oceanic islands

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oceanic islands Oceanic islands are best defined by geography rather than geology: as islands in the ocean basins, away from the margins of continents and certainly not within them. This excludes Guernsey and Jersey, the islands of the English Channel; it excludes Tasmania and Madagascar because of their clear continental connections, both geographical and geological. It includes all islands within the ocean basins and thus it includes both New Caledonia and the Seychelles, despite their continental origins. In the Atlantic, it includes the Canary and Cape Verde groups—favoured sites for an Atlantic Atlantis—together with most islands in the Caribbean. It includes the large islands of Japan, the masses of romanticized islands in the south and west Pacific; the rumbling volcanic islands of Hawaii, the Azores and Galapagos groups, and those in South-East Asia; and the monolithic outposts of land in the ocean basins whose names strike chords in the hearts of everyone curious about world geography: Pitcairn, Easter Island (Rapanui), Heard Island, Kerguelen, St Helena, Ascension, and Juan Fernandez.

Table 1. A genetic classification of oceanic islands

Level One	Level Two
Plate-boundary islands	Islands at divergent plate boundaries
	Islands at convergent plate boundaries
Islands along transform plate boundaries
Intraplate islands	Linear groups of islands
	Isolated islands
Clustered groups of islands

A genetic classification of islands is the most useful type of classification for geoscientists. This divides islands according to their association with either plate-boundary or intraplate locations and thence according to the boundary type or pattern, respectively (Table 1).

There are few islands associated with divergent plate boundaries for, although these are sites of active submarine volcanism, they are also sites of normal faulting and rifting associated with divergence. Iceland is an exception to this general rule; it forms an island because it coincides with a hot spot (mantle plume) as well as lying on the Mid-Atlantic ridge.

Islands along convergent plate boundaries are distinguished by their tectonic instability. Along the frontal arc (on the overriding plate nearest the trench) islands experience rapid uplift: islands like Nias on the Sunda frontal arc in South-East Asia are rising by as much as 1 mm/year; rates are higher elsewhere. Behind the frontal arc is the volcanic arc—a line of active volcanoes, andesitic or basaltic depending on the nature of the magmatic ‘mix’ beneath. Classic examples are found in the Caribbean and western Pacific; many are inactive, that of 'Ata in the Tonga volcanic arc being an example. Many islands on downgoing plates at convergent boundaries have been conspicuously elevated as the result of lithospheric flexure: Christmas Island in the eastern Indian Ocean and Niue in the Pacific are examples.

Islands can form along transform plate boundaries where there is significant convergence across them. Examples include Cikobia in Fiji and Clipperton in the eastern Pacific.

The best understood islands in intraplate situations are the linear chains, formed as a plate moves across a fixed hot-spot, a place where magma from the asthenosphere leaks to the surface. The Hawaii–Emperor island–seamount chain is the classic example; other chains are known, or have been proposed, for other parts of the oceans.

Isolated islands have a variety of origins; most appear to have been severed from mid-ocean ridges, others may mark nascent hot-spot chains. The origins of island clusters are unresolved; most authorities regard their volcanism as the product of fissuring produced by intraplate stress; the Galápagos and Cape Verde island groups are examples.

An important aspect of this classification is that it is based on island origins. By far the majority of oceanic islands have a volcanic origin but many now appear to us as atolls or high limestone islands whose igneous foundations have been swathed in great thicknesses of limestone. Atolls and high limestone islands are but two examples of how oceanic islands can develop: through subsidence in the case of mid-ocean atolls; through subsidence then uplift in the case of many high limestone islands.

Oceanic islands are of potential interest to all manner of geoscientists. They can inform us about the origin and ongoing development of many parts of the ocean basins, about former and current plate movements, about upper-Earth rheology, and about postglacial sea-level rise, among other processes. Oceanic islands are also of great interest in their own right. Charles Darwin stands out among nineteenth-century visitors to oceanic islands for the insights he reported into island origins and distribution. In the early decades of the twentieth century, Reginald Daly demonstrated his aptitude for independent thinking by studying a great number of oceanic islands across the world's oceans, amassing data on volcanic activity and a recent worldwide fall of sea level. W. M. Davis emulated Daly, largely in order to challenge his ideas, particularly about atoll origins, by visiting a large number of islands, a study which culminated in his 1928 classic, The coral reef problem.

Few Earth scientists carried on the tradition established by Daly and Davis until the 1960s, when people like Tuzo Wilson and H. W. Menard demonstrated how marine geologists should integrate ocean-floor and oceanic island data.

Arthur Bloom, in a seminal paper in 1967, showed how oceanic islands were by far the best places to interpret the record of past sea levels, since they acted like ‘dipsticks’ in the oceans. French workers in particular have followed up Bloom's lead, as have I in both the Atlantic and Pacific.

Pioneering work by John Chappell on the Huon Peninsula in Papua New Guinea led to the realization that the tectonic component of an observed shoreline displacement must be known before its eustatic (sea-level) component can be isolated. This was accompanied by and, indeed, led to an upsurge in studies of tectonically active oceanic islands; for example, in Vanuatu, on Barbados, on the Indonesian island of Atauro, and in various parts of Japan. This in turn stimulated interest in oceanic island tectonics. Many oceanic islands are in the coral seas and thus record slight and/or long-term changes of level with uncommon precision. Much recent work in the south and west Pacific has focused on the nature of uplift along the region's island arcs: either aseismic (vertical creep) or coseismic.

Interpretation of oceanic island landscapes is potentially of great value, particularly for bridging the gap between volcanic extinction of a particular island and the period of recorded human observation. Oceanic island landscapes also have considerable potential for testing models of landscape change, owing to the simplicity of these landscapes (and the controls on their development) compared to continental landscapes.

Throughout the history of the Earth sciences, oceanic islands have been considered of little importance because they are commonly small and remote from the traditional centres of intellectual enquiry. Since the rise of marine geology, it might have been expected that oceanic islands would become a more pressing topic for study, but most marine geologists are interested only in what is below the sea surface and most terrestrial geologists remain secure in their continental strongholds. This will change in the future as oceanic islands become more accessible and better known, but at present, for many Earth scientists, they remain at the frontiers of understanding.

Patrick D. Nunn

Bibliography

Darwin, C. R. (1844) Geological observations on the volcanic islands and parts of South America visited during the voyage of HMS ‘Beagle’. Smith, Elder, London.
Menard, H. W. (1964) Marine geology of the Pacific. McGraw-Hill, New York.
Menard, H. W. (1986) Islands. Scientific American Books, New York.
Nunn, P. D. (1994) Oceanic islands. Blackwell, Oxford.
Nunn, P. D. (1998) Pacific Island Landscapes. The University of the South Pacific, Suva.