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45 Geomorphology Patrick D. Nunn 4 The Pacific Islands region extends over 130° of longitude and 70° of latitude. Some islands are more than 100,000 km2 in size; others are miniscule. Some islands are pieces of ancient continent, hundreds of millions of years old; other islands are still growing, and periodic volcanic eruptions give subaerial landforms little chance to develop. Some tropical islands are so high they have ice caps; others are so low they can barely be seen on approach by sea. Some islands are rain-soaked; others sometimes go for years without rain. Prevailing climatic and geological controls produce seemingly infinite permutations and militate against sweeping generalizations . Yet generalize we must to get some appreciation of Pacific Islands landscapes. This chapter begins by looking in detail at the principal causes of landscape diversity in the Pacific Islands, along with some pertinent examples. This is followed by a systematic account of landscapes on distinct island types. The chapter concludes with a discussion of rates of change and two key issues in Pacific Islands landscape study. Controls on Landform Development Climate and geology are the principal controls on global landform development. Climatic and oceanographic controls, particularly precipitation, are important causes of landform variation throughout the Pacific Islands. Geological controls are also important, but less so relative to continents because of the more restricted range of geologic structures and histories on islands (Nunn 1987). Many Pacific islands are seismically and volcanically active. In these cases, vertical tectonics (land-level movements) may overwhelm the influence of other factors in landscape evolution. Long-term changes of climate and sea level (Figure 4.1) have also brought about changes in landforms on certain Pacific islands. Some landforms are relict in character, formed in the past when conditions were significantly different. On some islands, people have been the principal agents of landscape change, often blithely unaware of the long-term effects of their actions. Climatic and Oceanographic Controls Climate in the Pacific varies mostly with latitude. Islands nearest the equator are generally hotter and wetter and experience more tropical cyclones (hurricanes, typhoons) than those farther from the equator. Changing amounts of solar radiation in the course of a year are responsible for significant seasonal variations of temperature and rainfall. On Tarawa, Kiribati, twice-yearly crossings by the Intertropical Convergence Zone (ITCZ) produce two peaks in precipitation. Geomorphic processes on such islands are affected by periodic changes in rainfall and water-table level. For example, it is possible that seasonal shifts in the relative amounts of fresh and saline groundwater in the intertidal zone are important in beachrock formation (Schmalz 1971). Various aspects of climate also act as limiting factors in the development of particular landforms. Amphitheater-headed valleys , for example, can develop only on those high parts of volcanic islands where annual precipitation exceeds 2,000 mm (Nunn 1994a). The development of phosphate rock on many Pacific reef islands is one reason why they have survived so long. Phosphate rock of this kind requires deposition of guano and low annual precipitation levels to prevent its decomposition and leaching (Stoddart and Scoffin 1983). Figure 4.1. Changes in temperature and sea level during the past 150,000 years (after Nunn 1997). Temperature changes are derived from oxygenisotope analyses of marine microrganisms; sea-level changes are from studies of emerged reefs on the Huon Peninsula, Papua New Guinea. 46 ■ The Pacific Islands In some of the westernmost Pacific islands, the reversal of winds associated with the Asian and Austral monsoons brings large amounts of orographic moisture to opposing sides of islands during summer and winter. Landform development on both sides may be similar, though during any season a different set of climatic processes dominates. The aridity of Easter Island is associated with the stationary high-pressure cell in the eastern Pacific. This has given rise to a set of semiarid landforms quite different from those of the nearest islands in French Polynesia, 2,400 km to the west. The ocean circulation of the Pacific is dominated by two large gyres. The gyre in the North Pacific involves clockwise movement of water; that in the South Pacific involves counterclockwise movement . The impact of ocean currents on landform development is well represented by the presence of coral at unusually high latitudes in the western Pacific, where warm equatorial water is driven poleward. A final consideration involves the gross distribution of islands within the Pacific. Most islands are within or just outside the tropics ; there is a conspicuous paucity...

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