Krakatoa, in its destruction, became known to the entire world. The eruption gave rise to a hundred-foot wave that wiped out villages along the Strait and killed people by tens of thousands. The wave was felt on the shores of the Indian Ocean and at Cape Horn; rounding the Cape into the Atlantic, it sped northward and retained its identity even as far as the English Channel. The sound of the explosions was heard in the Philippine Islands, in Australia, and on the Island of Madagascar, nearly 3000 miles away. And clouds of volcanic dust, the pulverized rock that had been torn from the heart of Krakatoa, ascended into the stratosphere and were carried around the globe to give rise to a series of spectacular sunsets in every country of the world for nearly a year.
Although Krakatoa’s dramatic passing was the most violent eruption that modern man has witnessed, Krakatoa itself seems to have been the product of an even greater one. There is evidence that an immense volcano once stood where the waters of Sunda Strait now lie. In some remote period a titanic explosion blew it away, leaving only its base represented by a broken ring of islands. The largest of these was Krakatoa, which, in its own demise, carried away what was left of the original crater ring. But in 1929 a new volcanic island arose in this place—Anak Krakatoa, Child of Krakatoa.
Subterranean fires and deep unrest disturb the whole area occupied by the Aleutians. The islands themselves are the peaks of a thousand-mile chain of undersea mountains, of which volcanic action was the chief architect. The geologic structure of the ridge is little known, but it rises abruptly from oceanic depths of about a mile on one side and two miles on the other. Apparently this long narrow ridge indicates a deep fracture of the earth’s crust. On many of the islands volcanoes are now active, or only temporarily quiescent. In the short history of modern navigation in this region, it has often happened that a new island has been reported but perhaps only the following year could not be found.
The small island of Bogoslof, since it was first observed in 1796, has altered its shape and position several times and has even disappeared completely, only to emerge again. The original island was a mass of black rock, sculptured into fantastic, tower-like shapes. Explorers and sealers coming upon it in the fog were reminded of a castle and named it Castle Rock. At the present time there remain only one or two pinnacles of the castle, a long spit of black rocks where sea lions haul out, and a cluster of higher rocks resounding with the cries of thousands of sea birds. Each time the parent volcano erupts, as it has done at least half a dozen times since men have been observing it, new masses of steaming rocks emerge from the heated waters, some to reach heights of several hundred feet before they are destroyed in fresh explosions. Each new cone that appears is, as described by the volcanologist Jaggar, ‘the live crest, equivalent to a crater, of a great submarine heap of lava six thousand feet high, piled above the floor of Bering Sea where the Aleutian mountains fall off to the deep sea.’
One of the few exceptions to the almost universal rule that oceanic islands have a volcanic origin seems to be the remarkable and fascinating group of islets known as the Rocks of St. Paul. Lying in the open Atlantic between Brazil and Africa, St. Paul’s Rocks are an obstruction thrust up from the floor of the ocean into the midst of the racing Equatorial Current, a mass against which the seas, which have rolled a thousand miles unhindered, break in sudden violence. The entire cluster of rocks covers not more than a quarter of a mile, running in a curved line like a horseshoe. The highest rock is no more than sixty feet above the sea; spray wets it to the summit. Abruptly the rocks dip under water and slope steeply down into great depths. Geologists since the time of Darwin have puzzled over the origin of these black, wave-washed islets. Most of them agree that they are composed of material like that of the sea floor itself. In some remote period, inconceivable stresses in the earth’s crust must have pushed a solid rock mass upward more than two miles.
So bare and desolate that not even a lichen grows on them, St. Paul’s Rocks would seem one of the most unpromising places in the world to look for a spider, spinning its web in arachnidan hope of snaring passing insects. Yet Darwin found spiders when he visited the Rocks in 1833, and forty years later the naturalists of H.M.S. Challenger also reported them, busy at their web-spinning. A few insects are there, too, some as parasites on the sea birds, three species of which nest on the Rocks. One of the insects is a small brown moth that lives on feathers. This very nearly completes the inventory of the inhabitants of St. Paul’s Rocks, except for the grotesque crabs that swarm over the islets, living chiefly on the flying fish brought by the birds to their young.
St. Paul’s Rocks are not alone in having an extraordinary assortment of inhabitants, for the faunas and floras of oceanic islands are amazingly different from those of the continents. The pattern of island life is peculiar and significant. Aside from forms recently introduced by man, islands remote from the continents are never inhabited by any land mammals, except sometimes the one mammal that has learned to fly—the bat. There are never any frogs, salamanders, or other amphibians. Of reptiles, there may be a few snakes, lizards, and turtles, but the more remote the island from a major land mass, the fewer reptiles there are, and the really isolated islands have none. There are usually a few species of land birds, some insects, and some spiders. So remote an island as Tristan da Cunha in the South Atlantic, 1500 miles from the nearest continent, has no land animals but these: three species of land birds, a few insects, and several small snails.
With so selective a list, it is hard to see how, as some biologists believe, the islands could have been colonized by migration across land bridges, even if there were good evidence for the existence of the bridges. The very animals missing from the islands are the ones that would have had to come dry-shod, over the hypothetical bridges. The plants and animals that we find on oceanic islands, on the other hand, are the ones that could have come by wind or water. As an alternative, then, we must suppose that the stocking of the islands has been accomplished by the strangest migration in earth’s history—a migration that began long before man appeared on earth and is still continuing, a migration that seems more like a series of cosmic accidents than an orderly process of nature.
We can only guess how long after its emergence from the sea an oceanic island may lie uninhabited. Certainly in its original state it is a land bare, harsh, and repelling beyond human experience. No living thing moves over the slopes of its volcanic hills; no plants cover its naked lava fields. But little by little, riding on the winds, drifting on the currents, or rafting in on logs, floating brush, or trees, the plants and animals that are to colonize it arrive from the distant continents.
So deliberate, so unhurried, so inexorable are the ways of nature that the stocking of an island may require thousands or millions of years. It may be that no more than half a dozen times in all these eons does a particular form, such as a tortoise, make a successful landing upon its shores. To wonder impatiently why man is not a constant witness of such arrivals is to fail to understand the majestic pace of the process.