There are several ways in which a volcano can form, just as there are several different kinds of volcanoes. Volcanism is part of the process by which a planet cools off.

Hot magma, rising from lower reaches of the Earth, eventually, but not always, erupts onto the surface in the form of lava. During the eruption of a volcano, flowing lava and ash form a large cone. This cone is what we know as a volcano.

Among the different kinds of volcanoes are:

-shield volcanoes
-cinder cones
-composite volcanoes or stratovolcanoes

The most prevalent volcanoes on the Earth's surface are the kind which form the "Pacific Rim of Fire" near subduction zones.

Magma is the molten material beneath the Earth’s crust. It usually collects in a magma chamber beneath a volcano, and can then be injected into cracks in rocks or issue out of volcanoes in eruptions. The temperature of magma ranges between 700- 1300 degrees celsius.

When it reaches the surface and comes out of a volcano, magma becomes lava. There are actually different kinds of lava depending on its thickness or viscosity. The thinnest lava can flow downhill for many kilometers, creating a gentle slope. The thicker lavas will pile up around the volcanic vent and hardly flow at all. The thickest lava doesn’t even flow. It just plugs up the plumbing of a volcano, and can be the cause of violent explosions.

Volcanoes form primarily at convergent boundaries where there is a subduction zone, such as the Ring of Fire region in the Pacific ocean. Volcanoes also form at divergent boundaries, such as spreading ridges, or over a hot spot.

As an oceanic crust descends to hotter depths beneath the overriding plate, it is heated and releases water. The presence of hot, water-rich liquids causes the overlying rock to begin melting. The resulting magma is less dense than the surrounding rock, so it rises up toward the surface where it collects in magma chambers. Volcanic eruptions happen when magma from these chambers reaches the surface. Over millions of years, repeated eruptions build volcanic mountains on the overriding plate.

The Ring of Fire is an area where a large number of earthquakes and volcanic eruptions occur in the basin of the Pacific Ocean. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. The Ring of Fire has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes. It is sometimes called the circum-Pacific belt or the circum-Pacific seismic belt.

Stratovolcanoes, also known as composite cones, are the most picturesque and the most deadly of the volcano types. Their lower slopes are gentle, but they rise steeply near the summit to produce an overall morphology that is concave in an upward direction. The summit area typically contains a surprisingly small summit crater. This classic stratovolcano shape is exemplified by many well-known stratovolcanoes, such as Mt. Fuji in Japan, Mt. Mayon in the Philippines, and Mt. Agua in Guatemala.

Stratovolcanoes typically form at convergent plate margins, where one plate descends beneath an adjacent plate at the site of a subduction zone. Examples of subduction-related stratovolcanoes can be found in many places in the world, but they are particularly abundant along the rim of the Pacific Ocean, a region known as Ring of Fire.

Pyroclastic flows, most commonly from stratovolcanoes, are fluidized masses of rock fragments and gases that move rapidly in response to gravity.Pyroclastic flows can form in several different ways. They can form when an eruption column collapses, or as the result of gravitational collapse or explosion on a lava dome or lava flow (Francis, 1993 and Scott, 1989). These flows are more dense than pyroclastic surges and can contain as much as 80 % unconsolidated material. The flow is fluidized because it contains water and gas from the eruption, water vapor from melted snow and ice, and air from the flow overriding air as it moves downslope (Scott, 1989). The image shows the formation of pyroclastic flows during a 1980 eruption of Mount St. Helens (photo courtesy of J.M. Vallance).

Shield volcanoes are broad, low-profile features with basal diameters that vary from a few kilometers to over 100 kilometers. The image is of the Mauna Loa shield volcano in Hawaii. Their heights are typically about 1/20th of their widths. The lower slopes are often gentle (2-3 degrees), but the middle slopes become steeper (~10 degrees) and then flatten at the summit. Their overall broad shapes result from the extrusion of very fluid (low viscosity) basalt lava that spreads outward from the summit area, in contrast to the vertical accumulation of airfall tephra around cinder cone vents, and the build-up of viscous lava and around stratovolcanoes. Pyroclastic deposits are minor (

A volcanic "hotspot" is an area in the mantle from which heat rises as a thermal plume from deep in the Earth. High heat and lower pressure at the base of the lithosphere (tectonic plate) facilitates melting of the rock. This melt, called magma, rises through cracks and erupts to form volcanoes. As the tectonic plate moves over the stationary hot spot, the volcanoes are rafted away and new ones form in their place. This results in chains of volcanoes, such as the Hawaiian Islands.

Cinder cones are simple volcanoes which have a bowl-shaped crater at the summit and only grow to about a thousand feet, the size of a hill. They usually are created of eruptions from a single opening, unlike a strato-volcano or shield volcano which can erupt from many different openings.

Cinder cone volcanoes are usually made of piles of lava, not ash. During the eruption, blobs ("cinders") of lava, blown into the air from the central vent, break into small fragments that fall around the opening to the volcano. The pile forms an oval-shaped small volcano.