More than 500 volcanoes are known to have erupted on the earth's surface since historic times, and many more have erupted on the ocean floor unobserved by humans. Fifty volcanoes have erupted in the United States, which ranks third, behind Indonesia and Japan, in the number of historically active volcanoes. Of the world's active volcanoes, more than half are found around the perimeter of the Pacific, about a third on midoceanic islands and in an arc along the south of the Indonesian islands, and about a tenth in the Mediterranean area, Africa, and Asia Minor.
Evidence of extraterrestrial volcanic activity also has been found. Space probes have detected the remnants of ancient eruptions on earth's moon, Mars (which has the largest volcano in the solar system, Olympus Mons, 340 mi/550 km across and 15 mi/24 km high), and Mercury; these probably originated billions of years ago, since these bodies are no longer capable of volcanic activity. Triton (a satellite of Neptune), Io (a satellite of Jupiter), and Venus are known to be volcanically active. The volcanic processes that occur in the outer portion of the solar system are very different from those in the inner part. Eruptions on earth, Venus, Mercury, and Mars are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds, such as methane or nitrogen, driven by seasonal heating from the sun.
Terrestrial volcanic eruptions may take one or more of five chief forms, or phases, known as Hawaiian, Strombolian, Vulcanian, Peleean, and Plinian. In the Hawaiian phase there is a relatively quiet effusion of basaltic lava unaccompanied by explosions or the ejection of fragments; the eruptions of Mauna Loa on the island of Hawaii are typical. The Strombolian phase derives its name from the volcano Stromboli in the Lipari, or Aeolian, Islands, N of Sicily. It applies to continuous but mild discharges in which viscous lava is emitted in recurring explosions; the ejection of incandescent material produces luminous clouds. A more explosive volcanic eruption is the Vulcanian, where the magma (lava before emission) accumulates in the upper level of the vent but is blocked by a hardened plug of lava that forms between consecutive explosions. When the explosive gases have reached a critical pressure within the volcano, masses of solid and liquid rock erupt into the air and clouds of vapor form over the crater. The Peleean, derived from Mt. Pelée, is more violent, emitting fine ash; hot, gas-charged fragments of lava; and a characteristic superheated pyroclastic flow that travels downhill at great speed. Plinian, or Vesuvian eruptions, derives its name from Pliny the Younger, who described the eruption of Vesuvius in A.D. 79. The Plinian eruption is similar to Strombolian and Vulcanian eruptions with significant ash and pumice and pyroclastic flows, but it also produces a characteristic massive, sustained eruptive column of hot ash that can reach 28 mi (45 km) in height.
Eruptions are often accompanied by torrential rains caused by the condensation of steam. The erupted fragments vary in size, including minute particles of volcanic dust and ash, lapilli (cinders or pellets), bombs (rounded or ellipsoidal masses of hardened magma), and huge masses called blocks. Minute dust and ash and aerosols carried high into the earth's atmosphere can have a cooling effect on the climate, and significant amounts of chlorine and bromine gases ejected in large eruptions can reach the stratosphere and deplete the ozone layer. The dust and ash can also be a hazard to air travel. The 1783 eruption of Laki, S Iceland, had devastating effects on local livestock and, as result, the populace; the resulting sulfur dioxide haze that spread over parts of Europe is believed to have negatively affected the health of the inhabitants.