1911 Encyclopædia Britannica/Pumice

From Wikisource
Jump to navigation Jump to search

PUMICE (Lat. pumex, spumex, spuma, froth), a very porous, froth-like, volcanic glass. It is an igneous rock which was almost completely liquid at the moment of effusion and was so rapidly cooled that there was no time for it to crystallize. When it solidified the vapours dissolved in it were suddenly released and the whole mass swelled up into a froth which immediately consolidated. Had it cooled under more pressure it would have formed a solid glass or obsidian (q.v.); in fact if we take fragments of obsidian and heat them in a crucible till they fuse they will suddenly change to pumice when their dissolved gases are set free. Hence it can be understood that pumice is found only in recent volcanic countries. Artificial substances resembling pumice can be produced by blowing steam through molten glass or slag, and when a mass of slag is suddenly cooled by being tipped into the sea (as is the case at the blast furnaces of Whitehaven in Cumberland) it swells up into a pumiceous form so light and full of vesicles that it will float on water. Any type of lava, if the conditions are favourable, may assume the pumiceous state; but basalts and andesites do not so often occur in this form as do trachytes and rhyolites. Pumices are most abundant and most typically developed from acid rocks; for which reason they usually accompany obsidians, in fact in Lipari and elsewhere the base of a lava flow may be black obsichan while the upper portion is a snow white pumice.

Small crystals of various minerals occur in many pumices; the commonest are felspar, augite, hornblende and zircon. If they are abundant they greatly diminish the economic value of the rock, as they are hard and wear down more slowly than the glassy material; consequently they produce scratches. The cavities of pumice are sometimes rounded, but may, also be elongated or tubular owing to the flowing movement of the solidifying lava. The glass itself forms threads, fibres and thin partitions between the vesicles. Rhyolite and trachyte pumices are white, contain 60 to 75% of silica and the specific gravity of the glass is 2.3 to 2.4; andesite pumices are often yellow or brown; while pumiceous basalts, such as occur in the Sandwich Islands, are pitch black when perfectly fresh.

Good pumice is found in Iceland, Hungary, Nevada, Teneriffe, New Zealand, Pantellaria and the Lipari Islands. The last-named are the chief sources of pumice for the arts and manufactures. At Campo Bianco in Lipari there is an extinct volcanic cone with a breached crater from which a dark stream of obsidian has flowed. For industrial purposes the best varieties are obtained from Monte Pelato and Monte Chirica. The pumice is extracted by means of shafts and tunnels driven through the soft incoherent stone. It is brought out in blocks of irregular shape and size and is trimmed into slabs and graded into several qualities before it is exported to Canneto, which is the centre of the pumice trade. The workmen say that the good pumice occurs in beds or veins, which are probably lava flows and are separated by valueless rock or by obsidian. The value depends entirely on the regularity, size and shape of the steam cavities and on the absence of minute crystals. From' time immemorial the extraction and sale of pumice have been one of the principal sources of wealth to the inhabitants of this island. An inferior pumice, known in Lipari as Alessandrina, is used for smoothing oilcloth. Though all the Aeolian Isles are volcanic no pumice is exported from any of the others. In Iceland, Teneriffe and Hungary pumice also occurs, but not in sufficient quantity, or of such quality as to render it worth working on a large scale. It is estimated that in Lipari there are 170 pumice quarries (or mines) giving employment to 1200 persons and producing 6000 tons of pumice per annum. The price varies with the quality: from 3 lire per 100 kilogrammes for the commonest sorts to 200 or 300 lire for the best pieces, the average being about 15 lire. Much pumice is also used nowadays in the form of a fine powder, produced by crushing the rock, and forms an ingredient of metal polishes and some kinds of soap. It is often confounded with diatom earth or tripoli powder, but can easily be recognized by the aid of the microscope or by simple chemical tests.

Among the older volcanic rocks pumice occurs, but usually has its cavities filled up by deposits of secondary minerals introduced by percolating water; hence it is of no value for industrial pun, poses. Pumice, in minute fragments, has been shown to have an exceedingly wide distribution over the earth's surface at the present day. It occurs in all the deposits which cover the floor of the deepest portion of the oceans, and is especially abundant in the abysmal red clay. In some measure this pumice has been derived from submarine volcanic eruptions, but its presence is also accounted for by the fact that pumice will float on water for months, and is thus distributed over the sea by windsand currents. After a long time it becomes waterlogged and sinks to the bottom, where it gradually disintegrates and is incorporated in the muds and oozes which are gathering there. After the great eruption of Krakatoa in 1883 banks of pumice covered the surface of the sea for 'many miles and rose in some cases for four or five ft. above the water level. In addition to this much finely broken pumice was thrown into the air to a great height and was borne away by the winds, ultimately settling down in the most distant parts of the continents and oceans.  (J. S. F.)