Volcanoes occur when magma and heated gases from below the Earth's crust are channeled to the surface through fissures in the crust layer. Magma is molten rock that is called lava once it reaches the surface.
Volcanoes usually occur near the point where two or more tectonic plates meet. Tectonic plates are sections of the Earth's lithosphere that include the crust and upper mantle. The different types of volcanoes that exist are determined by the type of interaction that exists between the tectonic plates below it. Where divergent plate boundaries exist, it is common to find ridge volcanoes. These start at the bottom of the ocean at the point where the tectonic plates are pulling away from each other and new seafloor is being made. As the lava cools and accumulates, it can rise above sea level and give rise to volcanic islands.
Where convergent plates meet, one submerges underneath the other. The force created by this action creates magma. The magma that fails to solidify subterraneously travels toward the surface. The magma may trace a fissure to vent out directly as lava on the surface, which creates shield volcanoes. Alternatively, the magma may accumulate underground, forming a magma chamber. When pressure accumulates within the chamber, a volcanic eruption occurs, creating a conical volcano. These types of volcanoes can eventually develop into stratovolcanoes.