Fig. 119.—Artificial production of the structure of a cinder cone with use of colored sands carried up in alternation by a current of air (after G. Linck).
The term lapilli, or sometimes rapilli, is applied to the ejected lava fragments when of the average size of a finger joint. This is the material which still partially covers the unexhumed portions of the city of Pompeii. Volcanic sand, ash, and dust are terms applied in order to increasingly fine particles of the ejected lava. The finest material, the volcanic dust, is often carried for hundreds and sometimes even for thousands of miles from the crater in the high-level currents of the atmosphere. Inasmuch as this material is deposited far from the crater and in layers more or less horizontal, such material plays a small rôle in the formation of the cinder cone. The coarser sands and ash, on the other hand, are the materials from which the cinder cone is largely constructed.
The manner of formation and the structure of cinder cones may be illustrated by use of a simple laboratory apparatus ([Fig. 119]). Through an opening in a board, first white and then colored sand is sent up in a light current of air or gas supplied from suitable apparatus. The alternating layers of the sand form in the attitudes shown; that is to say, dipping inward or toward the chimney of the volcano at all points within the crater rim, and outward or away from it at all points outside ([Fig. 119]). If the experiment is carried so far that at its termination sand slides down the crater walls into the chimney below, the inward dipping layers will be truncated, or even removed entirely, as shown in [Fig. 119 b].
Fig. 120.—Diagram to show the contrast between a lava dome and a cinder cone. AAA, cinder cone; BabC, lava dome; DE, line of low cinder cones above a fissure (after Thoroddsen).
The profile lines of cinder cones.—The shapes of cinder cones are notably different from those of lava mountains. While the latter are domes, the mountains constructed of cinder are conical and have curves of profile that are concave upward instead of convex ([Fig. 120]). In the earlier stages of its growth the cinder cone has a crater which in proportion to the height of the mountain is relatively broad ([Fig. 99], [p. 104]).
Fig. 121.—Mayon volcano on the island of Luzon, P.I. A remarkably perfect high cinder cone.
Speaking broadly, the diameter of the crater is a measure of the violence of the explosions within the chimney. A single series of short and violent explosive eruptions builds a low and broad cinder cone. A long-continued succession of moderately violent explosions, on the other hand, builds a high cone with crater diameter small if compared with the mountain’s altitude, and the profile afforded is a remarkably beautiful sweeping curve ([Fig. 121]). Toward the summit of such a cone the loose materials of which it is composed are at as steep an angle as they can lie, the so-called angle of repose of the material; whereas lower down the flatter slopes have been determined by the distribution of the cinder during its fall from the air. When one makes the ascent of such a mountain, he encounters continually steeper grades, with the most difficult slope just below the crest.