A volcano is an opening in the earth's crust through which gases, lava, and solid fragments are discharged. There are many different types of volcanoes, such as, conical (e.g., Mt. Ranier); fissure; submarine; and shield (e.g., the Hawaiian islands).

Volcanoes are described as active, dormant, or extinct. On Earth about 500 volcanoes are known to be active. Belts of volcanoes are found along the crest of the mid-ocean ridge system under the ocean and at converging crustal plate boundaries.

Eruptions range from the quiet type (Hawaiian) to the violently explosive (Krakatoa). Recently, pictures taken in space have revealed that the planets Mars and Venus have volcanoes too. Volcanoes have been observed on Io, a satellite of Jupiter as well.

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