Lava-cones are built up of streams of lava which have consolid ated around the funnel of escape. Associated with the lava, how ever, there is usually more or less fragmentary matter, so that the cones are composite in structure and consequently more acute in shape than if they were composed wholly of lava. As the streams of lava in a volcano run at different times in different directions, they radiate from the centre, or flow from lateral or eccentric orifices, as irregular tongues, and do not generally form continuous sheets covering the mountain. When lava is the sole or chief element in the cone, the shape of the hill is determined to a great extent by the viscosity of the lava, its copiousness and the rapidity of flow. If the lava be highly basic and very mobile, it may spread to a great distance before solidifying, and thus form a hill covering a large area and rising perhaps to a great height, but remarkably flat in profile. Were the lava perfectly liquid, it would indeed form a sheet without any perceptible slope of surface. As a matter of fact, some lavas are so fluent as to run down an incline of i°, and flat cones of basalt have in some cases a slope of only le or even less. The colossal mass of Mauna Loa, in Hawaii, forms a remarkably flat broad cone, spreading over a base of enormous area and rising 13,90o feet.
If the lava be less basic and less fusible, the hill formed by its accumulation instead of being a low dome will take the shape of a cone with sides of higher gradient ; in the case of andesite cones, for instance, the slope may vary from 25° to 35°. Acid rocks, or those rich in silica, such as rhyolites and trachytes, may be emitted as very viscous lavas tending to form dome-shaped or bulbous masses.
The Spine of Pelee.—A peculiar volcanic structure appeared at Mont Pelee in the course of the eruption of 1902, and was the subject of careful study by Professor A. Lacroix, Dr. E. 0. Hovey, A. Heilprin and other observers. It appears that from fissures in the floor of the tang Sec a viscous andesitic lava, partly quartzif erous, was poured forth and rapidly solidified superficially, form ing a dome-shaped mass invested by a crust or carapace. Accord ing to Lacroix, the crust soon became fractured, partly by shrinkage on consolidation and partly by internal tension, and the dome grew rapidly by injection of molten matter. Then there gradually rose from the dome a huge monolith or needle, forming a terminal spine, which in the course of its existence varied in shape and height, having been at its maximum in July 1903, when its absolute height was about 5,276 ft. above sea-level. The walls of the spine, inclined at from 75° to 90° to the horizon, were apparently slickensided, or polished and scratched by friction; masses were occasionally detached and vapours escaped.
The Crater.—The eruptive orifice in a normal volcano—the bocca of Italian vulcanologists—is usually situated at the bottom of a depression or cup, known as the crater. This hollow is formed
and kept open by the explosive force of the elastic vapours, and when the volcano becomes dormant or extinct it may be closed, partly by rock falling from its crumbling walls and partly by the solidification of the lava which it may contain. If a renewed out burst occurs, the floor of the old crater may reopen or a new outlet may be formed at some weak point on the side of the mountain; hence a crater may, with regard to position, be either terminal or lateral. The position of the crater will evidently be also changed on any shifting of the general axis of eruption. Vesuvius suffered a reduction of several hundred feet during the great eruption of 1906, the east side of the cone having lost, according to V. R. Matteucci, nearly 400 feet.
Whilst in many cases the crater is a comparatively small circular hollow around the orifice of discharge, it forms in others a large bowl-like cavity, such as is termed in some localities a "caldera." In Hawaii the craters are wide pits bounded by nearly vertical walls, showing stratified and terraced lavas and floored by a great plain of black basalt, sometimes with lakes of molten lava. Prof. W. H. Pickering compares the lava-pits of Hawaii to the crater rings in the moon. Some of the pit-craters here are of great size, but none comparable with the greatest of the lunar craters. Dr. G. K. Gilbert, however, has suggested that the ring-shaped pits on the moon are not of volcanic origin, but are depressions formed by the impact of meteorites. Similarly the "crater" of Coon Butte, near Canyon Diablo, in Arizona, which is 4,000 ft. in diameter and Soo ft. deep, has been regarded as a vast pit due to collision of a meteorite of prodigious size. Probably the largest terrestrial volcanic crater is that of Aso-san, in the isle of Kiushiu (Japan), which is a huge oval depression estimated by some ob servers to have an area of at least ioo sq.m.
On the floor of the crater ejected matter may accumulate as a conoidal pile ; and if such action be repeated in the crater of the new cone, a succession of concentric cones will ultimately be formed. The walls of a perfect crater form a ring, giving the cone a truncated appearance, but the ring may suffer more or less de struction in the course of the history of the mountain. A familiar instance of such change is afforded by Vesuvius. The mountain now so called, using the term in a restricted sense, is a huge com posite cone built up within an old crateral hollow, the walls of which still rise as an encircling rampart on the N. and N.E. sides, and are known as Monte Somma ; but the S. and S.W. sides of the ancient crater have disappeared, having been blown away during some former outburst, probably the eruption of 79.