Hawaiian Islands Formation and Geology
The Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles (2,400 km) from the island of Hawaiʻi in the south to northernmost Kure Atoll (the northwesternmost island in Hawaii is Green Island, which is joined to the Kure Atoll). Excluding Midway, which is an unincorporated territory of the United States, the Hawaiian Islands form the U.S. state of Hawaii. Once known as the "Sandwich Islands", the archipelago now takes its name from the largest island in the cluster.
The islands are the exposed peaks of a great undersea mountain range known as the Hawaiian-Emperor seamount chain, formed by volcanic activity over a hotspot in the Earth's mantle. At about 1,860 miles (3,000 km) from the nearest continent, the Hawaiian Island archipelago is the most isolated grouping of islands on earth.
The 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, Loʻihi. 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.
The Hawaiian Islands have many earthquakes, generally, caused by volcanic activity. Most of the early earthquake monitoring took place in Hilo, by missionaries Titus Coan, Sarah J. Lyman and her family. From 1833 to 1896, approximately 4 or 5 earthquakes were reported per year.
Hawaii accounted for 7.3% of the United States' reported earthquakes with a magnitude 3.5 or greater from 1974 to 2003, with a total 1533 earthquakes. Hawaii ranked as the state with the third most earthquakes over this time period, after Alaska and California.
The Hawaiian Islands are subject to tsunamis, great waves that strike the shore. Tsunamis are most often caused by earthquakes somewhere in the Pacific. The waves produced by the earthquakes travel at speeds of 400–500 miles per hour and can affect coastal regions thousands of miles away.
The Hawaii hotspot is the volcanic hotspot that created the Hawaiian Islands in the central Pacific Ocean, and is one of Earth's best-known and most heavily-studied hotspots.
The Hawaii hotspot has created the Hawaiian – Emperor seamount chain comprising at least 129 volcanoes arranged in a line with a sharp bend. More than 123 of these are extinct volcanoes, seamounts, and atolls, four are active volcanoes, and two are dormant volcanoes.
Seven shield volcanoes created the island of Maui Nui. In Hawaiian, "Nui" means "great" or "large", and Maui is the name of Hawaii's second largest island (which formed Maui Nui's backbone).
Almost all magma created by the hotspot is igneous basalt; the volcanoes are constructed almost entirely of this or the similar, coarse-grained gabbro and diabase. Rarely, there are different igneous rocks, such as nephelinite; these occur often on the older volcanoes, most prominently Detroit Seamount. Most eruptions are runny because basaltic magma is more fluid than magmas typical in more explosive eruptions such as the andesitic magmas that produce spectacular and dangerous eruptions around Pacific Basin margins. Volcanoes fall into several eruptive categories. Hawaiian volcanoes are called "Hawaiian-type". Hawaiian lava spills out of craters and forms long streams of glowing molten rock, flowing down the slope, covering acres of land and replacing ocean with new land.
The possibility that the Hawaiian islands became older as one moves to the northwest was suspected by ancient Hawaiians long before Europeans arrived. During their voyages, sea-faring Hawaiians noticed differences in erosion, soil formation, and vegetation, allowing them to deduce that the islands to the northwest (Niʻihau and Kauaʻi) were older than those to the southeast (Maui and Hawaii). The idea was handed down the generations through the legend of Pele, the fiery Hawaiian Goddess of Volcanoes. This dynamic view contrasted with the static Genesis creation myth taught by Europeans at the time.
See also: Hawaiian Islands
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