How Many Islands?, Island Formation, Coral Islands, Island Biogeography, Island EconomicsIsland types
An island is an area of land, smaller than a continent, that is entirely surrounded by water. That distinction, although somewhat artificial, suggests different geologic forces acting to create and maintain islands versus continents. Islands further differ from continents in their natural environments—in the biological systems they support, in their rate of response to change, in their ability to recover from ecological disaster.
The plants and animals found on islands often seem an odd assemblage. Some in fact are odd, in the sense that they live nowhere else. The seemingly skewed distribution of populations on islands, compared with that on a mainland, results in part from the small size of islands; they cannot carry a representative zoo, and, without migration in from outside, the extinction of one life form might leave a gaping hole in the biota. Also, the water surrounding islands acts as a barrier to the passage of some life forms, particularly large mammals, but encourages the migration of others, such as birds and insects. Because of their relative isolation and the potentially unique biota that may be established on them, islands have been known, at least since the time of Charles Darwin, as natural laboratories of evolution.
The islands discussed here are of three kinds: continental islands, oceanic islands, and coral islands. Not discussed are inland islands, such as islands found in the middle of a lake.
Continental islands are parts of the continental shelf that rise above the surrounding water. That is, they are situated on the shallow water margin of a continent, usually in water less than 600 ft (200 m) deep. Greenland, the largest island in the world, and Newfoundland are examples of continental islands. A drop in sea level would be sufficient to connect these islands to the North American continent.
Another, rarer kind of continental island consists of small pieces of continental material that broke away from a land mass. These islands are now part of a separate crustal plate that is following an independent path. The Seychelles in the Indian Ocean were once associated with the Madagascar-India portion of the supercontinent Pangaea. With the breakup of Pangaea about 200 million years ago, the Seychelles began their independent existence. Their continental basement structure, however, clearly associates them with the continents rather than with oceanic islands of volcanic origin.
Oceanic islands arise from volcanic action related to the movement of the lithospheric, continent-bearing crustal plates. Unlike continental islands, oceanic islands grow from oceanic crust. Oceanic islands are not scattered haphazardly about the deep ocean waters but are aligned along converging oceanic plate boundaries or along the mid-ocean ridges, or diverging oceanic plate boundaries, associated with sea-floor spreading. In addition, some arose as oceanic plates moved over fixed hot spots in the deeper mantle.
Coral islands are distinct from both continental islands and oceanic islands in that they are formed of once living creatures, the corals, which colonize in place to form coral reefs.
Unrelated to this three-part classification of islands are islands of a fourth kind, barrier islands. Barrier islands occur in shallow-water coastal areas and are composed of unconsolidated sediment, usually sand. Barrier islands form 15% of all the coastline in the world, including most of the coastline of the continental United States and Alaska, and also occur off the shores of bays and the Great Lakes. Some barrier islands are stable enough to support houses or an airport runway; others are short-lived, ripped up annually by winter storms and reestablished by wave and tidal action. As their name suggests, they afford some protection to the mainland from erosion.
The development of plate tectonics theory in the 1960s greatly aided scientists' understanding of the genesis of islands. Oceanic islands originate in volcanic action typically associated with the movement of the lithospheric plates.
The lithosphere is the major outer layer of the earth. It consists of the crust—both continental and oceanic-and upper mantle, and ranges from the surface to 60 m (100 km) deep, although subducted crust has been remotely detected at depths of 620 mi (1,000 km). (For comparison, the average radius of the earth is 4,173 mi [6,731 km].) The lithosphere is divided into rigid, interlocking plates that move with respect to one another.
There are 11 major plates (two of which seem to be fracturing) and many smaller ones. The plates move over the next lower layer, the asthenosphere (a sort of crystalline sludge), perhaps by thermal convection, at an average rate of 4-4.5 in (10-11 cm) per year. The plate boundaries tend to move away from each other at mid-ocean ridges and to approach each other at the edges of continents.
At the mid-ocean ridges, magma wells up and cools, forming mountains. At the same time, existing sea floor spreads apart, and new sea floor is created. As sea-floor spreading continues, the mountains, which sit on ocean crust, are carried away from the mid-ocean ridge and therefore away from the source of new lava deposits. Many of these underseas mountains, as a result, never grow tall enough that their tops could emerge as islands. These submarine mountains are known as seamounts. It is possible to date with some accuracy the age of seamounts by measuring their distance from the ridge where they were born.
A major exception to the nonemergence of mountains formed at mid-ocean ridges is the volcanic island of Iceland, which has had a more complicated history; it was formed both by upwelling magma from the mid-Atlantic ridge and by volcanism over a hot spot deeper in the mantle, which also contributed to upwelling magma in the same area. The hot spot, active for about 55 million years, has since cooled, and the mid-Atlantic ridge has shifted abruptly from its previous position to a position somewhat eastward. It is still active, producing lava from volcanoes, the flows of which sometimes close harbors. Iceland is one of the few places on Earth where a mid-ocean ridge has risen to the land's surface and become visible.
Volcanoes producing lava flows and occasional seamounts, and more rarely emerged islands, characterize divergent plate boundaries at the mid-ocean ridges. However, converging plate boundaries—two plates coming together—are characterized by volcanoes that often produce emerged islands, as well as by forceful earthquakes and deep oceanic trenches, such as the Marianas Trench. Most plates that converge do so at the edges of continents.
When two plates meet, the plate carrying the heavier oceanic crust dips under, or subducts, and the plate carrying the lighter continental crust rides over it. At the point of subduction a deep trench develops; and parallel to it, on the lighter plate, volcanic action produces a row of islands. (The magma involved in the volcanism comes from melting of the oceanic crust as it is subducted.) These island groups are called island arcs, after their curved pattern. An island arc with active volcanism is called a back arc. Between the back arc of the system and its associated trench there may be a second, nonvolcanic arc of islands, called the front arc, that is thought to be caused by upthrust of crust from the lighter plate. The front arc may lie below the surface of the water and not be readily visible.
Island formation occurs at intraplate locations (anywhere between the boundaries) as well as at plate boundaries. It is reasoned, with strong scientific support, that some of these mid-plate volcanic islands resulted from passage of a lithospheric plate over a thermal plume rising from a fixed hot spot deeper in the mantle layer.
The formation of the Hawaiian-Emperor island chain in the north Pacific is attributed to this mechanism. The hot spot is thought to be located near the Loiki Seamount and to be causing the currently active volcanoes of Mauna Loa and Kilauea on the island of Hawaii. The chain trends northwestward, with the oldest islands at the northwestern end. As the Pacific plate drifts, at a rate of 4 in (10 cm) per year, areas that had been positioned over the hot spot and so subject to volcanism, lava accumulation, and island formation, move off the hot spot and cool down. Their position is then taken by new areas of lithosphere that drift over the thermal plume and a new island begins to form on the sea floor.
This scenario would explain why some of the animals populating the Hawaiian islands are older than the islands themselves. Because of the relative closeness of the islands to each other and the leisurely pace of the Pacific plate's drift, animals theoretically could have had time to raft, swim, or fly from the older, now submerged islands at the northern end of the chain toward the younger, more southerly islands, which are not submerged. By island hopping over geological time, some of the original species may eventually have made their way to islands that had not yet come into existence at the time the animals established a presence on the islands.
The hot spot model works well for the Hawaiian-Emperor island chain but does not explain all intraplate clusters or chains of islands. Indeed, one of the problems facing oceanographers is the association of islands whose formation can easily be described by global and local conditions with islands that appear not to have been produced by the same processes.
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