A volcano is a gap in Earth’s crust from which molten (melted) rock, ash, and steam flow—or burst, depending on the type of volcano and the pressure involved.
Volcanologists—people who study volcanoes—have discovered several ways in which a volcano comes into existence. Some volcanoes occur at the edges of the huge tectonic plates that make up Earth’s crust.
When these plates collide, one gets pushed down underneath the other, or subducted. As the rocky material is subducted, it melts to form magma. This magma can burst up through the surface, creating a volcano.
Other volcanoes form under the oceans, when magma gushes up from underneath the crust that makes up the ocean floor.