A volcano occurs when molten rock wells up from the mantle to breach the Earth's crust. Most commonly, this occurs around the edges of tectonic plates. When two plates collide, one may slide underneath its neighbor, opening a channel for magma to reach the surface. If two plates pull away from each other, the resulting gap may allow multiple channels for magma to escape.
In addition to volcanoes that occur at the edges of tectonic plates, some may form when a weak spot appears in the center of a plate. These often produce a series of volcanoes over a long period, illustrating the tectonic shift. For example, a hot spot currently exists under Mount Kilauea in Hawaii, providing the magma to fuel that volcano. Each of the islands in the Hawaiian chain is a dormant volcano created by that same hot spot as the plate moved. The chain of volcanoes created by this one flaw extends all the way to the island of Midway, covering more than 15 million years of volcanic development.
When a channel opens for magma to reach the surface, the molten rock pools around the vent and hardens. Over time, this rock builds up, forming the traditional cone shape of a volcano. In 1943, a vent in a cornfield in Mexico began spewing magma, forming the volcano Paricutin. By 2014, Paricutin had grown to form a cinder cone more than 9,000 feet high.