Volcanoes are formed from previous magma (molten rock from the Earth's interior) emissions that slowly cool and harden over time, forming a visible vent in the Earth's surface. When magma and gas from the Earth's mantle (a region of molten rock that exists between Earth's core and surface) build enough pressure within the vent, a volcanic eruption occurs and causes hot magma, gas and ash to spewed onto the Earth's surface and atmosphere at temperatures reaching 2,000 degrees Fahrenheit.
Volcanoes exist because of magma chambers that allow lighter, liquid rock to rise up and breach the surface along weakness in the Earth's surface. These weak points in the surface are found between the Earth's tectonic plates (the slabs of rock that make up the Earth's surface and very slowly drift together and apart). Of all the volcanoes on the Earth's surface, approximately 90 percent exist within the "Ring of Fire" located along the edges of the Pacific Ocean.
Volcanic eruptions vary largely in scale and depend largely on the type of pressure that builds within the volcano itself. When large amounts of gas and thick magma build up beneath the surface, the pressure typically causes a large eruption to spew large amounts of material with tremendous force. Pressure caused by liquid magma and lesser amounts of gas typically cause much-less-violent eruptions in which magma simply oozes out from the vent.