VULCANO. — In common usage, volcanoes are elevations of the Earth’s crust through which, during periods of eruption, incandescent materials such as lava, ash, and lapilli are discharged to the surface. More generally, all discontinuities in the Earth’s crust through which the products of endogenous magmatic activity emerge in various forms should be considered volcanoes.
Volcanoes are in fact direct evidence of the existence, in the deeper zones of the lithosphere, of natural molten silicate masses known as magmas (see ROCKS). Owing to the grandeur of their manifestations and their sometimes devastating effects, volcanic phenomena have attracted the attention of scholars since antiquity. Ancient philosophers soon sought to explain them, and their theories sometimes contain striking insights. Thus Plato (Phaedo) posited the existence of an underground river of fire, the Pyriphlegethon, which would find an outlet through volcanoes. Seneca believed that earthquakes and eruptions were caused by the penetration of water into the subsurface, where, coming into contact with incandescent matter, it would generate high-pressure steam: “Ignem causam motus quidam et quidem non eamdem indicat.... videmus aquam spumare, igne subiecto. Quod in hac aqua facit inclusa et angusta multo magis illum facere credamus quum violentus et vastus, ingentes aquas excitat.” Aristotle clearly asserted the connection between eruptions and earthquakes. A classic account is provided by Pliny the Younger, who described the famous eruption of Vesuvius in 79 AD that buried Pompeii, Herculaneum, and Stabiae and in which Pliny the Elder heroically perished.
Nevertheless, a true science of volcanology cannot be said to have existed until the 16th century. By the mid-17th century, the attention of naturalists was aroused by the eruptions of Vesuvius (1631) and Etna (1669), for which accurate descriptions exist. In the 17th and 18th centuries, good naturalistic studies were produced, especially by Spallanzani (1788), and finally in the 19th century volcanology made decisive progress with the aid of petrography. Various theories were advanced to explain the origin of volcanoes: opposed to the elevation-crater theory of von Buch—which held that volcanic reliefs were formed by magma lifting the strata, which then broke at the top to form the crater—was the external accumulation theory supported by Scrope and Stoppani. According to this view, volcanic cones and reliefs were due to the accumulation of solid materials emitted or ejected from the volcanic conduit. Some authors emphasized the role of seawater infiltrations, which, reaching deep hot zones, would produce steam whose pressure would cause volcanic explosions. Later, the idea of a molten igneous Earth’s core covered by a thin crust was proposed, but soon the notion of a single feeding hearth for all volcanoes on Earth was replaced by the conviction that there must be multiple magma reservoirs within the lithosphere itself, generally shallow and behaving independently; only in some cases must direct effusions of subcrustal magma be admitted.
Volcanic activity takes many forms; the most typical and well-known are the simple projection of ash and scoriae or lava, the effusion or extrusion of lava, explosions, and the continuous or periodic emission of vapors. Often the forms of activity are mixed. Particularly catastrophic are the glowing avalanches and nuées ardentes that accompany certain types of eruptions, such as the nuée ardente of Mount Pelée in Martinique, which destroyed the city of St. Pierre and claimed 36,000 lives. In lava ejections, the projected material often assumes characteristic forms, such as cauliflower bombs, bread-crust bombs, and Pele’s hair. Various designations have been used, especially in the past, to denote particular forms of eruptions, taking as types those of historically known volcanoes; thus we have Plinian eruptions, strongly explosive (Vesuvius, 79 AD); Vulcanian eruptions (Vulcano, 1888), also explosive, with the ejection of semi-vitreous blocks, pumice, and bombs of coeval material; Strombolian eruptions (normal activity of Stromboli), with periodic ejection of ash and scoriae; and Hawaiian eruptions, purely effusive, with fluid magma and the formation of lava lakes.
The type of volcanic apparatus varies according to the type of activity and the emitted material. Purely explosive activity can produce explosion channels or funnels, or cinder cones. Purely effusive activity produces lava volcanoes of the shield type, domes or stagnation mounds, and extrusion spines, depending on the fluidity of the magma. From mixed activity—partly explosive and partly effusive—strato-volcanoes or composite volcanoes arise, consisting alternately of lava flows and incoherent materials of varying complexity.
The origin of volcanoes, the various types of their activity, and the different forms and structures of volcanic apparatus are essentially to be related to the modalities of magma consolidation. The essential components of magmas, which can vary in their quantitative ratios, are oxides of silicon, aluminum, iron (bi- and tri-valent), calcium, magnesium, potassium, sodium, and titanium; these represent the so-called fixed components of magma, to which are added elements or combinations thereof characterized, even at ordinary temperature, by a high vapor tension. These latter therefore tend to assume the gaseous state and are called volatile components. Among these, water predominates, but there are many others, such as carbon dioxide, hydrochloric acid, and hydrogen sulfide. When, as a result of cooling, magma begins to consolidate, predominantly silicate minerals separate out; since these generally contain little or no volatile components, it is clear that the latter will become increasingly concentrated in the residual melt, whose vapor tension will progressively increase. It is evident that when the vapor tension of the residual melt exceeds the external pressure (atmospheric pressure plus the load of the masses overlying the magma reservoir), an explosive phenomenon will occur, and the magma will boil. A new volcano may thus originate through what is commonly called an initial perforation, or an ancient dormant volcano may resume activity after a long interruption, as in the case of the eruption of Vesuvius in 79 AD. If the explosion occurs at an early stage of magmatic crystallization, the activity will be predominantly effusive, with abundant lava outflows and the formation of lava volcanoes. Typical in this regard are the shield volcanoes of Iceland and Hawaii. If, on the other hand, the explosion occurs at a very advanced stage of magma consolidation, the activity will be predominantly explosive, with the formation of cones of incoherent materials or, in extreme cases, explosion channels or funnels. Under intermediate conditions, stratovolcanoes are formed, of which Vesuvius and Etna are examples.
They are variously distributed over the Earth’s surface, where it is known that there exist zones that are typically volcanic (Japan, the Cordilleras, the Mediterranean); but more than their geographical distribution, it is important to consider the distribution of volcanoes in relation to tectonic structure. Thus it is possible to distinguish between volcanism in oceanic zones (parts of the lithosphere lacking or poor in sial) which is predominantly effusive, volcanism in continental massifs characterized by repeated effusions of lava from fissures that give rise to vast and powerful basaltic coverings (Deccan, Siberia, Abyssinia), and finally volcanism in orogenic zones (Japan, the Cordilleras, the Aegean) which tends to be explosive in character.
The activity of a single volcano may extend over a very variable span of time, from a few days to several tens of millennia; it may be considered that volcanism as a whole has been active on Earth since the most remote epochs.