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| Geology |
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| Written by Webmaster | |
| Wednesday, 18 June 2008 | |
GeologyThe chain of islands or archipelago formed as the Pacific plate moved slowly northwestward over a hotspot in the Earth's mantle at about 32 miles (51 km) per million years. Hence the islands in the northwest of the archipelago are older and typically smaller, due to longer exposure to erosion. The only active volcanism in the last 200 years has been on the southeastern island, Hawaiʻi, and on the submerged but growing volcano at the extreme southeast, Loihi. The Hawaiian Volcano Observatory of the U. S. Geological Survey documents recent volcanic activity and provides images and interpretations of the volcanism.Almost all magma created in the hotspot has the composition of basalt, and so the Hawaiian volcanoes are constructed almost entirely of this igneous rock and its coarse-grained equivalents, gabbro and diabase. A few igneous rock types with compositions unlike basalt, such as nephelinite, do occur on these islands but are extremely rare. The majority of eruptions in Hawaiʻi are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.  Eruptions from the Hawaiʻi hotspot have left a trail of underwater mountains across the pacific over millions of years, called the Emperor Seamounts Hawaiʻi (the Big Island) is the largest and youngest island in the chain, built from five different volcanoes. Mauna Loa, comprising over half of the Big Island, is the largest shield volcano on the planet. The measurement from sea level to summit is more than 2.5 miles (4 km), from sea level to sea floor about 3.1 miles (5 km). [2]
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