The processes by which molten rock material, or magma, rises from the interior of the earth on to or towards its surface, and by which associated gases are released into the atmosphere are called Volcanism. The study of these processes, and of the structures, deposits, and landforms they create is called volcano logy.
Magma and gases exploit weak zones in the earth's outermost layer, the lithosphere, in order to reach the surface. Such weaknesses are found primarily along the boundaries between the earth's tectonic plates, and this is where most volcanism occurs. Where magma and gases do reach the surface, through vents or fissures in the earth's crust, they form geological structures known as volcanoes, of which there are several types. The classic picture of a volcano, exemplified by Mount Fuji in Japan or Mount Mayon in the Philippines, is of a conical structure with a hole (crater) at the top, from which (in the case of active volcanoes) ash, steam, gases, molten rock, and solid fragments erupt, often explosively. In fact, volcanoes of this type, though not uncommon, account for less than 1 percent of the earth's volcanic activity.
At least 80 per cent of volcanism takes place through lengthy vertical fissures in the earth's crust. Such fissure volcanism occurs predominantly along the constructive boundaries between the plates into which the lithosphere is divided. Oceanic ridges where new lithosphere is continuously being created and the plates pushed apart mark constructive boundaries. Indeed, it is the rising, cooling magma produced by fissure volcanism that makes the new ocean floor. Most of the world's volcanism therefore takes place unseen, beneath the oceans.
Surface Volcanism. Surface, or continental, volcanism is much less important than sub-oceanic volcanism in terms of the volume of magma ejected, but much more is known about it because it is visible and directly affects human beings. It has been known since ancient times that volcanic activity ranges from violent explosions to the gentle extrusion of magma, which becomes known as lava when it is on the earth's surface.
Fissure Volcanoes. Fissure volcanism is mostly associated with oceanic ridges, but it also occurs on land, and in some cases has led to spectacular results. Fissure volcanoes emit large volumes of very fluid material, which spreads out to cover large areas; successive eruptions can build up great plains or plateaux. Today, fissure volcanoes are probably best seen in Iceland, which straddles the Mid-Atlantic Ridge. However, fissure volcanism on land is most associated with the past, with the great plateaux to be found on most continents. Plateau basalts, flood basalts, or ignimbrites, as they are called, have formed, among others, the Deccan Plateau of west-central India; the Paraná Basin of southern Brazil, Argentina, and Uruguay; the Columbia Plateau of the north-western United States; the Drakensberg Plateau of South Africa; and the central plateau of the North Island, New Zealand.
Central Volcanoes. The majority of surface volcanic activity, however, is associated with more-or-less circular vents, or clusters of vents, in the earth's crust, rather than fissures. These vents give rise to central volcanoes, of which there are two basic types. The steep-sided conical volcano mentioned above is occasionally constructed entirely from solid material, or tephra, which ranges in size from ash and cinders to rocks and boulders. The tephra have been ejected explosively in an eruption, or series of eruptions, and have fallen back to the ground in the immediate vicinity of the crater, the external outlet of the vent. A well-known example of such a volcano is Parícutin, in Mexico, which first erupted in a field on February 20, 1943, and within six days had built a cinder cone 150 m (492 ft) high; by the end of the year the cone was more than 336 m (1,100 ft) high.
However, very few conical volcanoes eject only tephra in every eruption, to become cinder-cone volcanoes. Lava is likely to be extruded in some eruptions, in which case the resulting volcanic structure will comprise alternating layers of tephra and lava. Such volcanoes are called composite volcanoes, or strato-volcanoes. Most of the world's largest and best-known volcanoes, including Stromboli and Vesuvius in Italy, Popocatépetl in Mexico, Cotopaxi in Ecuador, and Kilimanjaro in Tanzania, as well as Fuji and Mayon, are of this type. Although most conical and near-conical volcanoes generally have a single central vent, this does not preclude volcanic material sometimes emerging from secondary, often temporary, vents on the flanks of a volcano.