What Causes a Volcano to form?

On this page, you can learn about how volcanoes are formed.

To learn about volcanoes, we first need to know about tectonic plates. These are basically big pieces of Earth's crust that float on top of molten mantle (the layer of Earth beneath the crust). There are two types of crust, oceanic crust (found under large bodies of water) and continental crust (found under large bodies of land).

The image to the left shows a map of the tectonic plates. Think of these plates as boats on water. They don't just stay still, they move around, a lot. The arrows on the map show the directions that these plates move.

The places where the tectonic plates meet are called plate boundaries. Plate boundaries are defined by what directions the plates are moving.

Convergent Plate Boundaries

When two tectonic plates collide, it is called a convergent plate boundary. Different things can happen at these boundaries depending on what type of crust is there.

Oceanic & Oceanic

Volcanic island arcs and trenches can be formed when oceanic crust bumps into oceanic crust. One piece of crust gets pushed beneath the other (otherwise known as subduction) and pushes the magma underneath it, forcing it to pop out the other side.

Oceanic & Continental

As you can see in the picture, when a piece of continental crust comes crashing into a piece of oceanic crust, volcanic mountains can form. The Cascades are a good example of this.

Continental & Continental

When two a piece continental crust collides with another piece of continental crust, mountain ranges often form. Volcanoes do not form here.

The volcanoes that generally form at convergent boundaries are called Stratovolcanoes. These volcanoes are the cone shaped ones you usually think about when you picture a volcano. To learn more about these, click the button to the left.

Divergent Plate Boundaries

Divergent plate boundaries are the opposite of convergent plate boundaries. At a divergent boundary, the two plates are moving away from each other.

Continental & Continental

When two pieces of continental crust pull away from each other, the magma in underneath the plates has the chance to move up to the surface. As you probably guessed, this forms volcanoes. These plate boundaries can also create new seafloor, and create ridges.

Oceanic & Oceanic

Similar to the continental and continental divergent plate boundary, when two pieces of oceanic crust divide, they leave room for magma to flow up between them, forming volcanoes.

Divergent plate boundaries form a specific type of volcano called a Shield Volcano. These volcanoes are much more rare than Stratovolcanoes, but are still supper cool. The image to the left shows one of these Shield Volcanoes. If you are interested in learning more about shield volcanoes, click the image.

Transform Boundaries

This is the third type of plate boundary. Transform boundaries are formed when two tectonic plates slide past each other. Earthquakes often occur on these boundaries. Transform boundaries do not form volcanoes.

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