Popular Science Monthly/Volume 77/December 1910/Two Active Volcanoes of the South Seas
|TWO ACTIVE VOLCANOES OF THE SOUTH SEAS|
BARNARD COLLEGE, COLUMBIA UNIVERSITY, AND THE AMERICAN MUSEUM OF NATURAL HISTORY
IN the course of a fourth journey among the islands of the Pacific Ocean, during the year 1909, the rare opportunity was presented of making an ascent of the remarkably active volcano formed about five years ago on the island of Savaii, the largest member of the Samoan group. In addition, during a short stay in the Hawaiian Islands, a visit was made to Kilauea, a volcano which in contrast with the former, has a long geological history, for records of its intermittent periods of activity cover more than a century. It is the purpose of the present article to give a short general description of these two volcanoes.
Under any circumstances such works of nature would arouse the interest of a student of natural phenomena; but in my own case the opportunity to study them was valuable for additional reasons. My investigations of the distribution and evolution of the land snails of Polynesia demanded a thorough exploration of volcanic islands of greater age, islands that for many centuries have been sculptured by the elements so as to display alternating ridges and valleys radiating from their high central peaks. Tahiti is perhaps the most beautiful example of such an island. One finds that the several islands of the Pacific groups are of various geological ages, and consequently exhibit different degrees of weathering. They thus constitute a series showing how ancient rugged islands like Tahiti and Moorea have been derived from newly formed volcanic mountains like those of the Hawaiian and other groups, which possess relatively even sides of lava fields unfurrowed by erosion. Furthermore the various islands scattered throughout the vast areas of the Pacific Ocean are interesting to the naturalist because of the evidences they give of great changes in the level of the ocean bed, and also on account of the role played by corals in the construction of many types of islands. With few exceptions the islands occur in groups or chains suggesting the conclusion that they are the peaks of a range of mountains formerly connected by lowlands but now separated as the result of a subsidence of the ocean's floor. Every one is familiar with the theory that a coral atoll, consisting of a living reef bearing a more or less extensive series of coral islets, is built upon such a volcanic peak, which, according to Darwin and Dana, has been withdrawn below the water's level and overgrown by coral as it slowly subsided. It may be, as Agassiz contends, that a coral
The cone is 400 feet from base to crest.
Cinders and lava, in superimposed layers.
atoll is built upon a submarine volcanic mountain upheaved from the ocean's floor; but in either case the relation between coral reefs and volcanic peaks is one that possesses a real importance for the zoologist.
The two volcanoes of Savaii and Kilauea occur in island groups that are in every way typical of the so-called "high" islands of the Pacific Ocean. The Samoan Islands, including Savaii, lie almost on a straight line running nearly east and west. Upon examination they prove to be of various ages, for the westernmost, Savaii, bears the active volcano and displays other indications that it is more recent in origin than its neighbor, Upolu; this island, in its turn, is younger than the more rugged Tutuila and Manua to the east. The Hawaiian Islands, containing Kilauea, also range with some regularity along a line, which in this case runs west-northwest and east-southeast; but one very interesting difference consists in the fact that the newest island, Hawaii, lies at the eastern end.of the group, while the relative geological ages of the other islands correspond with their serial geographical order westward to Kauai, the oldest and most sharply sculptured member of the group. In all other essential respects, the Samoan and Hawaiian Islands are closely similar. Our interest centers about the peculiar features of their two active volcanoes, and the ways in which these agree and differ.
The new volcano on the island of Savaii is assuredly the more impressive of the two. Its total mass is great, but this feature is not so striking as its remarkably rapid development in the short period of five years; this development and the continual flow of fiery lava from its vast crater entitle it to supreme place in the array of volcanoes now in activity. It lies about eleven miles back from the coast nearly opposite the middle of the north shore of Savaii, which is roughly rhomboidal in outline and forty miles long. Approaching this part of the island by day, the most striking features of the panorama are the two vast clouds of steam that rise from the places where molten lava pours in cascades into the ocean (Fig. 1). Upon the glistening black slopes beyond, jets of vapor mark the vents in the roofs of the tunnels through which the fluid lava runs upon its seaward journey from the crater; and from the crater itself, two thousand feet above sea level, rises a similar fountain of thin steam that quickly merges with the dense clouds above.When one looks upon the enormous mass of this new mountain, it seems impossible that five years could be sufficient for its formation, yet this is actually the case. The first crater appeared in August, 1905, upon the floor of a beautiful green valley. As cinders and lava were cast forth, they gradually built up a larger dome and spread out to form the first strata of the great volcanic field. The flow followed the valley
The even slopes, bearing secondary cones, rise slowly and grandly to a high summit.
The jet of vapor marks the fire-pit of incandescent lava
to the ocean, but as wave after wave of fluid lava or steam-charged ash swept downward, more and more territory was devastated, while the lava, already cooled to form ridges and hillocks, diverted the later lava rivers into irregular and wider-spreading channels. Reaching the ocean, the molten rock poured into the depths of the sea over the coral reef, building ever outward, at the same time that it followed the reef and shore so as to spread laterally over a sector of the island with a shore-ward arc of five miles. Naturally the seaward wall of the whole lava field is highest near its center (Fig. 4) where it measures eighty or ninety feet. This wall displays a regular series of strata of prismatic blocks or tables, formed by the cooling of successive sheets of flowing lava. These strata sometimes lie between masses of cinders, showing how the eruptive output varied in character during succeeding weeks and months. Toward either side, the whole field gradually thins out, and at its western edge (Fig. 2) it ends in a series of rough rocky billows, seared and broken by their contraction in cooling. Yet their materials reached this point as red-hot fluid lava, having traversed a route that must have been nearly fifteen miles in length.
As the molten lava swept down the valley and along the strand, its destructive effects were rapid and complete. The wooden huts of the seaside villages were entirely consumed and only where there were walls of coral limestone, like those of the churches and traders' warehouses (Fig. 5), was there anything to withstand the flood of rock. Yet so quickly did the surface of the plastic mass become cool, that the cocoanut and other trees, felled by the burning through of their bases, were rarely consumed.
We began the ascent of the volcano early in the afternoon in order to reach the crater before dusk. Proceeding through the undestroyed woods of a neighboring valley we entered upon the lava field at a point some miles from the coast, thus obviating the necessity of traversing its whole extent from sea to crater. Our natives, bearing food and water, now tied the husks of cocoanuts to their naked feet for protection in walking over the broken lava, and after a final pause for rest, we left the shade and tempered heat of the tropical forest for the open glare of the volcano's slope. Viewed from afar, this slope seems even and smooth, but in reality it is like a tempestuous ocean suddenly arrested in its movements and turned into stone. Here and there wide sheets of lava with corrugated rippling surfaces formed still rivers between massive banks of cinders through which their molten substance had earlier ploughed its way; larger and smaller tables of crust, like broken floes of the Arctic Ocean, were tilted up and piled in strange heaps. And so vitreous was the material of this sea of black broken rock that the light was reflected from millions of crystal surfaces and facets as from so many fragments of ice or glass.
Progress over this field was necessarily slow, but by following the
The photographic film was exposed four seconds.
general trend of the less broken lava streams, we gradually worked upward and inward toward the main axis of the whole lava mass, indicated by vents which gave egress to steam and gases discharged by fluid lava running through tunnels beneath the surface.
The great crater (Fig. 3) is a perfectly typical cone of cinders and lava, with a height from base to summit of four hundred feet as measured by the aneroid barometer. On three sides it is composed mainly of ashes and pumice, but toward the sea its surface displays smoother areas of rock where the lava formerly welled over the edge before the tunnels were formed by which the discharge now takes place. Large bombs, rounded masses of rock hurled from the crater during some explosive eruption, occur on the slopes, sometimes covered as by a sheet of tar with a later-extruded layer of lava (Fig. 6).
When we stood upon the extreme edge of the jagged margin and looked down upon the immense lake of fiery lava, four hundred feet below, it was hard to realize that the scene was actual and not an imaginary panorama of Dantesque infernal regions. The yawning cavity of the crater extended a full half mile in length, and its width was more than four hundred yards. Almost perpendicular and sometimes undercut, the crater walls dropped hundreds of feet to the lake of molten lava, which was in such violent commotion that it seemed to be liquid flame rather than a mass of fused and fiery rock. At certain places it boiled with greater activity, sending huge jets and fountains high into the air. Its waves moved variously at different times, but ever and again they would surge heavily to dash against the wall where the tunnels opened to give exit for the flow to the ocean. And always from this surface, thin steam-like vapor charged with acid gases swirled upward in the draught caused by the strongly-blowing trade winds, making it excessively unpleasant to look over the edge even from the windward side.
Magnificent though it was by day, the scene at night was far beyond human powers of description. With the darkness, the lake glowed almost as a continuous incandescent mass. Its light was reflected upon the clouds above, making a beacon that we had often seen from a distance of forty miles and which was said to have been visible at a distance of seventy miles during the period of the volcano's greatest activity about two years earlier. Looking seaward, the rosy vapors above the tunnel vents outlined the course of the lava down to the shore of the island where the fire of the final lava cascades gave color to two huge clouds of steam. Again and again through the night we climbed from our camp at the base of the cone to look down upon the fascinating but awful marvel, whose fires illuminated the scene so as to give ample light to guide a way over the broken lava.
Leaving now the volcano of Savaii, which is a veritable classic in its regularity of structure and mode of origin, we pass to the Hawaiian Islands and the active volcano of Kilanea that is in many respects quite different from the new one of Samoa. It is an accessory outlet upon the side of the giant volcanic mountain of Mauna Loa, whose main crater at the summit, more than thirteen thousand feet above the sea, is active only at very long intervals. It is a journey of two hundred miles from Honolulu to the island of Hawaii on which Mauna Loa occurs; viewed from the ocean (Fig. 7) the even slopes of the mountain rise slowly and grandly to the high summit, bearing numerous secondary or "parasitic" cones which have been formed by sporadic local eruptions.
The first view of Kilauea itself is somewhat disappointing to one who has recently witnessed the grandeur of the eruption at Savaii, but closer acquaintance reveals many features of great interest. Kilauea lies about four thousand feet above the level of the sea, and is about twenty miles back from the coast. In general structure (Fig. 8) it is a wide shallow basin over three miles in diameter, depressed below the general level of the slopes of Mauna Loa. At quite a little distance from the geometric center of the lava field which forms the floor of this basin is the active fire-pit, marked during the day, as at Savaii, by a cloud of vapor, and at night by a pillar of fire.
The well-beaten trail to this center of activity leads down along the terraced wall of one side to the almost level floor of the main basin. In the strongest contrast to Savaii, Kilauea's lava field is remarkably even; indeed, the best areas of the former are far more broken than the most irregular parts of the latter. The surface undulates more or less, it is true, while here and there broken masses form hillocks and ridges, but the active vent has given forth the molten lava with comparative regularity. Since the middle of the nineteenth century enough rock has poured out into this wide basin to reduce the height of its vertical walls from more than eight hundred feet to about four hundred.
In December last, Kilauea was unusually active after a period of relative quiet. The fire pit (Fig. 9) is nearly circular in outline and its walls fall in two terraces to the small pool of molten lava, about two hundred feet below the natural level of the whole basin. Its general structure has varied more or less in past decades, as well as its degree of violence, but it has been a permanent center of eruptive activity for more than a hundred years, well deserving the native name of "Halemaumau," the "House of Perpetual Fire."
Here as at Savaii the surface of the pool is in constant commotion, but the areas of incandescence are much restricted and run in parallel or forking lines. Cakes of congealed lava float between these lines, and when in their movements they reach the neighboring areas of greater activity, they are redissolved and their fragments are thrown into the air together with jets of more fluid lava. Photographs taken at night (Fig. 10) exhibit with great distinctness the major and minor areas of greater activity that form a network upon the surface of the whole pool.