VOLCANO, an opening in the earth's crust, through which heated matter is brought to the surface, where it usually forms a hill, more or less conical in shape, and generally with a hollow or crater at the top. This hill, though not an essential part of the volcanic mechanism, is what is commonly called the volcano. The name seems to have been applied originally to Etna and some of the Lipari Islands, which were regarded as the seats of Hephaestus, a Greek divinity identified with Vulcan, the god of fire in Roman mythology. All the phenomena connected with volcanic activity are comprised under the general designation of volcanism or vulcanicity ; whilst the study of the phenomena forms a department of natural knowledge known as vulcanology.
Volcanic Phenomena.—A volcanic eruption is usually pre ceded by certain symptoms, of which the most common are local earthquakes, subterranean noises, changes in the flow and tempera ture of springs and evolution of various gases in and near the crater. Where a crater has been occupied by water, forming a crater-lake, the water becomes warm, and may even boil.
Of all volcanic phenomena the most constant is the emission of vapour. It is one of the earliest features of an eruption; it persists during the paroxysms, attain ing often to prodigious volume ; and it lingers as the last relic of an outburst. The well-known "pine-tree appendage" of Vesu vius (pino vulcanico), noted by the younger Pliny in his first letter to Tacitus on the eruption in A.D. 79, is a vertical shaft of vapour terminating upwards in a canopy of cloud, and compared popularly with the trunk and spreading branches of the stone pine. During the eruption of Vesuvius in April 1906 the steam and dust rose to a height of from 6 to 8 miles, while at Krakatoa in 1883 the column reached an altitude of nearly 20 miles.
Volcanic Rain and Mud.—The steam given out from the crater soon condenses to rain which mixes with the ashes and loose material to form mud, which may rush down the cone and spread far and wide. Herculaneum was buried beneath a flood of mud swept down from Vesuvius during the eruption of 79, and the hard crust which thus sealed up the city came in turn to be covered by lava-flows from subsequent eruptions.
It sometimes happens that volcanic mud is formed by the mingling of hot ashes not directly with rain but with water from streams and lakes, or even, as in Iceland, with melted snow. A
torrent of mud was one of the earliest symptoms of the violent eruption of Mont Pelee in Martinique in 19o2.
a volcano after a long period of re pose starts into fresh activity, the materials which have accu mulated in the crater are ejected, often mixed with blocks from the walls of the volcanic pipe, or from the older rocks on which the volcano stands. Masses of limestone ejected from Sothma are scattered through the tuffs on the slopes of Vesuvius and contain many interesting minerals due to heating of the lime stone. Similarly at Etna blocks of sandstone are changed to quartzite. A rock consisting of an aggregation of coarse ejected materials, including many large blocks, is known as a "volcanic agglomerate." Cinders, Ashes and Dust.—After the throat of a volcano has been cleared out and a free exit established, the copious discharge of vapour is generally accompanied by the ejection of fresh lava in a fragmentary condition. If the ejected masses bear obvious resemblance to the products of the hearth and the furnace, they are known as "cinders" or "scoriae," whilst the small cinders not larger than walnuts often pass under their Italian name of "lapilli" (q.v.). When of globular or ellipsoidal form, the ejected masses are known as "bombs" (q.v.) ; if the lava has become granulated it is termed "volcanic sand"; when in a finer state of division it is called ash, or if yet more highly comminuted it is classed as dust; but the latter terms are sometimes used interchangeably. After an eruption the country for miles around the volcano may be covered with a coating of fine ash or dust, sometimes nearly white, like a fall of snow, but often greyish, looking rather like Portland cement; this dust insinuates itself into every crack and cranny, reaching the interior of houses even when windows and doors are closed. A heavy fall of ash or cinders may cause great structural damage, crushing the roofs of buildings by sheer weight, as was markedly the case at Ottajano and San Giuseppe during the eruption of Vesuvius in April 1906. On this occasion the dry ashes slipped down the sides of the volcanic cone like an ava lanche, forming great ash-slides with ridges and furrows rather like barrancos, or ravines, caused by rain. The burial of Ottajano and San Giuseppe in 1906 by Vesuvian ejecta, mostly lapilli, has been compared with that of Pompeii in 79.