The majority of volcanoes in the world form along the boundaries of Earth’s tectonic plates—massive expanses of our planet’s lithosphere that continually shift, bumping into one another.
When tectonic plates collide, one often plunges deep below the other in what’s known as a subduction zone.
As the descending landmass sinks deep into the Earth, temperatures and pressures climb, releasing water from the rocks.
The water slightly reduces the melting point of the overlying rock, forming magma that can work its way to the surface—the spark of life to reawaken a slumbering volcano.
Not all volcanoes are related to subduction,
Another way volcanoes can form is what’s known as hotspot volcanism.
In this situation, a zone of magmatic activity—or a hotspot—in the middle of a tectonic plate can push up through the crust to form a volcano.
Although the hotspot itself is thought to be largely stationary, the tectonic plates continue their slow march, building a line of volcanoes or islands on the surface. This mechanism is thought to be behind the Hawaii volcanic chain.