There are three ways that volcanoes form. Subduction Zone volcanoes form at the boundaries of two plates, one overriding the other. Subduction zone volcanoes are the most violent and destructive of the volcanic types. Mt. St. Helens, Mt. Pinatubo, Krakatoa, and Mt. Vesuvius are all famous explosive subduction zone volcanoes. Mid-ocean rift volcanoes form where two oceanic plates are spreading apart. There are more rift zone volcanoes than any other type.

These mid-ocean or rift zone volcanoes are the world's longest continuous mountain chain. This mountain chain encircles the entire Earth. It is more than 40,000 miles long.

The third way that volcanoes form occurs at a Hot Spot. Hot spots are usually found under oceanic crust, but can be located under continental crust. You will learn more about Hot Spot volcanoes in the lesson "Hot Spots-Yellowstone and Hawaii".

The diagram to your left shows the three ways that volcanoes form.

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