A naturally occurring vent or fissure at the Earth's surface through which molten, solid, and other gaseous materials are expelled. Volcanic eruptions inject large quantities of dust, gas, and aerosols into the atmosphere. A major component of volcanic clouds is sulfur dioxide; a strong absorber of ultraviolet radiation. Chemical interactions between sulfur dioxide and water form sulfuric acid aerosols which can then scatter some of the incident solar radiation back to space; thus causing a global cooling effect. For example, Mt. Pinatubo in the Philippines erupted in June 1991, and in the following year the global surface temperature was observed to decrease by approximately 0.3 degrees

Source: NASA (http://earthobservatory.nasa.gov/Glossary)

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