The Basics	In-Depth	Experiments	Witness Account
Words to Know	Experts Say	Be Ready!	Explore the Ring of Fire

In-Depth

Keep reading to find out...
What causes a volcano to erupt?
What happens at hot spots?
How do volcanoes create rock?
What are the different types of volcanoes?

What causes a volcano to erupt?
Throughout history volcanic eruptions have filled people with awe and fear. Ancient Romans believed volcanoes were controlled by a fire god, and Polynesians believed the beautiful goddess Pele caused the flowing lava and terrifying eruptions. Today, scientists are able to explain — and sometimes even predict — volcanic activity, but that doesn't make them any less awe-inspiring.

More than 500 volcanoes are known to have erupted in the time since people started recording history, with more "hidden" eruptions occurring under the sea. The U.S. has had 50 volcanoes erupt, putting it third, behind Indonesia and Japan, in the number of active volcanoes.

As the tectonic plates slide together, rock is pushed toward the interior of the Earth. This pressure heats up the rock and turns it into magma.

A map of the world's volcanoes shows that there are clusters of these extraordinary mountains in specific regions. Most of the world's active volcanoes are located along or near the boundaries between the shifting tectonic plates that make up the earth's surface. These plate-boundary volcanoes include those along the edges of the Pacific Ocean in the "Ring of Fire."

The tectonic plates are constantly shifting. If they collide, one plate may be squeezed down into the earth's mantle (the layer just below the crust). When this happens, rocks in the plate are pushed towards the interior of the earth where they become very hot and rise up as magma. If the pressure continues to increase, this boiling magma will shoot out of its underground chamber through volcano vents and erupt as lava.

Lava appears different depending on how quickly it flows. Pahoehoe, fast moving lava flows, are smooth. Aa, slow moving flows, are rough and blocky.

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