A lava is a more or less completely melted rock; the degree of fluidity varies greatly in different lavas, but is rarely, if ever, perfect. Instead of being a true liquid, a lava ordinarily consists of larger and smaller crystals, embedded in a pasty mass, which is saturated with steam and gases. The degree of fluidity depends upon several factors, the most obvious of which is temperature; the more highly heated the mass is, the more perfectly will it be melted. The quantity of imprisoned gases and vapours present has also an important effect, and some lavas appear to owe nearly all their mobility to these vapours. A third and most significant factor is the chemical composition. Those lavas which contain high percentages of silica (Si02), the acid lavas, are much less readily fusible than the basic lavas, in which the percentage of silica is lower. The difference in the proportion of silica present is associated with other chemical differences which have a similar effect upon fusibility, the basic kinds having much more lime, magnesia and iron in them, which act as fluxes.
Fig. 21. - Sunset Butte, Arizona. An extinct volcano, with scoriaceous block-lava in foreground. (Photograph by A. E. Hackett, Flagstaff, Ariz).
The experiments of Barus on the fusibility of lavas, which he divides into three groups, resulted as follows: (1) Certain lavas fuse readily (22500 F.); these are of basic composition and are made up of lime-soda felspars, the augitic and allied ferro-magnesian minerals, and iron oxide, but rarely have quartz. (2) A second group is of medium fusibility (25200 F.), and is made up of lime-soda felspars, augitic or hornblende minerals, and frequently quartz. (3) The third series melts with difficulty (27000 F.),and remains pasty at even 31000 F. These are acid lavas, and are composed of potash felspars, with quartz, hornblende, or mica. Lavas which, like those of the Sandwich Islands, are notably fluid, are always of basic composition.
Fig. 22. - Lava-tunnel, and " Spatter-cone" formed by escaping steam, Kilauea.
(Photograph by Libbey).
When a lava stream reaches the surface of the ground, the imprisoned vapours immediately begin to escape and the surface of the molten mass to cool and harden. The surface layers are blown by the steam bubbles into a light, frothy or slaggy consistency, forming "scoriae" or cindery masses. The motion of the lava breaks up this thin crust into loose slabs and blocks, and on the advancing front of the stream these loose masses rattle down over one another in the wildest confusion. The less perfectly fused lavas are soon covered with heaped-up cindery blocks, while the more completely fluid lavas are characterized by curiously twisted, ropy surfaces, such as may be observed in the slag from an iron furnace. The front of a lava stream advances, not by gliding over the ground, but by rolling, the bottom being retarded by the friction of the ground and the top moving faster, so that it is continually rolling down at the curved front end and forming the bottom. Thus, the scoriae, though formed mostly on the top of the stream, are rolled beneath it, and the whole is enclosed in a cindery envelope. Or the flow may be checked by the mass of cinders, until the fluid lava bursts through them in a fresh stream.
The scoriaceous mass is a non-conductor of heat, and greatly retards the cooling of the interior mass, which may remain hot for rhany years. The arched surface of cindery blocks may become self-supporting, and then the still fluid mass will flow away from beneath it, leaving long tunnels or caverns. These tunnels are especially well shown in Iceland and the Sandwich Islands.
Fig. 23. - Lava stalactites and stalagmites in lava-tunnel, Kilauea. (Photograph by Libbey).
The distance to which lava streams extend and the rapidity .with which they move are determined by the abundance and fluidity of the lava and the slope over which it flows. Some lavas are so liquid that they flow for many miles, even down moderate slopes, while others are so pasty that they stiffen and set within a short distance of the vent, even on steep grades. Ordinarily the motion soon becomes very slow, though thoroughly melted masses pouring down steep slopes may, for a short time, move very swiftly. One of the lava floods from Mauna Loa moved fifteen miles in two hours, and for shorter distances much higher rates of speed have been observed; but this is very exceptional.
The cooling of the surfaces of the lava stream takes place rapidly, while the interior cools but slowly, and great thicknesses require very long periods of time to become entirely cold. The differences in the rate of cooling produce very strongly marked varieties in the appearance and texture of the resulting rock. The portions which have chilled and solidified very quickly are glassy and form the volcanic glass, obsidian. If the swiftly cooling portions have been much disturbed by the bubbles of steam and vapours, they are made light and frothy; in some cases, as in pumice, they will float upon water. Otherwise, the glass is solid and is usually very dark in colour, resembling an inferior bottle glass in appearance. Microscopic examination shows minute, hair-like bodies in the glass, which are called crystallites, and represent the incipient stages of crystallization.
Passing inward from the surface of the lava stream, we find the steam bubbles becoming rarer, until they cease altogether, the vapours having escaped while the lava was still so soft that the bubble holes soon collapsed. At the same time the glassy texture of the rock is replaced by a stony character, which the microscope shows to be due to the formation of crystals too minute to be recognized by the unaided eye. Still deeper in the rock the stony texture passes gradually into an obviously crystalline one; and the slower the cooling, the larger will these crystals be, though in lava streams which have cooled on the surface of the ground, the whole mass, even of the deeper parts, is never coarsely crystalline.
Fig. 24. - A hand-specimen of obsidian, showing the glassy lustre and fracture.