why do volcanos form near subduction zones?can anyone also draw a diagram to assist your explanation?
147972
Water.\n\nWater is one of the key parts of the formation of magma in subduction zones. The water that is in the subducting plate lowers the melting point of the rock by a few hundred degrees C. The water in the subduction plate will boil and migrate upwards into the overlying wedge of mantle rock where it will reduce the melting point of the overlying mantle. This magma will migrate upwards and form a volcano. \n\nAnother part of this process is that within rock there are many different minerals, all with different melting points, so the ones that melt at the lowest temperatures tend to form magma first, so the resulting magma comes from a partial melt of the source rock.\n\nHere's another diagram (I'd draw you my own but I can't post anything but text here):\nhttp://earth.leeds.ac.uk/dynamicearth/subduction/cordillera_islandarc.htm
728798
The website below will take you to a well-illustrated USGS pub on plate tectonics. It is called: This Dynamic Earth--the story of plate tectonics. By Kious and Tilling.\nBecause it is a government publication, it isn't copyrighted, but you should give appropriate credit if you reuse the figures, etc.\n\nVolcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific "Ring of Fire." In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.-From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication.   \nThe ring of fire is associated with convergent plate boundaries.
149677
Subduction zones occur where oceanic crust collides with continental crust.  Since oceanic crust has more density, it is always subducted underneath continental crust.
296747
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.
26232
Absoutly, there is a ridge subduction zone off the pacific coast that is moving at a good clip.  We know through laser tomography, that the surface of this subduction zone isn't as smooth as many other fault lines.  We know what will happen as soon as it 'buckles', a huge earthquake, followed, of course by a sub surface wave, making a tsunami.\n\nAs to when, no clue.  The plate at the subduction zone isn't moving at a constant clip, but random.  So hard to say.  Will it happen? Absoutly, when? Dunno.
950107
Ah... something called a caldera forms.\n\nhttp://vulcan.wr.usgs.gov/Glossary/Caldera/description_caldera.html\nhttp://en.wikipedia.org/wiki/Caldera\nhttp://www.agu.org/pubs/crossref/2003/2003GL018380.shtml\nhttp://hvo.wr.usgs.gov/volcanowatch/1996/96_04_26.html\nhttp://www.yellowstonenationalpark.com/calderas.htm\nhttp://www.mvo.ms/glossary.htm\n\nGosh, if these don't help... I don't know what will.
284815
The structural integridy of the plate itself prevents this. When the earth cooled down after its formation the plates were created like large crystals on the molten surface. They can slowly shift around as a result of the magma currents but to have them break would require a large catastrophic event like a meteor or the earth heating up to levels hot enough to lower their integridy. This of course would be a lethal level and would only occur if, for example, the earth got really close to the sun.\n\nThere is the remote chance that an earthquake at just the right fault line might cause just the right fracture that could move through an entire continent but the probability is very remote.
1081316
The reponse is related to plate tectonics. The earth crust is composed of a larger number of plates that float, very slowly of course, over the liquid interior. Continents therefore move relative to each other. When two plates meet, generally one moves under the other and it is here that we find mountains and volcanoes.\n\nWhat relation with oceans? Well, between some of the plates there is an upwelling of new magma, new crust. This is for instance the case in the middle of the Atlantic Ocean. The new crust needs room and pushes the plates on the left and right away: the Americas are moving to the west, while Europe and Africa are moving to the east. \n\nOceans (ah, at last!) are watermasses in which such upwelling of fresh crust takes place. An ocean therefore is partly in two or more different plates. On the other hand, seas are bodies of water that are on one continental plate only, or on two plates were no new crust is formed.\n\nHere is a pictures of the different continental plates:\n http://en.wikipedia.org/wiki/Image:Plates_tect2_en.svg
56322
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.
616929
1) when the plates collide - eg himalayas\n2) when the plates pull apart - eg mid oceanic ridge, it is underwater but still a mountain chain\n3) when one subsides under another, when that plate melts, it can cause a chain of volcanoes, which can also make mountains - pacific rim
305725
Yes, the seismic activity that causes one can also cause the other.  When the Earth's techtonic plates shift but experience some resistance, they shake, jump, rattle the area around that resistance.  Sometimes a crack in the Earth's crust is created or widened which creates the eventual environment for a volcano.  Othertimes volcanoes are created over a hot spot (thin crust) in the Earth's crust, like the Hawaiian islands.  This is somewhat different, although some tremors can be felt.
278928
Composite cone volcanoes are often found near convergent plate boundaries. And at all fault lines there are earthquakes so yes, there is.
73951
In order for volcanoes to form, there has to be some way for magma to form and pool up.  The "ripping" of plates at spreading zones, and the melting of a subducted plate in subduction zones, can both result in a pool of molten rock.  But there's nothing to cause this to happen at a transform fault, just two plates passing by each other.
263800
Yellowstone volcanics are a localized feature that has shot through the overlying crust.  This is determined largely through geochemestry and heatflow studies.  The Hawiaan Islands are formed as a result of a hotspot.  You can see that the overlying plate has been moving with respect to the hotspot because the islands formed one at a tine in a line, showing plate direction.\n\nMt St. Helens is the result of a process called subduction.  The oceanic Pacific plate is being shoved underneath the North American plate.  As that plate is pushed deeper and deeper, heat and pressure create a partial melt that can rise to the surface and create volcanoes.  Look at the Aleutian chain, those are also subduction volcanoes, only there you see oceanic crust shoved under oceanic crust
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...And the ring of fire is located around the Pacific.  \n\nVolcanoes can also be found in Iceland, where it lies above a hot spot and a Mid Ocean Ridge.  They also tend to coincide with mantle plumes surging from the core mantle boundary as well as plate convergent (subduction) and divergent (rifts and MOR) settings, but further details can be found in various websites --> search on google.\n\nWikipedia or the United States Geologic Survey (usgs.gov?) websites may have some useful info about volcanoes.
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4 ways:\n\nDivergent plate boundaries\nAt the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling down and solidifying. In these places, the crust is very thin and eruptions occur frequently because of the pull by the tectonic plates. The main part of the mid-oceanic ridges are at the bottom of the ocean, and most volcanic activity is submarine. Black smokers are a typical example of this kind of volcanic activity. Where the mid-oceanic ridge comes above sea-level, volcanoes like the Hekla on Iceland are formed. Divergent plate boundaries create new seafloor and volcanic islands, such as Cjertce and Hawaii.\n\nConvergent plate boundaries\nIn places where one tectonic plate submerges beneath another, the crust melts and becomes magma. This surplus amount of magma generated in one location causes the formation of the volcano. Typical examples for this kind of volcano are the volcanoes in the Pacific Ring of Fire, and also Mount Etna and Mount Vesuvius.\n\nHotspots\nHotspots are not located on the ridges of tectonic plates, but on top of mantle plumes, where the convection of Earth's mantle creates a column of hot material that rises until it reaches the crust. The temperature of the plume causes the crust to melt and form pipes, which can vent magma. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes extinct after a while and a new volcano is then being formed as the plate shifts over the hotspot. The Hawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the current part of the North American plate over the hotspot.\n\nPetitspots\nIn July 2006, volcanoes were discovered that did not fit in any of the above-mentioned categories, since they are located far from the plate boundary, but are too small to be the result of a mantle plume. A new theory suggests that submergence of tectonic plates causes stress all over the plate, which causes the plate to crack in some places. However, other scientists believe the mantle plume theory to be incorrect, and consider this discovery a confirmation of their ideas.
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OK, first of all, inhale. Your body needs oxygen crisis or no crisis.\n\nIt's not that this plate movement re-awakens a bunch of volcanoes, these volcanoes exist in the first place because of the process of subduction, which has been going on--[gasp]--for your entire lifetime and a few billion years before your birth.\n\nThere are several tectonic plates at play in the Pacific. The Pacific Plate seems to be moving in a northwesterly direction, judging by the formation and erosion of the Hawaiian Islands over a hot spot that stays stationary under the moving plate. This plate apparently changed direction abruptly many millions of years ago, judging by the direction of the Emperor Seamounts in relation to the Hawaiian Islands. The first link below shows the other oceanic and continental plates in the world, including those on the Pacific side.\n\nAs a general rule, the oceanic plates in the Pacific are subducted under the surrounding continental plates and into the mantle. If you placed a couple of tables edge to edge and seated a few kindergarten students around each one, the kindergarten students would push on the tables (it's just a thing that they do) until the edge of one managed to slip over or under the edge of the other. \n\nThat's similar to what happens with continental and oceanic plates, too. If an oceanic plate and a continental plate come edge to edge, it's usually the denser seafloor that will go under the other plate. The odds are closer to even if two continental plates collide (as in the collision of the Indian subcontinent with Asia that created the Himalayas) or if two ocean plates are pushing up against each other.\n\nAs far as giant or tsunami waves, we are unable to predict exactly where the next tsunami will come from, but here are three possibilities to keep us up at night:\n\n1) Huge earthquake off the coast of British Columbia/ Washington State/Oregon/ northern California. Such an earthquake hit along 600 miles in January of 1700, and it was about the magnitude of the one that created the Indian Ocean tsunamis in 2004. The 1700 quake also caused devastating tsunamis that did hit Japan and probably hit other places as well. Computer models have shown a series of tsunamis from such a quake hitting the Hawaiian islands on their way across the Pacific, and again on the way back, a "ricochet" effect, if you will. Unfortunately, I think that animation was taken off the web a year or two ago, or I would share it with you here.\n\n2) A volcano on the island of La Palma may collapse, creating a megatsunami hundreds of meters high to scour the coast of Africa and tsunamis of 10-25 meters to batter the eastern coast of the US.\n\n3) A 4,760 cubic mile chunk of the Big Island (Hawaii) falls into the ocean, creating a devastating tsunami around the Pacific. The island of Hawaii suffered two deadly tsunamis in the middle of the last century from earthquakes around the Pacific Rim (Alaska and Chile). The Pacific Rim would be just as vulnerable to tsunami's originating in Hawaii. \n\nSuch "slumps" have occured in Hawaii before. I live on the lower slopes of an extinct volcano that lost half its mass to a landslide a million and a half years ago.