A volcano forms when magma pushes up through the Earth's crust from below, depositing lava on the surface. This lava cools, creating volcanic rock. Over time, repeated eruptions of lava build a cone-shaped mountainous structure, producing a volcano.
Volcanoes typically form around three tectonic features. Plate boundaries are common spots for volcano formation, either divergent plate boundaries where two plates are separating or convergent boundaries where two plates collide. The resulting damage can produce weak spots in the plate, allowing magma to flow up from beneath. Divergent boundaries tend to produce shallow flows of lava and occur regularly on the ocean floor. Convergent boundaries produce thick, viscous lava that may not reach the surface and is responsible for many of the undersea mountain ranges on the planet.
Hot spots are areas where a tectonic plate is weakened, allowing an upwelling of magma from below. Since tectonic plates move over time, these hot spots can create a series of volcanoes, each going dormant when it moves away from the hot spot. The Hawaiian islands are believed to be a result of a moving hot spot underneath the Pacific that created a chain of islands and undersea mountains from Midway to the currently active volcano of Kilauea.