Volcano

VOLCANO. - In common usage, volcanoes are elevations of the earth's crust through which, during periods of eruption, incandescent materials such as lava, ashes, and lapilli pour out onto the surface. More generally, all the discontinuities of the earth's crust through which, with various manifestations, the products of endogenous magmatic activity make their way toward the surface should be understood as volcanoes.

Volcanoes are in fact the direct evidence of the existence, in the deep zones of the lithosphere, of natural molten silicate masses called magmas (v. ROCCE). Because of the grandeur of their manifestations and their sometimes deleterious effects, volcanic phenomena attracted the attention of scholars since antiquity. The ancient philosophers soon concerned themselves with finding an explanation for them, and their theories are sometimes striking for the happy intuitions they contain. Thus Plato (Phaedo) admitted the existence of an underground river of fire, the Pyriphlegethon, which would have found an outlet in volcanoes. Seneca thought that earthquakes and eruptions were due to the penetration of water into the subsoil where, coming into contact with incandescent materials, it would generate high-tension 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." In Aristotle the link between eruptions and earthquakes is clearly asserted. Classic is the description that Pliny the Younger gives of the famous eruption of Vesuvius which in 79 A.D. buried Pompeii, Herculaneum and Stabiae and in which Pliny the Elder heroically met his death.

Nevertheless, one cannot speak of a true volcanological science until the 16th century. Toward the middle of the 17th century the attention of naturalists was awakened by the eruptions of Vesuvius (1631) and of Etna (1669), of which accurate descriptions exist. In the 17th-18th centuries there were good studies of a naturalistic character, especially through the work of Spallanzani (1788), and finally in the 19th century volcanology made decisive progress with the help of petrography. Various theories were put forward to explain the origin of volcanoes: to De Buch's theory of craters of elevation, according to which volcanic reliefs would originate from magma, which would lift the strata forming cones that would then break open at the top forming the crater, was opposed the theory of external accumulation, upheld by Scrope and by Stoppani. According to this, the volcanic reliefs and cones would be due to the accumulation of solid materials emitted or projected from the volcanic conduit. Some authors gave fundamental importance to the infiltrations of sea water, which, reaching the deep hot zones, would produce vapor whose tension would determine the volcanic explosions. Later the existence was postulated of a terrestrial core in a state of igneous fusion, covered by a thin layer of crust; but soon the idea of a single hearth feeding all the volcanoes of the earth was replaced by the conviction that various magmatic reservoirs must exist within the lithosphere itself, generally not very deep, which behave independently; only in some cases must one admit that direct effusions of subcrustal magma occur. The manifestations of volcanic activity are manifold; the most typical and best known are the simple projection of ashes and scoriae or of lava, the effusions or extrusions of lava, the explosions, the continuous or periodic emission of vapors. Often the forms of activity are mixed. Particularly catastrophic are the glowing clouds and avalanches that accompany certain types of eruptions, such as the glowing avalanche of Mount Pelée in Martinique that destroyed the city of St-Pierre causing 36,000 human victims. In the ejections of lava the projected material often assumes characteristic forms. Thus there are the tailed bombs, the bread-crust bombs, Pele's hair, and so on. Various names were used, especially in the past, to designate particular forms of eruptions, taking as types those historical ones of known volcanoes; thus there are the Plinian eruptions, strongly explosive (Vesuvius, 79 A.D.); the Vulcanian (Vulcano, 1888), still explosive, with the ejection of semi-vitreous blocks, pumice and bombs of contemporaneous material; the Strombolian (normal activity of Stromboli), with periodic ejection of ashes and scoriae; the Hawaiian, purely effusive, with fluid magma and formation of lava lakes.

The type of volcanic apparatus varies according to the type of activity and the material emitted. From purely explosive activity there may be explosion channels or funnels, or ash volcanoes (cones). From purely effusive activity there are shield-type lava volcanoes, domes or ridges of stagnation, spines of extrusion according to the degree of fluidity of the magma. From mixed activity, partly explosive and partly effusive, originate the strato-volcanoes or stratovolcanoes, consisting alternately of lava flows and of incoherent materials, of varying degrees of complexity.

The origin of volcanoes, the various types of their activity, the various forms and structures of volcanic apparatuses are essentially to be related to the modes of consolidation of magma. The essential components of magmas, which may vary in their quantitative ratios, are the oxides of silicon, of aluminum, of iron (bi-and tri-valent), of calcium, of magnesium, of potassium, of sodium, of titanium, which represent the so-called fixed components of magma, to which are added elements or combinations of them, characterized, even at ordinary temperature, by a high value of vapor tension. The latter therefore have a tendency to assume the gaseous state and are called volatile components. Among these water predominates, but there are numerous others such as carbon dioxide, hydrochloric acid, hydrogen sulfide, and so on. When, following cooling, the consolidation of magma begins, minerals predominantly silicates separate from it; since they generally do not contain the volatile components except in traces, it is clear that the latter will become ever more concentrated in the molten residue, whose vapor tension will progressively increase. It is evident that when the vapor tension of the molten residue exceeds the external pressure (atmospheric pressure plus the load of the masses overlying the magmatic reservoir), the explosive phenomenon will occur, and the magma will enter into ebullition. Thus a new volcano may originate through what is commonly called an initial perforation, or an ancient quiescent volcano may resume with an initial eruption its long-interrupted activity. This is the case of the eruption of Vesuvius in 79 A.D. If the explosion occurs at an early stage of magmatic crystallization, the activity will be predominantly effusive with abundant lava effusions and formation of lava volcanoes. Typical in this regard are the shield volcanic apparatuses of Iceland and of Hawaii. If the explosion occurs conversely at a very advanced stage of the consolidation of magma, the activity will be predominantly explosive with the formation of cones of incoherent materials or, in extreme cases, of explosion channels or funnels. In intermediate conditions the strato-volcanoes will form, of which Vesuvius and Etna are the type.

Volcanoes are variously distributed over the earth's surface, where it is known that there exist typically volcanic zones (Japan, the Cordilleras, the Mediterranean); but more than the distribution in a geographical sense, it is of interest to consider the distribution of volcanoes in relation to tectonic structure. It is thus possible to distinguish the volcanism of oceanic zones (parts of the lithosphere lacking or poor in Sial), predominantly effusive, from the volcanism of continental massifs, characterized by repeated lava outflows from fractures that give rise to vast and thick basaltic coverings (Deccan, Siberia, Abyssinia), from the volcanism, finally, of orogenic zones (Japan, the Cordilleras, the Aegean) of a tendentially explosive character.

The activity of a single volcano may have a very variable duration of time, from a few days to several tens of millennia; it may be held that volcanism has on the whole been active on the earth since the most remote epochs.

Mario Fornasei