Volcanoes mostly form along the edges of tectonic plates where plates collide, separate, or slide past each other.

The Earth's outer shell is made up of large pieces called tectonic plates. These plates are always moving slowly. When they crash into each other, one can dive below the other, creating a subduction zone. The sinking plate heats up and releases water, which melts the rock above it, forming magma. This magma then rises to the surface, creating volcanoes. Volcanoes can also form where plates pull apart, like at mid-ocean ridges, allowing magma to rise and create new crust. Hotspots, which are places where magma rises through a plate, can also make volcanoes far from plate edges, like the Hawaiian Islands.