening at the top of a volcano is called a crater.\n\nA caldera is where a magma chamber once existed but collapsed during an eruption. Calderas are much larger than craters.
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Plates collide in areas called subduction zones.  At these zones, oceanic crust is subducted underneath continental crust due to its higher density.  Subduction results in great amounts of heat and pressure building up in the zone.  When these factors become strong enough, they are forced to the Earth's surface in the form of volcanoes.  That is why volcanoes typically occur along plate boundaries (e.g., the ring of fire).  However, volcanoes are not exclusive to plate boundaries.  They can also form over hot spots within the interior of the Earth.  The islands of Hawaii are an example.  The islands are formed on the Pacific Plate.  The plate is moving in a northwest direction, while the hot spot underneath the crust is stationary.  As the plate moves over the hot spot, volcanic eruptions occur.  The result is a new island being created.  The islands are actually the products of a series of shield volcanoes that grow in age as one moves to the northwest.
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Mt. St. Helens was formed from the subduction of the Juan de Fuca Plate underneath the North American Plate.  Subsequent eruptions, producing basalt and andesitic lava flows, have either reshaped or built up the volcano over time.
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Volcanoes occur because of cracks or gaps in the Earths crust.Heat rises ,so the hot molten rock comes through the gaps and builds up.WHen the lava builds up enough to come through the last bit of crust the emense pressure makes a volocano.Explode!
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Curiously, I could not find a comprehensive listing.  This site might be helpful, however, or at least give you a start.\n\nhttp://dir.yahoo.com/Science/Earth_Sciences/Geology_and_Geophysics/Volcanology/Volcanoes