What is a Volcano?
A volcano is a large mound of earth and rock which is actually a passage or rupture through the earth's crust leading to liquid hot deposits of magma within the earth's mantle. It is believed that volcanoes are the primary source of land development in the planet's history. This can be seen in action on the big island of Hawaii where the continuously active volcano there has been slowly erupting, sending out lava. This lava runs down the mound of the volcano to the very edges of the island, where it is cooled into solid form upon touching the ocean. This new layer of hardened rock serves to increase the overall mass of the island. One day, many years from now, after the volcano has stabilized and ceased to erupt, this new land will be in perfect condition for habitation.
A volcano is created and enlarged through a continuous process by which magma under high pressure comes up from the earth's molten mantle. When this happens, the presence of the magma increases the mass of the volcano's outer mound, causing it to bulge outward and upward. If the pressure of the magma is insufficient to cause the volcano to erupt, it will cool over time, forming hard rock which the newest part of the volcano. This process repeats continuously all over the planet, often without us knowing it. But this is only one portion of volcano formation.
Formation by Subduction
The first method which can place magma under pressure and in turn create a volcano or cause one to erupt is known as subduction. Subduction is reliant on the tectonic plates which form the top layer of the earth's mantle. These plates support the weight of the earth's continents and seabed. They drift atop a thick layer of molten pyrolitic rock. This rock is under intense pressure and heat. As the tectonic plates drift, one edge of a plate occasionally slips beneath the edge of another. This creates a hole through which material from the earth's crust slips. This occurs most on the sea floor, taking millions of tons of seawater with it. The seawater slips between the tectonic plates and flash boils into steam. This creates a pathway of high pressure for the pyrolitic rock or magma to follow in one big surge. Subduction is believed to be responsible for forming the islands of Japan millions of years ago.
Formation by Distension
Distension is responsible for the chains of "wandering" volcanoes patterned across the planet. Essentially it's caused by a "hot spot" near the earth's outer core. This is a continuous area where the pyrolitic rock is significantly hotter than the rock in surrounding areas. The cause of a hot spot is still unknown. Because the hot spot occurs beneath the tectonic plates, the place where this pressurized and superheated rock vents to the planetary surface is static, even though the tectonic plates above shift over time.
What this means is that a volcano is formed, remains active for a short time, and dies as the tectonic plates shift the land above away from the hotspot. Adjacent to the dead volcano, a live one forms and repeats the cycle.