Volcanic Cones are built up by the material which the volcanoes eject, and vary in shape according to the character of those materials and to the violence of the eruptions. Those vents which yield only lavas build up cones of solid rock, the steepness of which corresponds to the degree of fluidity of the flows. The remarkably liquid lavas of the Sandwich Islands have formed cones of exceedingly gentle slope, 30 to 10° (see Fig. 28, the cone of Mauna Loa). Very stiff lavas which consolidate rapidly form very steep-sided cones. The cones which are constructed principally out of fragmental materials are steep (300); the more so, the coarser the fragments which compose them, and often beautifully symmetrical, as in the noble mountains of our Pacific States, such as Mt. Shasta, Mt. Hood, and Mt. Rainier. Most cones are built up of scoriae, ashes, and lava flows, while the fissures that radiate from the crater are filled by dykes, greatly strengthening the mountain, as in the case of Vesuvius. The latter is noted for its double head, Monte Somma being part of an ancient crater ring, one side of which was destroyed by an explosion before the cone of Vesuvius was built up.

It is usually stated that the explosion which destroyed part of Monte Somma was that of 79 a.d., but it is not improbable that the destruction took place at a much earlier date.

Fig. 28. - Mauna Loa, seen from a distance of 40 miles. (Photograph by Libbey).

Violent explosions occurring within a volcano blow off more or less of the top, thus producing the truncated cones and crater rings so often seen among volcanic mountains.

Volcanoes, like other mountains, are subject to the destructive activity of the atmosphere, of rivers and of the sea, and, when eruptions have ceased, this destruction may go on with great rapidity, especially in the case of cones made up of loose materials.

Fig. 29. - Mt. Shasta, California. (U. S. G. S).

Very ancient cones can seldom be found, for this reason, and often the lava-filled pipe is the only record left of an ancient volcano.