Volcanoes form when magma, a mixture of hot gas and molten rock, flows out of fissures in Earth's crust. The powerful eruption creates vents in the crust, and mountainous formations take shape above ground as molten rock spreads out in layers and gradually cools down. Each successive eruption occurs when more gas and magma accumulate in the vents, building pressure below the crust.
Beneath Earth's outer surface, the fragmented crust layer is made up of tectonic plates that slowly shift position. Steaming magma helps the tectonic plates move and fills the space between the crust and the mantle, an inner layer of Earth. If plates move too far apart, pressurized magma can shoot up through the crust layer and form a volcano. When tectonic plates shift closer and forcefully bump into one another, their edges may be pushed so deep into Earth's interior that the solid rock heats up, turns into melted magma and rises back up through the crust as a volcano.
Every type of volcano has a distinct formation and eruption pattern. Shield volcanoes are shallow, low-lying formations with wide bases. They form when mild, centralized eruptions release lava that quickly spreads out in thin layers. Stratovolcanoes, also known as composite volcanoes, are highly explosive and contain multiple vents, leading to massive eruptions that cause molten rock to stack up in steep, sloped layers. Scoria volcanoes, or cinder cones, are usually symmetrical and have a large crater at the summit formed by repeated eruptions from a single vent.