Fig. 461.—Active volcanic area at the junction of the continental segments of North and South America, and of the abysmal segments of the Atlantic and Pacific. Jones Relief Globe. (Photo. by R. T. Chamberlin.)
There is perhaps some significance in the fact that the most active regions of vulcanism to-day lie at the angular junctions of the great earth-segments. The Antillean and Central American volcanic region, that has recently been so demonstrative, lies where the southern angle of the North American continental block joins the northern angle of the South American continental block, and where the western angle of the North Atlantic abysmal segment closely approaches one of the eastern angles of the great Pacific abysmal segment. The complex and very active Java-Philippine volcanic region lies where the southeastern angle of the great Asian segment projects toward the Australian block, and where the western angle of the Pacific block approaches the northeastern angle of the Indian oceanic segment. The active Alaskan volcanic area lies at the angles of the North American, Asian, Pacific, and Arctic segments. The Mediterranean volcanic area falls less notably under this generalization, but it lies where the continental blocks of Europe and Africa come into peculiar relations to each other on either side of the remarkable Mediterranean trough. The eastern angle of the North Atlantic segment is near by, but not in very close relations. The Icelandic region, small but vigorous, lies near the junction of the North American, European, North Atlantic, and Arctic segments, and the New Zealand volcanic region is somewhat less closely related to the approach of the Australian, Antarctic, Pacific, and Southern oceanic segments. Nearly all of these angular conjunctions involve two depressed segments joining two relatively elevated segments. This relationship suggests a causal connection between the intensified movements at these angular conjunctions and the intensified volcanic action of these regions. There are enough volcanoes, however, that do not fall into these groups, or apparently into any other grouping, to suggest that the development of volcanoes is not wholly dependent on any surface relationship, but that it is connected with deep-seated causes that are indeed modified, but not wholly controlled, by surface conditions, or even by the movements of the master segments of the earth’s crust.
Fig. 462.—Active volcanic area at the junction of the continental segments of Asia and Australia, and the abysmal segments of the Pacific and Indian oceans. Jones Relief Globe. (Photo. by R. T. Chamberlin.)
4. In latitude.—The distribution of volcanoes appears to have no specific relation to latitude. Mounts Erebus and Terror, amid the ice-mantle of Antarctica, and Mount Hecla in Iceland, as well as the numerous volcanoes of the Aleutian chain, give no ground for supposing that volcanoes shun the frigid zones. On the other hand, the numerous volcanoes of the equatorial zone do not imply that they avoid the torrid belt. Their distribution appears to be independent of latitude. This is not cited because of any supposed effects of external temperature, for that must be trivial, but because it bears on the question whether strains are now arising from the supposed slackening of the earth’s rotation, which have any connection with volcanic action. If the oblateness of the earth is decreasing, the equatorial belt must be sinking and growing shorter, and hence must be under lateral pressure, while the polar caps must be rising, and increasing their curvatures, and should be under tension. These conditions, if real, might be supposed to have something to do with the extrusion of lava. Nothing in the present or the past distribution of igneous action seems to afford much support to this hypothetical inference.
5. In curved lines.—In the Antilles, the Aleutian Islands, the Kurile Islands, and in other instances, there is a notable linear arrangement of volcanoes with appreciable curvature. It has been noted that the convexity of the curves is turned toward the adjacent ocean. In some cases, however, there is a notable linear arrangement without appreciable curvature, as in the Hawaiian range, in the recently extinct line of cones of the Cascade Range, and in others. Less often, volcanoes are bunched irregularly, as in some of the groups of volcanic islands of the Pacific ([Fig. 460]).
Relations of Volcanoes.
1. Relations to rising and sinking surfaces.—So far as observations cover this point, the area immediately adjacent to active volcanoes is rising (Dutton). This is shown by raised beaches, terraces, coral deposits, etc. Whether this is wholly due to the expansional effect of the heating of the subterrane by the rising lava, or whether it has a wider significance, is not known. If a broader view is taken, it does not appear that there are sufficient data to connect volcanic action exclusively with either the rising or the sinking of the general surface. It is certain that the great mountain ranges and plateaus in which so much of the more recent volcanic action has taken place have been recently elevated relatively, but they have also undergone more or less of oscillation, involving some relative depression. The question whether the Pacific basin as a whole has been relatively elevated or depressed in modern times is a mooted one. Darwin[278] and Dana,[279] as the result of their early studies on its coral deposits and on other phenomena, concluded that the Pacific was a sinking area, but this view has been recently challenged by Murray[280] and Agassiz[281] with at least some measure of success. From the fiords on the borders of the Pacific and other physical phenomena, the inference has been drawn that relative sinking of the land has recently taken place. Raised beaches on the coasts are interpreted as indicating a relative rise of the land or a sinking of some ocean basin, for the withdrawal of the waters can only be the result of increasing the capacity of the oceanic basin as a whole. The most probable view is that the general areas of present and recent volcanic action are partly rising areas and partly sinking areas, and that movement of either kind may be connected with the extrusion of the lavas. The rising and sinking are but complementary phases of a deformation of the earth’s body, and involve a readjustment of stresses within the body of the earth. These stresses are possibly an essential factor in eruptions.
2. Relations to one another.—A most significant feature of volcanic action is the degree of concurrence or of independence of action in adjacent volcanoes. In some instances they act as though in sympathy, as in the recent outburst in Martinique and Saint Vincent, and the concurrent symptoms of activity in other places. On the other hand, the independence of neighboring vents is sometimes extraordinary. The group of volcanoes near the center of the Mediterranean, of which Vesuvius and Etna are the most conspicuous examples, usually act with measurable independence of one another, an eruption in the one not being habitually coincident with an eruption in the others. But the most conspicuous instance of independence is found in the great craters of Mauna Loa and Kilauea in Hawaii. They are only about twenty miles apart, the one on the top and the other on the side of the same great mountain mass. The crater of Mauna Loa is about 10,000 feet higher than the crater of Kilauea, and yet, while the latter has been in constant activity as far back as its history is known, the former is periodic. The case is the more remarkable because of the greatness of the ejections. The outflow of Mauna Loa in 1885 formed a stream from three to ten miles in width, and forty-five miles in length, with a probable average thickness of 100 feet, and some of its other outflows were of nearly equal greatness; indeed its outflows are among the most massive that have issued from volcanoes in recent times. Besides this massiveness there have been extraordinary movements of the lava within the crater, if the testimony of witnesses may be trusted. But throughout these great movements in the higher crater, the lava-column of Kilauea, 10,000 feet lower, continued its quiet action without sensible effects from its boisterous neighbor. The bearing of such extraordinary independence upon the sources of volcanic action is very cogent, for the lavas are of the same type, both being basalts, that of Mauna Loa being notably basic and probably as high in specific gravity as that in Kilauea. No difference in specific gravity that could at all account for a difference in height of 10,000 feet can be presumed, unless their ducts remain separate to extraordinary depths. Nor does it appear possible that a superior amount of gas within the column of Mauna Loa could account for such an extraordinary difference in height, for the hydrostatic pressure of such a column is not far from 10,000 pounds to the square inch. Even if the difference in the heights of the columns could be explained by differences in specific gravity, the agitation of the one should be communicated to the other, and an outflow of the one, particularly an outflow by a breakage through its walls sufficient to lower its surface hundreds of feet, as has repeatedly occurred in Kilauea, should change the surface of the other proportionately, if they were in hydrostatic equilibrium. It seems a necessary inference, therefore, that the two lava-columns have no connection with each other or with a common reservoir. The tops of some lava-columns stand about 20,000 feet above the sea, while others emerge on the sea-bottom far below sea-level. The total vertical range is, therefore, probably between 30,000 and 40,000 feet, a difference which tells its own story as to their relative independence.
