Plural volcanoes, or volcanos, any vent in the crust of the Earth or other planet or satellite, from which issue molten rock, pyroclastic debris, and steam. Volcanism (vulcanism) is the name given to the processes and phenomena associated with the surficial discharge of such material from volcanoes, geysers, and fumaroles. Volcanoes figure prominently in the mythology of many peoples who have learned to live with eruptions, but science was late in recognizing the important roleof volcanism in the evolution of the Earth.
One major 18th-century school of thoughtheld that molten rock and volcanoes were simply accidents caused by burning coal seams. Geologists today agree that volcanism is a profound process resultingfrom the thermal evolution of planetary bodies. Heat does not easily escape from large bodies by conduction or radiation. Instead, partial melting and buoyant rise of magma are major contributors to the process of heat flux from the Earth’s interior.
Volcanoes are the surface manifestation of this thermal process, which has its roots deep inside the Earth and which hurls its ashes high into the atmosphere. The term volcano can either mean the vent from which magma erupts to the surface, or it can refer to the landform created by the solidified lava and fragmental volcanic debris that accumulate near the vent. One could say, for example, that large lava flows are erupted from Kilauea Volcano in Hawaii, the word volcano here signifying a vent.
By contrast, one could say that Kilauea is a gently sloping volcano of modest size as Hawaiian volcanoes go, the reference in this case being to a landform. Broadly defined, all igneous rocks are the result of volcanism. If igneous rocks solidify from magmas that have not reached the surface, they are called intrusive igneous rocks, and this process in a more restricted sense is termed plutonism. Igneous rocks that cool and solidify at the Earth’s surface are known as extrusive igneous rocks, and these are unequivocally products of volcanism.
Volcanoes (and their products) are not the realm of any single scientific discipline. Rather, they require study by many scientists from several specialties: geophysicists and geochemists to probe the deep roots of volcanoes; geologists to decipher prehistoric volcanic activity; biologists to learn how life becomes established and evolves on barren volcanic islands; and meteorologists to determine the effects of volcanicdust and gases on the atmosphere, weather, and climate.
The 1984 eruption of Mauna Loa Volcano in Hawaii serves to illustrate a “quiet” eruption involving the effusion of lava. The widely varying composition of volcanic gases from fumaroles is explained by studies at Kilauea Volcano, and Yellowstone National Park in the western United States provides classic examples of hot springs and geysers. Volcanic landforms evolve from the cumulative effects of both constructive and destructive eruptions. Mt. Fuji exemplifies a stratovolcano and Mauna Loa typifies a shield volcano.
Iceland provides fine examples of volcanic plateaus, and since it sits astride the Mid-Atlantic Ridge, it is also a logical starting place for a discussion of submarine volcanic structures. Volcanoes are closely associated with tectonic activity. Most of them occur on either the overriding or the diverging margins of the enormous lithospheric plates that make up the Earth’s surface. The volcanoes of Japan provide an excellent example of the former, while those of the Mid-Atlantic Ridge the latter.
Intraplate volcanoes such as those of the Hawaiian chain provide important evidence as to the direction and rate of plate motion. Volcanoes affect humankind in many ways. Their destructiveness is awesome, but the risk involved can be reduced by assessing volcanic hazards and forecasting volcaniceruptions. Volcanism provides fertile soils, valuable mineral deposits, and geothermal energy. Over geologic time volcanoes recycle the Earth’s hydrosphere and atmosphere, and explosive eruptions can affect climate