Volcano

Volcanic Vapours.

It seems not unlikely that the vapours and gases exist in the volcanic magma in much the same way that they can exist in molten metal. It appears that many igneous rocks contain gases locked up in their pores, not set free by pulverization, yet capable of expulsion by strong heat. The gases in rocks have been the subject of elaborate study by R. T. Cham berlin, whose results appear in publication No. Io6 of the Car negie Institution of Washington.

After the surface of a lava-stream has become crusted over, vapour may still be evolved in the interior of the mass, and in seeking release may elevate or even pierce the crust. Small cones may thus be thrown up on a lava-flow, and when vapour escapes from terminal or lateral orifices they are known as "spiracles." The steam may issue with sufficient force to toss up the lava in little fountains. When the lava is very liquid, as in the Hawaiian volcanoes, it may of ter projection from the blow-hole fall back in drops and plastic clots, which on consolidation form, by their union, small cones.

Vapour-vents connected with volcanoes are often known as fumaroles (q.v.). (See also GEYSER and SOLFATARA.) In some volcanoes much sulphur, formed by a reaction between sulphuretted hydrogen and sulphur dioxide, is deposited. Chlorides also are sometimes formed in considerable quantity, especially ammonium chloride and common salt. The presence of various metallic chlorides and sulphides has often been observed. The pale flames sometimes seen in craters are due to the burning of hydro gen and various hydrocarbons, this being the only true combustion connected with vulcanicity : it is quite unimportant.

The Dust Cloud of Mt. Pelee.

The eruptions in Martinique and St. Vincent in the West Indies in 1902 furnished examples of a type of activity not previously recognized by vulcanologists, though, as Professor A. Lacroix has pointed out, similar phenom ena have no doubt occurred elsewhere, especially in the Azores. By Dr. Tempest Anderson and Sir J. S. Flett, who were com missioned by the Royal Society to report on the phenomena, this type of explosive eruption is distinguished as the "Pelean type." Its distinctive character is found in the sudden emission of a dense black cloud of superheated and suffocating gases, heavily charged with incandescent dust, moving with great velocity and accompanied by the discharge of immense volumes of volcanic sand, which are not rained down in the normal manner but descend like a hot avalanche. In its typical form, the cloud at Pelee appeared as a solid bank, opaque and impenetrable, but having the edge in places hanging like folds of a curtain, and apparently of brown or purplish colour. Rolling along like an inky torrent,

it produced in its passage intense darkness, relieved by vivid lightning. After leaving the crater, it underwent enormous ex pansion, and Anderson and Flett were led to suggest that pos sibly at the moment of emission it might have been partly in the form of liquid drops, which on solidifying evolved large volumes of gas held previously in occlusion. The deadly effect of the blast seems to have been mostly due to the irritation of the mucous membrane of the respiratory passages by the fine hot dust—a serious aggravation of the calamity.

Forms of Volcanoes.

Those volcanic products which are solid when ejected, or which solidify after extrusion, tend to form by their accumulation around the eruptive vent a hill, which, though generally more or less conical, is subject to much variation in shape. It occasionally happens that the hill is composed wholly of ejected blocks, not themselves of volcanic origin. This rather exceptional type is represented in the Eifel by certain monticules which consist of fragments of altered Devonian slate.

In the ordinary paroxysmal type of eruption, however, cinders and ashes are shot upwards by the explosion and then descend in showers, forming around the orifice a mound, in shape rather like the diminutive cone of sand in the lower lobe of an hour-glass, Lit tle cinder-cones of this character may be formed within the crater of a large volcano during a single eruption ; whilst large cones are built up by many successive discharges, each sheet of fragmentary material mantling more or less regularly round the preceding layer. The symmetry of the hill is not infrequently affected by disturbing influences—a strong wind, for example, blowing the loose matter towards one side. The sides of a cinder cone have generally a steep slope, varying from 30° to 45°, depending on the angle of repose of the ejectamenta. Excellent examples of small scoria cones are found among the puys of Auvergne in central France, whilst a magnificent illustration of this type of hill is furnished by Fuji-san, in Japan, which reaches an altitude of 12,000 ft. How such a cone may be rapidly built up was well shown by the forma tion of Monte Nuovo, near Pozzuoli—a hill 40o ft. high and 14 m. in circumference, which is known from contemporary evidence to have been formed in the course of a few days in Sept. 1538.