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In-Depth

How do volcanoes create rock?
Volcanoes are often identified as destructive forces, but they are also tools of natural creation. For example, their eruptions create different types of rock such as basalt, black shiny rock with only a few crystals. The Hawaiian Islands are mostly made up of basalts and are famous for their beautiful black-sand beaches. Rhyolite is a white shiny rock with many crystals and often many bubble holes inside. Volcanoes from the Andes Mountains in South America produce andesites, which are light gray and usually have large box-shaped crystals called plagioclase.

About 500 million people live close to active volcanoes! Many of them do not even realize it.

What are the different types of volcanoes?
In addition to forming various types of rocks, volcanoes themselves fall into different categories. Volcanologists usually group them into one of three types:

Mt. Shasta is one example of a composite volcano.

1) Composite Volcanoes. Sometimes called stratovolcanoes, some of Earth's grandest volcanoes are composite. These steep-sided mountains stand as high as 8,000 feet. They're built of layers of lava, ash, cinders, and other materials that rise from deep in the earth's crust during an eruption. Composite volcanoes usually have circular depressions at their summits. Depressions of less than a mile across are called craters; larger depressions are calderas.

Examples: Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount St. Helens and Mount Rainier in Washington

Kilauea's long gentle slopes are characteristic of shield volcanoes.

2) Shield Volcanoes. Resembling an upturned warrior's shield, these volcanoes have long gentle slopes. They build up slowly from numerous eruptions of basaltic lava that spreads widely over great distances.

Examples: Kilauea and Mauna Loa on the island of Hawaii

Cinder cones are the simplest type of volcano.

3) Cinder Cones. The simplest type of volcano, these are created by an accumulation of cinders and other volcanic debris that erupt from a single vent. The material falls back to the earth around the vent in a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a thousand feet or so above their surroundings.

Examples: Lava Butte Cinder Cone, Oregon

The rock debris carried by a lateral blast of Mount St. Helens traveled as fast as 250 miles per hour.

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