Growing Mountains and Infantile Islands on the Western Pacific Rim

Abstract

IN A description of the geomorphology of the Central Range of New Guinea, H. T. Verstappen (1960a) has drawn attention t the extreme youth of these mountains. Not only has their present-day high altitude been very recently attained, but even their initial upheaval seems to date not very far back. In contrast with the Alps, which are much older, these New Guinea mountains first arose at the end of the Pliocene period; Verstappen infers this from the age of fringing sedimentary deposits in the southern foothills of the range, these being correlated strata the materials of which were derived by erosion from the mountain mass during and very soon after its upheaval. By the time these youngest Upper Pliocene beds had accumulated, erosion had reduced the new mountain range to low relief, and extensive areas of the worn-down land surface then developed and since uplifted are present at high altitudes, though the surface survives only where the resistant nature of the rocks under it has slowed down erosion taking place in a new cycle and has thus allowed of its preservation. In areas of less resistant rocks the highland has been entirely destroyed and replaced by an intricately dissected land surface at a much lower level. This dissection or rejuvenation of the landscape, which must have gone on extremely rapidly and with kaleidoscopic replacement of successive assemblages of land forms, has followed as a consequence of renewed upheaval of the range. This latter uplift has converted remnants of the first-cycle erosion surface, which had developed at a low level, into plateaus 12,000 to 15,000 feet in altitude, thus making extensive glaciation possible in a Pleistocene episode of world-wide refrigeration. Verstappen claims, however, that in all probability the rising mountain summits did not reach high altitudes until late in Pleistocene time?after the Riss glacial age, that is to say perhaps less than 100,000 years ago. This is because no traces are found of more than one glaciation, presumably of Wurm age, though in this last glacial age plateaus with a combined area (in the eastern part of the Central Mountains) of about 350 square miles were covered by ice caps. This claim that mountains have arisen so late as to cut out the possibility of any glaciation except the last is by no means unique. Such a history has been attributed to the Apennines; and in New Zealand the whole of the uplift and mature dissection of mountains in western Nelson took place, according to Suggate (1957, pp. 24, 61), in the Pleistocene period, and Wellman (IQ55> P39) and others have demonstrated that the Southern Alps have recently been growing rapidly, and are still growing, in altitude.