Most volcanoes occur along plate margins or at 'hotspots' in the earth's crust. Volcano forms because when magma from the mantle rises to earth's surface through a certain spot in the middle of the plate.
The most common type occur at the edges of tectonic plates, the crust beneath the oceans and the continents. When one plate is pushed beneath another, it is heated by the hot mantle, a fluid rock layer beneath the crust. The rock is forced upward through cracks to form volcanoes. Volcanoes usually occur where there are collision of the tectonic plates. There are 52 tectonic plates which either pull (gap and crack) or push (volcano). Volcanoes are actually tectonic plates which have magmas beneath their outer surface. Why Volcanoes Occur. Magma is generated in the Earth's lithosphere, which is made up of the crust and upper mantle. (Description of Figure 1.). The roots of Mount St. Helen's are 110 to 330 kilometers (70 to 200 miles) below the Earth's surface. Mount St. Helens is typical of more than 80 percent of the volcanoes that have formed on land. Known as subduction zone volcanoes, they occur along the edges of continents where one plate dives, or subducts, beneath a second plate. (fig. 2). When the subducting plate reaches about 100 kilometers (60 miles) into the Earth's hot mantle, it triggers partial melting of the overlying plate and forms new magma Volcanoes occur where they do because of the position of the crust over a plate boundary or a 'hot spot' in the Earth's mantle. Volcanoes usually occur where there are collision of the tectonic plates. There are 52 tectonic plates which either pull (gap and crack) or push (volcano). Volcanoes are actually tectonic plates which have magmas beneath their outer surface. Map showing Earth's plate boundaries. Active volcanoes generally occur close to the major tectonic plate boundaries. Many are located along the Earth's plate boundaries. Active volcanoes are rare in Australia because there are no plate boundaries on this continent. However, there are two active volcanoes located 4000 kilometres south west of Perth on the Australian territories, Heard Island and the nearby McDonald Islands.  Volcanoes can also occur at hot spots, such as at the areas of the Hawaiian islands and Yellowstone in America. This type of volcano forms when magma from the mantle rises to Earth's surface through a certain spot in the middle of the plate (i.e., the Pacific Plate for the Hawaiian Islands). Volcanoes usually occur where there are collision of the tectonic plates. There are 52 tectonic plates which either pull (gap and crack) or push (volcano). Volcanoes are actually tectonic plates which have magmas beneath their outer surface. The greatest number of the Earth's volcanoes occur on the ocean floor. ROV Jason gets a close view of magma explosions and lava flows on West Mata volcano. Volcanic eruptions occur only in certain places and do not occur randomly. This is because the Earth’s crust is broken into a series of slabs known as tectonic plates. These plates are rigid, but they “float” on a hotter, softer layer in the Earth's interior. Although most of the active volcanoes we see on land occur where plates collide, the greatest number of the Earth's volcanoes are hidden from view, occurring on the ocean floor along spreading ridges.
http://www.answers.com/Q/Where_do_volcanoes_occur http://egsc.usgs.gov/isb//pubs/teachers-packets/volcanoes/lesson1/lesson1.html http://www.answers.com/Q/Where_do_volcanoes_occur http://www.ga.gov.au/scientific-topics/hazards/volcano/basics/where http://www.answers.com/Q/Where_do_volcanoes_occur http://oceanservice.noaa.gov/facts/volcanoes.html
why volcanoes occur