A Manual of Elementary Geology Part 61

In the Little c.u.mbray, one of the Western Islands, near Arran, the amygdaloid sometimes contains elongated cavities filled with brown spar; and when the nodules have been washed out, the interior of the cavities is glazed with the vitreous varnish so characteristic of the pores of slaggy lavas. Even in some parts of this rock which are excluded from air and water, the cells are empty, and seem to have always remained in this state, and are therefore undistinguishable from some modern lavas.[390-A]

Dr. MacCulloch, after examining with great attention these and the other igneous rocks of Scotland, observes, "that it is a mere dispute about terms, to refuse to the ancient eruptions of trap the name of submarine volcanos; for they are such in every essential point, although they no longer eject fire and smoke."[390-B] The same author also considers it not improbable that some of the volcanic rocks of the same country may have been poured out in the open air.[390-C]

Although the princ.i.p.al component minerals of subaerial lavas are the same as those of intrusive trap, and both the columnar and globular structure are common to both, there are, nevertheless, some volcanic rocks which never occur as lava, such as greenstone, clinkstone, the more crystalline porphyries, and those traps in which quartz and mica appear as const.i.tuent parts. In short, the intrusive trap rocks, forming the intermediate step between lava and the plutonic rocks, depart in their characters from lava in proportion as they approximate to granite.

These views respecting the relations of the volcanic and trap rocks will be better understood when the reader has studied, in the 33d chapter, what is said of the plutonic formations.

FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.

The origin of volcanic cones with crater-shaped summits has been alluded to in the last chapter (p. 368.), and more fully explained in the "Principles of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and Barren Island were described. The more ancient portions of those mountains or islands, formed long before the times of history, exhibit the same external features and internal structure which belong to most of the extinct volcanos of still higher antiquity.

The island of Palma, for example, one of the Canaries, offers an excellent ill.u.s.tration of what, in common with many others, I regard as the ruins of a large dome-shaped ma.s.s formed by a series of eruptions proceeding from a crater at the summit, this crater having been since replaced by a larger cavity, the origin of which has afforded geologists an ample field for discussion and speculation.

[Ill.u.s.tration: Fig. 455. View of the Isle of Palma, and of the entrance into the central cavity or Caldera. From Von Buch's "Canary Islands."]

[Ill.u.s.tration: Fig. 456. 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 s.p.a.ce, 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.

a.s.suming 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 d.y.k.es, 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 ma.s.s 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 const.i.tuting 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 ma.s.s 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 ma.s.ses 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 ma.s.ses 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 ma.s.s 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 princ.i.p.al 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 princ.i.p.al 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.

[Ill.u.s.tration: Fig. 457. Mount Loa, in the Sandwich Islands. (Dana)

_a._ Crater at the summit.

_b._ The lateral crater of Kilauea.

The dotted lines indicate a supposed column of solid rock caused by the lava consolidating after eruptions.]

From the summits of these lofty though featureless hills, and from vents not far below their summits, successive streams of lava, often 2 miles or more in width, and sometimes 26 miles long, have flowed. They have been poured out one after the other, some of them in recent times, in every direction from the apex of the cone, down slopes varying on an average from 4 degrees to 8 degrees; but at some places considerably steeper.[394-B]

Sometimes deep rents open on the sides of these cones, which are filled by streams of lava pa.s.sing over them, the liquid matter in such cases probably uniting in the fissure with other lava melted in subterranean reservoirs below, and thus explaining the origin of one great cla.s.s of lateral dikes, on Etna, Palma, and other cones.

If the flattened domes, such as those here alluded to in the Sandwich Islands, instead of being inland, and above water, were situated in mid-ocean, like the Island of St. Paul, and for the most part submerged (see figs. 458, 459, 460.), and if a gradual upheaval of such a dome should then take place, the denuding power of the sea could scarcely fail to play an important part in modifying the form of the volcanic mountain as it rose. The crater will almost invariably have one side much lower than all the others, namely, that side towards which the prevailing winds never blow, and to which, therefore, showers of dust and scoriae are rarely carried during eruptions. There will also be one point on this windward or lowest side more depressed than all the rest, by which the sea may enter as often as the tide rises, or as often as the wind blows from that quarter.

For the same reason that the sea continues to keep open a single entrance into the lagoon of an atoll or annular coral reef, it will not allow this pa.s.sage into the crater to be stopped up, but scour it out, at low tide, or as often as the wind changes. The channel, therefore, will always be deepened in proportion as the island rises above the level of the sea, at the rate perhaps of a few feet or yards in a century.

[Ill.u.s.tration: Fig. 458. Map of the Island of St. Paul, in the Indian Ocean, lat. 38 44' S., long. 77 37' E., surveyed by Capt. Blackwood, R.N., 1842.]

[Ill.u.s.tration: Fig. 459. View of the Crater of the Island of St. Paul.]

The island of St. Paul may perhaps be motionless; but if, like many other parts of the earth's crust, it should begin to undergo a gradual upheaval, or if, as has happened to the sh.o.r.es of the Bay of Baiae, its level should oscillate, with a tendency upon the whole to increased elevation, the same power which has cut away part of the cone, and caused the cliffs now seen on the north-east side of the island, would have power to undermine the walls of the crater, and enlarge its diameter, keeping open the channel, by which it enters into it. This ravine might be excavated to the depth of 180 feet (the present depth of the crater), and its length might be extended to many miles according to the size of the submerged part of the cone. The crater is only a mile in diameter, and the surrounding cliffs, where loftiest, only 800 feet high, so that the size of this cone and crater is insignificant when compared to those in the Sandwich Islands, and I have merely selected it because it affords an example of a cla.s.s of insular volcanos, into the craters of which the sea now enters by a single pa.s.sage. The crater of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged cone, like St. Paul, the excavating power of the ocean might in conjunction with gaseous explosions and co-operating with a gradual upheaving force, give rise to a caldera on as grand a scale as that exhibited by Palma.

[Ill.u.s.tration: Fig. 460. Side view of the Island of St. Paul (N.E. side).

Nine-pin rocks two miles distant. (Captain Blackwood.)]

If, after the geographical changes above supposed, the volcanic fires long dormant should recover their energy, they might, as in the case of Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old central vent, long sealed up at the bottom of the caldera, new floods of lava and clouds of elastic vapours. Should this happen, a new cone will be built up in the middle of the cavity or circular bay, formed, partly by explosion, partly perhaps by engulphment, and partly by aqueous denudation. In the island of Palma this last phase of volcanic activity has never occurred; but the subterranean heat is still in full operation beneath the Canary Islands, so that we know not what future changes it may be destined to undergo.

FOOTNOTES:

[378-A] I have been favoured with this drawing by Captain B. Hall.

[381-A] Cambridge Transactions, vol. i. p. 402.

[382-A] Cambridge Trans., vol. i. p. 410.

[382-B] Ibid. vol. ii. p. 175.

[382-C] Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172.

[382-D] Geol. Trans., 1st series, vol. iii. p. 210. and plate 10.

[382-E] Ibid. p. 201.

[383-A] Geol. Trans., 1st series, vol. iii. p. 205.

[383-B] Ibid. p. 213.; and Playfair, Ill.u.s.t. of Hutt. Theory, p. 253.

[383-C] Geol. Trans., 1st series, vol. iii. p. 206.

[383-D] Sedgwick, Camb. Trans. vol. ii. p. 37.

[383-E] Ill.u.s.t. of Hutt. Theory, -- 253. and 261. Dr. MacCulloch, Geol.

Trans., 1st series, vol. ii. p. 305.

[383-F] Syst. of Geol. vol. i. p. 206.

[384-A] Camb. Trans. vol. ii. p. 180.

[385-A] MacCul. Syst. of Geol. vol. ii. p. 137.

[385-B] Seale's Geognosy of St. Helena, plate 9.

[386-A] Fortis. Mem. sur l'Hist. Nat. de l'Italie, tom. i. p. 233. plate 7.

[387-A] Scrope, Geol. Trans. vol. ii. p. 205. 2d series.