Volcanic topography is primarily constructive. The cones vary in form and height in accordance with the amount and character of the material of which they are composed, and the nature of the eruptions. Thus*, we find lofty, steep-sided cinder cones, like those of the Pacific coast, or very gently sloping lava cones, like those of the Sandwich Islands, or truncated cones and crater-rings, due to violent explosions and to remelting and engulfment of the upper part of the cone. Lava flows may take the form of long, narrow streams, or great floods poured out one upon another, until immense volcanic plateaus are built up, like those of Oregon and Washington.

The progress of denudation sculptures these volcanic masses in characteristic ways, already described in Chapter XV (Unstratified Or Massive Rocks). Cones are first furrowed with ravines and valleys and then gradually degraded into necks, or into low hills of volcanic agglomerate. The infinite variety of combinations of soft tuffs, loose masses of scoriae and ash, hard sheets, streams, pipes and dykes of lava, give rise to the manifold forms of volcanic mountains and islands in the course of denudation, the harder elements resisting longer and standing in relief. Lava flows are generally harder than the stratified rocks upon which they rest, and therefore, aside from their original form, their topographical effect is much the same as that of an exceptionally hard stratum among softer beds. A surface stream may be, indeed eventually must be, cut up by erosion into isolated masses, which protect the underlying softer rocks and thus form flat-topped table mountains and mesas, the lava cap with nearly vertical sides, the stratified rocks below with gentler slopes, especially in pluvial climates.

A lava plateau is dissected by streams, first trenching steep canons and then atmospheric erosion widens the canons and narrows the divides, just as in ordinary plateaus of stratified rocks, but the great hardness of the volcanic masses renders the process very slow.