Part 2 (2/2)
Piazzi Smyth, who during a prolonged visit to this mountain in 1856 made a careful survey of its form and structure, shows that the great cone is surrounded by an outer ring of basalt enclosing two _foci_ of eruption, the lavas from which have broken through the ring of the outer crater on the western side, and have poured down the mountain. At the top of the peak its once active crater is filled up, and we find a convex surface (”The Plain of Rambleta”) surmounted towards its eastern end by a diminutive cone, 500 feet high, called ”Humboldt's Ash Cone.” The slope of the great cone of Teneriffe ranges from 28 to 38; and below a level of 7000 feet the general slope of the whole mountain down to the water's edge varies from 10 to 12 from the horizontal. The great cone is penetrated by numerous basaltic d.y.k.es.
The Cape de Verde Islands, which contain beds of limestone along with volcanic matter, possess in the island of Fuego an active volcano, rising to a height of 7000 feet above the surface of the ocean. The central cone, like that of Teneriffe, rises from within an outer crater, formed of basalt alternating with beds of agglomerate, and traversed by numerous d.y.k.es of lava. This has been broken down on one side like that of Somma; and over its flanks are scattered numerous cones of scoriae, the most recent dating from the years 1785 and 1799.[3]
[Ill.u.s.tration: Fig. 5.--View of the summit of the Peak of Teneriffe (12,225 feet) and of the secondary crater, or outer ring of basaltic sheets which surrounds its base; seen from the east.--(After Leopold von Buch.)]
The volcanoes of Lancerote have a remarkably linear arrangement from west to east across the island. They are not yet extinct; for an eruption in 1730 destroyed a large number of villages, and covered with lava the most fertile tracts in the island, which at the time of Leopold von Buch's visit lay waste and dest.i.tute of herbage.[4] In the island of Palma there is one large central crater, the Caldera de Palma, three leagues in diameter, the walls of which conform closely to the margin of the coast. Von Buch calls this crater ”une merveille de la nature,”
for it distinguishes this isle from all the others, and renders it one of the most interesting and remarkable amongst the volcanic islands of the ocean. The outer walls are formed of basaltic sheets, and towards the south this great natural theatre is connected with the ocean by a long straight valley, called the ”Barranco de los Dolores,” along whose sides the structure of the mountain is deeply laid open to view. The outer flanks of the crater are furrowed by a great number of smaller barrancos radiating outward from the rim of the caldera. Von Buch regards the barrancos as having been formed during the upheaval of the island, according to his theory of the formation of such mountains (the elevation-theory); but unfortunately for his views, these ravines widen outwards from the centre, or at least do not become narrower in that direction, as would be the case were the elevation-theory sound. The maps which accompany Von Buch's work are remarkably good, and were partly constructed by himself.
(_c._) _Volcanic Islands in the Atlantic south of the Equator._--The island of Ascension, formed entirely of volcanic matter, rises from a depth of 2000 fathoms in the very centre of the Atlantic. As described by Darwin, the central and more elevated portions are formed of trachytic matter, with obsidian and laminated ash beds. Amongst these are found ejected ma.s.ses of unchanged granite, fragments of which have been torn from the central pipe during periods of activity, and would seem to indicate a granitic floor, or at least an original floor upon which more recent deposits may have been superimposed. In St. Helena we seem, according to Daubeny, to have the mere wreck of one great crater, no one stream of lava being traceable to its source, while d.y.k.es of lava are scattered in profusion throughout the whole substance of the basaltic ma.s.ses which compose the island. Tristan da Cunha, in the centre of the South Atlantic, rises abruptly from a depth of 12,150 feet, at a distance of 1500 miles from any land; and one of its summits reaches an elevation of 7000 feet, being a truncated cone composed of alternating strata of tuff and augitic lava, surrounding a crater in which is a lake of pure water. The volcano is extinct or dormant.
Were the waters of the ocean to be drawn off, these volcanic islands would appear like stupendous conical mountains, far loftier, and with sides more precipitous, than any to be found on our continental lands, all of which rise from platforms of considerable elevation. The enormous pressure of the water on their sides enables these mid-oceanic islands to stand with slopes varying from the perpendicular to a smaller extent than if they were sub-aerial; and it is on this account that we find them rising with such extraordinary abruptness from the ”vasty deep.”
(_d._) _Volcanic Islands of the Pacific._--The volcanic islands of this great ocean are scattered over a wide tract on both sides of the equator. Those to the north of this line include the Sandwich Islands, the Mariana or Ladrone Islands, South Island, and Bonin Sima; south of the equator, the Galapagos, New Britain, Salomon, Santa Cruz, New Hebrides, the Friendly and Society Isles. While the coral reefs and islands of the Pacific may be recognised by their slight elevation above the surface of the waters, those of volcanic origin and containing active or extinct craters of eruption generally rise into lofty elevations, so that the two kinds are called the _Low_ Islands and _High_ Islands respectively. Amongst the group are trachytic domes such as the Mountain of Tobreonu in the Society Islands, rising to a height probably not inferior to that of Etna, with extremely steep sides, and holding a lake on its summit.[5] The linear arrangement of some of the volcanic islands of the Pacific is ill.u.s.trated by those of the Tonga, or Friendly, Group, lying to the north of New Zealand. They consist of three divisions--(1) the volcanic; (2) those formed of stratified volcanic tuff, sometimes entirely or partially covered by coralline limestone; and (3) those which are purely coralline. The first form a chain of lofty cones and craters, lying in a E.N.E. and W.S.W.
direction, and rising from depths of over 1000 fathoms. Mr. J. J.
Lister, who has described the physical characters of these islands, has shown very clearly that they lie along a line--probably that of a great fissure--stretching from the volcanic island of Amargura on the north (lat. 18 S.), through Lette, Metis, Kao (3030 feet), Tofua, Falcon, Honga Tonga, and the Kermadec Group into the New Zealand chain on the south. Some of these volcanoes are in a state of intermittent activity, as in the case of Tofua (lat. 20 30' S.), Metis Island, and Amargura; the others are dormant or extinct. The whole group appears to have been elevated at a recent period, as some of the beds of coral have been raised 1272 feet and upward above the sea-level, as in the case of Eua Island.[6] The greater number of the Pacific volcanoes appear to be basaltic; such as those of the Hawaiian Group, which have been so fully described by Professor J. D. Dana.[7] Here fifteen volcanoes of the first cla.s.s have been in brilliant action; all of which, except three, are now extinct, and these are in Hawaii the largest and most eastern of the group. This island contains five volcanic mountains, of which Kea, 13,805 feet, is the highest; next to that, Loa, 13,675 feet; after these, Hualalai, rising 8273 feet; Kilauea, 4158 feet; and Kohala, 5505 feet above the sea; this last is largely buried beneath the lavas of Mauna Kea. The group contains a double line of volcanoes, one lying to the north and west of the other; and as the highest of the Hawaiian Group rises from a depth in the ocean of over 2000 fathoms, the total elevation of this mountain from its base on the bed of the ocean is not far from 26,000 feet, an elevation about that of the Himalayas. Mauna Kea has long been extinct, Hualalai has been dormant since 1801; but Mauna Loa is terribly active, there having been several eruptions, accompanied by earthquakes, within recent years, the most memorable being those of 1852 and 1868. In the former case the lava rose from the deep crater into ”a lofty mountain,” as described by Mr. Coan,[8] and then flowed away eastward for a distance of twenty miles. The interior of the crater consists of a vast caldron, surrounded by a precipice 200 to 400 feet in depth, with a circ.u.mference of about fifteen miles, and containing within it a second crater, bounded by a black ledge with a steep wall of basalt--a crater within a crater; and from the floor of the inner crater, formed of molten basalt, in a seething and boiling state, arise a large number of small cones and pyramids of lava, some emitting columns of grey smoke, others brilliant flames and streams of molten lava, presenting a wonderful spectacle, the effect of which is heightened by the constant roaring of the vast furnaces below.[9]
[1] _Principles of Geology_, 11th edition, vol. ii. p. 48.
[2] Smyth, _Report on the Teneriffe Astronomical Experiment of 1856_.
Humboldt makes the elevation 12,090 feet. A beautiful model of the Peak was constructed by Mr. J. Nasmyth from Piazzi Smyth's plans, of which photographs are given by the latter.
[3] Daubeny, _loc. cit._, p. 460.
[4] _Iles Canaries_, p. 37.
[5] Daubeny, _loc. cit._, p. 426.
[6] Lister, ”Notes on the Geology of the Tonga Islands,” _Quart. Jour.
Geol. Soc._, No. 188, p. 590 (1891).
[7] Dana, _Characteristics of Volcanoes, with Contributions of Facts and Principles from the Hawaiian Islands_. London, 1890.--Also, _Geology of the American Exploring Expedition--Volcanoes of the Sandwich Islands_.
[8] Coan, _Amer. Jour. of Science_, 1853.
[9] W. Ellis, the missionary, has given a vivid description of this volcano in his _Tour of Hawaii_. London, 1826.--Plans of the crater will be found in Professor Dana's work above quoted.
PART II.
EUROPEAN VOLCANOES.
CHAPTER I.
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