What Is a Volcano?

Volcanoes are openings in the Earth's crust. Many volcanoes are mountains or hills. Deep inside a volcano is a pool of molten rock. When a lot of pressure builds up inside an active volcano, it erupts. These eruptions can cause mudslides and hot ash flows. They can also cause lava flows. Lava is a type of rock that is so hot it melts. It spews out of a volcano like a bright, orange river. When the lava cools down, it hardens. Then it forms many different types of rocks.

There are more than 500 active volcanoes in the world. More than half of these are around the edges of the Pacific Ocean. This area is called "the Ring of Fire." Much of .....

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