Volcanoes: What They are and What They Teach

by John W. Judd

When the lava issues from the vent in great quantities it tends to flow on all sides of it, and to build up a great conical heap above the orifice. If the lava be very liquid it flows to great distances, resting at a very slight slope. Thus we find that the volcanoes of Hawaii have been built up of successive ejections of very liquid lava, which have formed cones having a slope of only 6° to 8°, but of such enormous dimensions that the diameter of their bases is seventy miles and their height 14,000 feet.

Fig. 60.—Outlines of Lava-cones.
1. Mauna Loa, in Hawaii. Composed of very fluid lava. 3. The Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or viscid lava.
Click on image to see original negative view.

If, on the other hand, the lava be viscid, or very imperfectly liquid in character, it tends to accumulate immediately around the vent; fresh ejections force the first extruded matter outwards, in the manner so well illustrated by Dr. Reyer's experiments, and at last a more or less steep-sided bulbous mass is formed over the vent. Such bulbous masses, composed of imperfectly fluid lavas, occur in many volcanic districts, and constitute hills of considerable size. From the tendency of matters thus extruded to choke up the vents, however, these volcanoes composed of viscid lavas cannot be expected to attain the vast dimensions reached by some of those composed of very liquid lavas. The difference in the forms of lava-cones composed of very fluid or of somewhat viscid materials is illustrated in [fig. 58]. When the interior of such steep-sided volcanic mountains composed of viscid materials is exposed by the action of denuding forces, the peculiar internal structure we have described is displayed by them. In the Chodi-Berg of Hungary, a great bulbous mass of andesitic rock, this endogenous structure is admirably displayed. It is also well seen in the excavation of the hill of the Grand Sarcoui, a similar mass, composed of altered trachyte, which has been erupted in the midst of a scoria-cone in the Auvergne. See [fig. 44] (p. 126).

CHARACTERS OF COMPOSITE CONES.

Most of the great volcanic mountains of the globe belong to the class of 'composite cones,' and are built up by alternate ejections of fluid lava and fragmentary materials. The slope of the sides in such composite cones is subject to a wide range of variation, being determined in part by the degree of liquidity of the lavas, in part by the nature of the fragmentary materials ejected, and in part by the proportions which the fragmentary and lava-ejections bear to one another.

But there is another set of causes which tends to modify the form and character of these composite, volcanic cones. As we have already pointed out, the sides of such cones are liable to be rent asunder from time to time, and the fissures so produced are injected with masses of liquid lava from below. These fissures, rent in the sides of volcanic cones, often reach the surface and eruptive action takes place, giving rise to the formation of a cone, or series of cones, upon the line of the fissure ([fig. 61]). Such small cones thrown up on the flanks of a great volcanic mountain are known as 'parasitic cones'; though subordinate to the great mountain mass, they may be in themselves of considerable dimensions. Among the hundreds of parasitic cones which stud the flanks of Etna, there are some which are nearly 800 feet in height.

Fig. 61.—Diagram illustrating the formation of Parasitic Cones along lines of fissure formed on the flanks of a great volcanic mountain.