The Pacific War Online Encyclopedia
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U.S. Geological Survey
Geologists have long known that islands in the deep Pacific basin tend to lie in chains stretching from northwest to southeast, with the youngest islands to the southeast. The clearest example is Hawaii, where the islands to the northwest, such as Midway, have eroded nearly to sea level, while Hawaii Island itself, at the southeast end of the chain, has two of the most active volcanoes on earth.
This pattern is explained by the theory of plate tectonics. It is believed that the Hawaiian Islands were formed as the Pacific Plate moved northwest over a hot spot, or plume, in the Earth’s mantle, where a local concentration of radioactive elements (perhaps potassium from a piece of crust that had been drawn deep into the mantle) has heated the mantle rock. Although the mantle is nominally solid, it is so hot that it slowly creeps under pressure, like metal that has been softened in a furnace prior to being worked. The rocks in the plume slowly rise towards the Earth’s surface, where they eventually form liquid magma that erupts as the volcanoes of the Hawaiian chain. As the Pacific Plate moves towards the northwest over the fixed mantle plume, older islands are carried away from the hot spot and their volcanoes become extinct, while new islands are formed over the hot spot.
Hot spot island chains, unlike island
arcs, are characterized by a complete absence of any sedimentary
basement. The islands are almost entirely volcanic in
volcanoes typically erupt fluid basaltic lava, which is low in gas and
relatively nonviolently. This results in volcanoes with gently
flanks rather than sharp peaks. The lack of a sedimentary
basement or of
much volatiles in the erupted lava means that hot spot island chains
devoid of valuable mineral resources.
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