Fig. 466.—Spatter-cone and cavern. Kilauea, Hawaii. (Photo. by Libbey.)
Fig. 467.—Hollow spatter-cone. Oregon. (Russell, U. S. Geol. Surv.)
Subordinate cones.—Small or temporary vents formed as offshoots from the main vents often give rise to secondary or “parasitic” cones. These are sometimes numerous, as in the case of Etna, and they may be so important that the mountain becomes a compound cone. A still more subordinate variety consists of “spatter-cones” formed by small mildly explosive vents that spatter forth little dabs of lava which form chimneys, or cones, and sometimes completely curved domes over vents (Figs. [466] and [467]). Spatter-cones often arise from the lava-flows themselves.
Composite cones.—From most existing volcanoes there issue both lava-flows and fragmental ejecta, and the resulting cones are composite in material. The lava more frequently breaks through the side of the cone than overflows its summit, and this gives rise to irregularities of form and structure. The cones are also subject to partial destruction both by the outbursts of lava and by the explosions, and perhaps also by migration of the vents. As a result, many volcanic regions show old, partially destroyed craters, together with new and more perfect ones, and the history of volcanic action in a region may often be read in the succession of cone formations.
The form of the cone, when composed chiefly of lava, is also affected by the mass of the outflow and by its fluidity. The larger the outflow at a given time, other things being equal, the wider it distributes itself and the flatter is the cone. As a rule, the basic lavas are more fluid than the acidic, and the cones of basic lavas are flatter than the cones of acidic lavas.
Extra-cone distribution.—In violent eruptions, the steam, accompanied with much ash, is shot up to great heights, often rolling outwards in cumulus or cauliflower-like forms ([Fig. 458]). In the more violent explosions these columns are projected several miles. In the phenomenal case of Krakatoa the projection was estimated at seventeen miles. The steam, by reason of its great expansion and its contact with the colder regions of the upper air, is quickly condensed, and prodigious floods of rain frequently accompany the eruption. This rain, carrying down a portion of the ash and gathering up much that had previously fallen, gives rise to mud-flows, which in some cases constitute a large part of the final deposit. These mud-flows chiefly lodge on the lower slopes of the volcano or adjacent to its base, and give rise to rather flat cones, sometimes designated as tufa-cones to distinguish them from cinder-cones formed by the direct fall of fragmental material. Mud-flows appear also to be formed by the ejection of mud and water that had gathered in quiescent craters during intervals between stages of eruption.
A portion of the finer exploded material floats away in the air to greater or less distances, and forms widespread tufa-deposits. In. some cases beds of volcanic ash of appreciable thickness (as those of Nebraska)[282] are found far from any known volcanic center. The extremely fine ash from the great explosion of Krakatoa floated several times around the earth in the equatorial belt and spread northward into the temperate zones.
