A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Map of the Caldera of Palma and the great ravine, called "Barranco de las Angustias." From Survey of Capt. Vidal, R.N., 1837.

Von Buch, in his excellent account of the Canaries, has given us a graphic picture of this island, which consists chiefly of a single mountain ([fig. 455.]). This mountain has the general form of a great truncated cone, with a huge and deep cavity in the middle, about six miles in diameter, called by the inhabitants "the Caldera," or cauldron. The range of precipices surrounding the Caldera are no less than 4000 feet in their average height; at one point, where they are highest, they are 7730 feet above the level of the sea. The external flanks of the cone incline gently in every direction towards the base of the island, and are in part cultivated; but the walls and bottom of the Caldera present on all sides rugged and uncultivated rocks, almost completely devoid of vegetation. So steep are these walls, that there is no part by which they can be descended, and the only entrance is by a great ravine, or Barranco, as it is called (see b b', map, [fig. 456.]), which extends from the sea to the interior of the great cavity, and by its jagged, broken, and precipitous sides, exhibits to the geologist a transverse section of the rocks of which the whole mountain is composed. By this means, we learn that the cone is made up of a great number of sloping beds, which dip outwards in every direction from the centre of the void space, or from the hollow axis of the cone. The beds consist chiefly of sheets of basalt, alternating with conglomerates; the materials of the latter being in part rounded, as if rolled by water in motion. The inclination of all the beds corresponds to that of the external slope of the island, being greatest towards the Caldera, and least steep when they are nearest the sea. There are a great number of tortuous veins, and many dikes of lava or trap, chiefly basaltic, and most of them vertical, which cut through the sloping beds laid open to view in the great gorge or Barranco. These dikes and veins are more and more abundant as we approach the Caldera, being therefore most numerous where the slope of the beds is greatest.

Assuming the cone to be a pile of volcanic materials ejected by a long succession of eruptions (a point on which all geologists are agreed), we have to account for the Caldera and the great Barranco. I conceive that the cone itself may be explained, in accordance with what we know of the ordinary growth of volcanos[392-A], by supposing most of the eruptions to have taken place from one or more central vents, at or near the summit of the cone, before it was truncated. From this culminating point, sheets of lava flowed down one after the other, and showers of ashes or ejected stones. The volcano may, in the earlier stages of its growth, have been in great part submerged, like Stromboli, in the sea; and, therefore, some of the fragments of rock cast out of its crater may not only have been rolled by torrents sweeping down the mountain's side, but have also been rounded by the waves of the sea, as we see happen on the beach near Catania, on which the modern lavas of Etna are broken up. The increased number of dykes, as we approach the axis of the cone, agrees well with the hypothesis of the eruptions having been most frequent towards the centre.

There are three known causes or modes of operation, which may have conduced towards the vast size of the Caldera. First, the summit of a conical mountain may have fallen in, as happened in the case of Capacurcu, one of the Andes, according to tradition, in the year 1462, and of many other volcanic mountains.[393-A] Sections seem wanting, to supply us with all the data required for judging fairly of the tenability of this hypothesis. It appears, however, from Captain Vidal's survey (see [fig. 456.]), that a hill of considerable height rises up from the bottom of the Caldera, the structure of which, if it be any where laid open, might doubtless throw much light on this subject. Secondly, an original crater may have been enlarged by a vast gaseous explosion, never followed by any subsequent eruption. A serious objection to this theory arises from our not finding that the exterior of the cone supports a mass of ruins, such as ought to cover it, had so enormous a volume of matter, partly made up of the solid contents of the dikes, been blown out into the air. In that case, an extensive bed of angular fragments of stone, and of fine dust, might be looked for, enveloping the entire exterior of the mountain up to the very rim of the Caldera, and ought nowhere to be intersected by a dike. The absence of such a formation has induced Von Buch to suppose that the missing portion of the cone was engulphed. It should, however, be remembered, that in existing volcanos, large craters, two or three miles in diameter, are sometimes formed by explosions, or by the discharge of great volumes of steam.

There is yet another cause to which the extraordinary dimensions of the Caldera may, in part at least, be owing; namely, aqueous denudation. Von Buch has observed, that the existence of a single deep ravine, like the Great Barranco, is a phenomenon common to many extinct volcanos, as well as to some active ones. Now, it will be seen by Captain Vidal's map ([fig. 456.] [p. 391.]), that the sea-cliff at Point Juan Graje, 780 feet high, now constituting the coast at the entrance of the great ravine, is continuous with an inland cliff which bounds the same ravine on its north-western side. No one will dispute that the precipice, at the base of which the waves are now beating, owes its origin to the undermining power of the sea. It is natural, therefore, to attribute the extension of the same cliff to the former action of the waves, exerted at a time when the relative level of the island and the ocean were different from what they are now. But if the waves and tides had power to remove the rocks once filling a great gorge which is 7 miles long, and, in its upper part, 2000 feet deep, can we doubt that the same power may have cleared out much of the solid mass now missing in the Great Caldera?

The theory advanced to account for the configuration of Palma, commonly called the "elevation crater theory," is this. All the alternating masses of basalt and conglomerate, intersected in the Barranco, or abruptly cut off in the escarpment or walls of the Caldera, were at first disposed in horizontal masses on the level floor of the ocean, and traversed, when in that position, by all the basaltic dikes which now cut through them. At length they were suddenly uplifted by the explosive force of elastic vapours, which raised the mass bodily, so as to tilt the beds on all sides away from the centre of elevation, causing at the same time an opening at the culminating point. Besides many other objections which may be urged against this hypothesis, it leaves unexplained the unbroken continuity of the rim of the Caldera, which is uninterrupted in all places save one[394-A], namely, that where the great gorge or Barranco occurs.

As a more natural way of explaining the phenomenon, the following series of events may be imagined. The principal vent, from which a large part of the materials of the cone were poured or thrown out, was left empty after the last escape of vapour, when the volcano became extinct. We learn from Mr. Dana's valuable work on the geology of the United States' Exploring Expedition, published in 1849, that two of the principal volcanos of the Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened volcanic cones, 15,000 feet high (see [fig. 457.]), each equalling two and a half Etnas in their dimensions.

Fig. 457.