Fig. 122.—A series of breached cinder cones where the place of eruption has migrated along the underlying fissure. The Puys Noir, Solas, and La Vache in the Mont Dore Province of central France (after Scrope).
The composite cone.—The life histories of volcanoes are generally so varied that lava domes and the pure types of cinder cones are less common than volcanoes in which paroxysmal eruptions have alternated with explosions, and where, therefore, the structure of the mountain represents a composite of lava and cinder. Such composite cones possess a skeleton of solid rock upon which have been built up alternate sloping layers of cinder and lava. In most respects such cones stand in an intermediate position between lava domes and cinder cones.
Fig. 123.—The bocca or mouth upon the inner cone of Mount Vesuvius from which flowed the lava stream of 1872. This lava stream appears in the foreground with its characteristic “ropy” surface.
Regarded as a retaining wall for the lava which mounts in the chimney, the cinder cone is obviously the weakest of all. Should lava rise in a cinder cone without an explosion occurring, the cone is at once broken through upon one side by the outwelling of the lava near the base. Thus arises the characteristic breached cone of horseshoe form ([Fig. 122]).
Fig. 124.—A row of parasitic cones raised above a fissure which was opened upon the flanks of Mount Etna during the eruption of 1892 (after De Lorenzo).
Quite in contrast with the weak cinder cone is the lava dome with its rock walls and relatively flat slopes. Considered as a retaining wall for lava it is much the strongest type of volcanic mountain, and it is likely that the hydrostatic pressure of the lava within the crater would seldom suffice to rupture the walls, were it not that the molten rock first fuses its way into old stream tunnels buried under the mountain slopes (see ante, p. 112). Composite cones have a strength as retaining walls for lava which is intermediate between that of the other types. Their Vulcanian eruptions of the convulsive type are initiated by the formation of a rent or fissure upon the mountain flanks at elevations well above the base, the opening of the fissure being generally accompanied by a local earthquake of greater or less violence.
From one or more such fissures the lava issues usually with sufficient violence at the place of outflow to build up over it either an enlarged type of driblet cone, referred to as a “mouth”, or bocca[1] ([Fig. 123]), or one or more cinder cones which from their position upon the flanks of the larger volcano are referred to as parasitic cones ([Fig. 124]). The lava of Vesuvius more frequently yields bocchi at the place of outflow, whereas the flanks of Etna are pimpled with great numbers of parasitic cinder cones, each the monument to some earlier eruption ([Fig. 125]).