How is a volcano formed?

A volcano forms when magma rises through weakened areas of the crust from a magma reservoir many miles deep within the earth, pushing itself up through a vent. This vent acts as a release valve for pressure building up below, and when the new volcano erupts, thousands or millions of pounds of ash and molten rock slowly accumulate, forming a volcanic mountain.

From the magma reservoir, the molten rock is pushed up through cracks in the solid rock above it by the force of gas pressure that builds up, as well as buoyancy because the molten material is lighter than the solid formations around it. As the material exits the vents at the surface of the Earth and accumulates in sufficient quantities, the force of gravity causes some of the material to fall down the sides toward ground level, eventually creating the conical shape of the classic volcano.

While this process typically takes hundreds or thousands of years, some volcanic mountains can be formed almost overnight. Paricutin is a volcano in Mexico that appeared on February 20, 1943, and within 10 years, it had grown to a height of 1,391 feet. As of 2014, there are over 500 active volcanoes on Earth. An active volcano is defined as one that has erupted within the span of human history, and every one of them is still in the "formation" process, adding more material to itself every time rock and ash is expelled through its vents.