Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

The spine of Pelé.—The disastrous eruption of Mont Pelé upon Martinique in the year 1902 is of importance in connection with the interesting problem of the upward growth of volcanic chimneys during the cone-building period of a volcano. After the conclusion of this great Vulcanian eruption, a spine of lava grew upward from the chimney of the main crater until it had reached an elevation of more then a thousand feet above its base, a figure of the same order of magnitude as the probable height of the upper section of the Vesuvian chimney previous to the eruption of 1906. The Pelé spine ([Fig. 142]) did not grow at a uniform rate, but was subject to smaller or larger truncations, but for a period of 18 days the upward growth was at the rate of about 41 feet per day. Later, the mass split upon a vertical plane revealing a concave inner surface, and was somewhat rapidly reduced in altitude to 600 feet ([Fig. 143]), only to rise again to its full height of about 1000 feet some three months later.

While apparently unique as an observed phenomenon, and not free from uncertainty as to its interpretation, the growth of this obelisk has at least shown us that a mass of rock can push its way up above the chimney of an active volcano even when there are no walls of tuff about it to sustain its outward pressures.

Fig. 144.—Corrugated surface of the Vesuvian cone after the mud flows which followed the eruption in 1906 (after Johnston-Lavis).

The aftermath of mud flows.—When the late Vulcanian explosions of Vesuvius had come to an end, all slopes of the mountain, but especially the higher ones, were buried in thick deposits of the cocoa-colored ash, included in which were larger and smaller projectiles. As this material is extremely porous, it greedily sucks up the water which falls during the first succeeding rains. When nearly saturated, it begins to descend the slopes of the mountain and soon develops a velocity quite in contrast with that of the slow-moving lava. The upper slopes are thus denuded, while the fields and even the houses about the base are invaded by these torrents of mud (lava d’acqua). Inasmuch as these mud flows are the inevitable aftermath of all grander explosive eruptions, the Italian government has of late spent large sums of money in the construction of dikes intended to arrest their progress in the future. It was streams of this sort that buried the city of Herculaneum after the explosive eruption of 79 A.D.

After the mud flows have occurred, the Vesuvian cone, like all similar volcanic cones under the same conditions, is found with deep radial corrugations ([Fig. 144]), such as were long ago described as “barrancoes” and supposed to support the “elevation crater” theory of volcano formation.

The dissection of volcanoes.—To the uninitiated it might appear a hopeless undertaking to attempt to learn by observation the internal structure of a volcano, and especially of a complex volcano of the composite type. The earliest successful attempt appears to have been made by Count Caspar von Sternberg in order to prove the correctness of the theory of his friend, the poet Goethe. Goethe had claimed that a little hill in the vicinity of Eger, on the borders of Bohemia, was an extinct volcano, though the foremost geologist of the time the famous Werner, had promulgated the doctrine that this hill, in common with others of similar aspect, originated in the combustion of a bed of coal. The elevation in question, which is known as the Kammerbühl, consists mainly of cinder, and Goethe had maintained that if a tunnel were to be driven horizontally into the mountain from one of its slopes, a core or plug of lava would be encountered beneath the summit. The excavations, which were completed in 1837, fully verified the poet’s view, for a lava plug was found to occupy the center of the mass and to connect with a small lava stream upon the side of the hill ([Fig. 145]).

Fig. 145.—The Kammerbühl near Eger, showing the tunnel completed in 1837 which proved the volcanic nature of the mountain (after Judd).

It is not, however, to such expensive projects that reference is here made, but rather to processes which are continually going on in nature, and on a far grander scale. The most important dissecting agent for our purpose is running water, which is continually paring down the earth’s surface and disclosing its buried structures. How much more convincing than any results of artificial excavation, as evidence of the internal structure of a volcano, is the monument represented in [Fig. 146], since here the lava plug stands in relief like a gigantic thumb still surrounded by a remnant of cinder deposits. Such exposed chimneys of former volcanoes are found in many regions, and have become known as volcanic necks, pipes, or plugs.