Volcano

Lava.—The volcanic cinders, sand, ashes and dust described above are but varied forms of solidified lava, which is the most characteristic product of volcanic activity. It is composed of various silicates, or their constituents, in a state of mutual solu tion, and heavily charged with certain vapours or gases, principally water-vapour, superheated and under pressure. The lava differs from the magma before eruption, inasmuch as water and various volatile substances may be expelled on extrusion.

The rapidity of a lava flow is determined partly by the slope of the bed over which it moves and partly by the consistency of the lava, this being dependent on its chemical composition and on the conditions of cooling. In an eruption of Mauna Loa, in Hawaii, in 1855, the lava was estimated to flow at a rate of 4o m. an hour; and at an eruption of Vesuvius in i8o5 a velocity of more than 5o m. an hour, at the moment of emission, was recorded. The rapidity of flow is, however, rapidly checked as the stream ad vances, the retardation being very marked in small flows. Where lava travels down a steep incline there is naturally a great ten dency to form a rugged surface, whilst a quiet flow over a flat plane favours smoothness. If the lava meet a precipice it may form a cascade of great beauty.

If, after a stream of lava has become crusted over, the under lying magma should flow away, a long cavern or tunnel may be formed. Should the flow be rapid the roof may collapse and the fragments, falling on to the stream, may be carried forward or become absorbed in the fused mass. The walls and roof of a lava cave are occasionally adorned with stalactites, whilst the floor may be covered with stalagmitic deposits of lava. The volcanic stalactites are slender, tubular bodies, extremely fragile, often knotted and rippled. Beautiful examples of lava stalactites from Hawaii have been described by Prof. E. S. Dana.

Physical Structure of Lavas.—An amorphous vitreous mass may result from the rapid cooling of a lava on its extrusion from the volcanic vent. The common type of volcanic glass is known as obsidian (q.v.). In many cases the lava brings up myriads of crystals that have been developed during slow solidification in the heart of the volcano. Showers of crystals of leucite have occurred at Vesuvius, of anorthoclase at Mt. Erebus, of labradorite at Etna and of pyroxene at Vesuvius, Etna• and Stromboli. These

"intratelluric crystals" were floating in the molten magma, and had they remained in suspension, this magma would have envel oped them as a ground-mass or base. A rock so formed is gener ally known as a "porphyry," and the structure as porphyritic. In such a lava the large crystals, or phenocrysts, represent an early phase of consolidation and the minerals of the matrix a later stage. For a discussion of the chemical and mineralogical corn position of lavas see PETROLOGY.

In the course of the life of a volcano the lava which it emits may vary within moderate limits, being at one time more acid at another more basic. Such changes are sometimes connected with a shifting of the axis of eruption. Thus at Etna the lavas from the old axis of Trifoglietto in the Val del Bove were andes ites, with about 55% of silica, but those rising in the present conduit are basaltic, with a silica-content of only about 50%. Other instances could be given.

Capillary Lava.

A filamentous form of lava well known at Kilauea, in Hawaii, is termed Pele's hair, after Pele, the goddess of the Hawaiian volcanoes. It resembles the artificial material known as "slag wool"—a material formed by injecting steam into molten slag from a blast-furnace.

Pumiceous Lava.

The copious disengagement of vapour in a glassy lava gives rise to the light cellular or spongy substance, full of microscopic pores, known as pumice (q.v.). It is usually, though not invariably, produced from an acid lava, and may be regarded as the solidified foam of an obsidian. During the eruption of Krakatoa in 1883 enormous quantities of pumice were ejected, and were carried by the sea to vast distances, until they ultimately became water-logged and sank. Professor Judd found the pumice to consist of a vitreous lava greatly inflated by im prisoned vapours.

Water in Lavas.

Whether an eruption is of an explosive or a tranquil character must depend largely, though not wholly, on the chemical composition of the magma, especially on the water content. By relief of pressure on the rise of the column in the volcanic channel, or otherwise, more or less steam will be dis engaged, and if in large quantity this must become, with other vapours, a projectile agency of enormous power. The precise physical condition in which water exists in the magma is a matter of speculation.