Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/266

Rh 252 (5.) Cones of T-uﬁand Lava.—These are by far the most frequent. They may be taken, indeed, as the typical form of volcanoes. Around the central vent the successive showers of scorize and ashes gather, with occasional streams of lava, which from time to time descend different sides of the growing cone. The component strata have thus the usual inclination outwards, though, when seen in section along the crater-walls, they look approximately horizontal. By continued explosions ﬁssures are formed in the sides of the cone. These are injected with lava, producing a net- work of dykes which strengthen the cone, or they serve as channels of escape for lava, and give rise to lateral erup- tions and minor or parasitic cones. As the main cone increases in diameter by the accumulation of fragmentary and molten ejections, it rises also in height, until at last few or no eruptions take place from its summit, but only from openings on its ﬂanks. (6.) Submarine Volcanoes.—It is not only on the surface of the land that volcanic action shows itself. It takes place likewise under the sea, and as the geological records of the earth’s past history are chieﬂy marine formations, the characteristics of submarine volcanic action have no small interest to the geologist. Unfortunately, the phenomena of recent volcanic eruptions under the sea are for the most part inaccessible. Here and there, as among the islands of the Greek Archipelago and at Tahiti, elevation of the sea- bed has taken place, and brought to the surface beds of lava, which had been erupted and had consolidated under water. There does not appear to be any appreciable difference either in external aspect or in internal structure between such submarine lavas and those erupted on the land. Some of them are highly scoriaceous. There is no reason indeed why slaggy lava and loose scorize should not accumulate under the pressure of a deep column of the ocean. At the Hawaii Islands, on 25th February 1877, masses of pumice, during a submarine volcanic explosion, were ejected to the surface, one of which struck the bottom of a boat with con- siderable violence and then ﬂoated. At the same time, when we reﬂect to what a considerable extent the bottom of the great ocean basins is dotted over with volcanic cones, rising often solitary from profound depths, we can under- stand how large a proportion of the actual eruptions may take place under the sea. The foundations of these volcanic islands doubtless consist of submarine lavas and fragmentary materials, which, in each case, continued to accumulate to a height of two or three miles, until the pile reached the surface of the water and the phenomena became subaerial. The immense abundance and wide diffusion of volcanic detritus over the bottom of the Paciﬁc and Atlantic Oceans, even at distances remote from land, as has been made known by the voyage of the “ Challenger,” may indicate the prevalence and persistence of submarine volcanic action, though, at the same time, it must be admitted that an ex- tensive diffusion of volcanic debris from the islands is effected by winds and ocean-currents. § 4. Geographical Distribution of Volcanoes. The chief facts under this head may thus be summar- ized. (l.) Volcanoes occur along the margins of the ocean basins, particularly along lines of dominant mountain ranges. The vast hollow of the Paciﬁc is girdled with a wide ring of volcanic foci. (2.) They rise as a striking feature in the heart of the ocean basins. Most of the oceanic islands are vol- canic. Even the coral islands have in all likelihood been built upon the tops of submarine volcanic cones. (3.) Volcanoes are thus situated, as a rule, close to the sea. “Then they occur inland they sometimes appear in the neighbourhood of a large sheet of water. Yet so many instances have been observed where volcanoes have appeared at great distances from any sheet of water that the proximity of a lake or of I GEOLOGY [11 I. DYNAMICAL. the sea cannot be regarded as necessary for the evolution of volcanic phenomena. The dominant arrangement of volcanoes is in lines along subterranean lines of weakness, as in the chain of the Andes, the Aleutian Islands, and the Malay Archipelago. Where the linear arrangement does not hold, it gives place to one in groups, as in Italy, Iceland, and the volcanic islands of the great oceans. § 5. Distribution of l'olcanic Action in Time. Besides the existence of what are called extinct volcanoes, the geologist can adduce proofs of the former presence of active volcanoes in many countries where cones and craters and all ordinary aspects of volcanic mountains have long disappeared. Sheets of lava, beds of tuﬂ‘, dykes, and necks representing the sites of volcanic vents have been recog- nized abundantly. These manifestations of volcanic action, moreover, have as wide a range in geological time as they have in geographical area. Every great geological period, back at least as far as the Lower Silurian, has had its volcanoes. In Britain, for instance, there were active volcanic vents in the Lower Silurian period, whence the lavas and tuffs of Snowdon, Aran Mowddwy, and Cader Idris were ejected. The Lower Old Red Sandstone epoch was one of prolonged volcanic activity in central Scotland. The earlier half of the Carboniferous period likewise witnessed the outburst of innumerable small volcanoes over the same region. During the Permian period a few scattered vents existed in the south—west of Scotland, and in the time of the New Red Sandstone some similar points of eruption appeared in the south of England. The older Tertiary ages were distinguished by the outpouring of the enormous basaltic plateaus of Antrim and the Inner Hebrides. Thus it can be shown that, within the same comparatively limited geographical space, volcanic action has been rife at intervals during a long succession of geological ages. The existing active volcanoes of Iceland rise from amid Tertiary lavas and tutfs, which form part of a great volcanic ridge, extending down through the Faroe Islands into the west of Britain. Volcanic action, which now manifests itself so conspicuously along certain lines, seems to have continued in that linear development for protracted periods of time. The actual vents have changed, dying in one place and breaking out in another, yet keeping on the whole along the same lines. § 6. Causes of Volcanic Action. Volcanoes depend, of course, upon the internal heat of the planet as their prime source of energy. But the modus operandi whereby that internal heat manifests itself in volcanic action is a problem by no means of easy solution. Were this action merely an expression of the intensity of the heat, we might expect it to have manifested itself in a far more powerful manner in former periods, and to exhibit a regularity and continuity commensurate with the exceed- ingly slow diminution of the earth’s temperature. But there is no geological evidence in favour of greater volcanic intensity in ancient times than in more recent periods; on the contrary, it may be doubted whether any of the Palaeozoic volcanoes equalled in magnitude those of the Tertiary period, or whether any of the latter ever produced such stupendous changes as have been effected by modern volcanoes still active. On the other hand, no feature of volcanic action is more conspicuous than its spasmodic ﬁtfulness. The mere presence of a high internal temperature, there- fore, would probably not of itself produce the phenomena of volcanoes, at least in the present condition of the planet. There can be no doubt that one essential exciting cause of volcanic action is the descent of water from the surface. It has already been pointed out how invariably steam plays