The Hawaiian Islands and the many seamounts to the northwest represent the exposed peaks and submerged mountains of a great chain of extinct, dormant, or active volcanoes. This chain has been forming for many millions of years as vast outpourings of lava issue from a relatively fixed vent or “hot spot” of volcanic activity on the deep ocean floor. This hot spot is believed to have remained in its present general position for many millions of years. The large tectonic plate (see Plate Tectonics) that forms the floor of much of the Pacific Ocean appears to be moving slowly in a northwesterly direction at a rate of about 10 cm (4 in) a year. Lava flows pouring out of this vent over long periods of geological time have built a series of broad, gently sloping volcanoes. Each has subsequently migrated to the northwest along with the slowly moving tectonic plate. Eventually becoming distant from the hot spot, the volcanoes become dormant and then extinct. Over long periods of time the volcanoes submerge into the sea as their great mass causes them to sink back into the crust, leaving no volcanic rock above sea level.
Over time, coral growth produces first fringe and then barrier reefs, and the tops of the sinking volcanoes become completely covered with coral (see Coral Reef). In this process atolls, such as Laysan, Midway, and Kure, have formed at the northwest end of the Hawaiian Archipelago. Atolls are characterized by a large lagoon surrounded by a barrier reef which protects one or more small, low lying, sandy islets.
The still active, and therefore younger, volcanoes of Hawaii today are technically those that have erupted since written records have been kept. These active volcanoes include one on the island of Maui, three on the island of Hawaii, and a recently discovered submarine volcano, Loihi, about 35 km (about 20 mi) southeast of the island of Hawaii.
Above the surface of the ocean the lava and limestone rock has been subjected to erosion, and today the islands of Hawaii reflect the intensity and duration of these forces of erosion. The oldest islands, in the northwest, have been worn down to sea level and are now represented only by low atolls and coral reefs that rest on the submerged remnants of volcanoes. Farther southeastward are tiny lava islets. The southeastern end of the island chain is geologically the most recent section and includes the eight main islands.
The island of Hawaii, the most recent of all, is the highest and largest island and, compared with the other main islands, has not been heavily eroded since it is still in the formative stage.
On the other, older main islands the long dormant volcanoes have been heavily eroded and the mountain ranges are characterized by steep slopes and numerous sharp ridges. As the process of erosion continues, all the main islands are being slowly worn down. The effects of earth movements and changing sea levels have also altered the physical appearance of the islands. For example, lower and higher sea levels, and perhaps some subsidence, or the sinking of the land, has caused the formation of Pearl Harbor. Uplifting, or the rising of the land, has left former beaches along the Oahu coast high above the sea. In addition, higher sea levels and uplifting of some of the ancient coral reefs that fringe part of the coast has resulted in deposits of limestone along the coast.
The volcanoes of the Hawaiian Islands are all so-called shield volcanoes, or lava domes. Unlike the volcanoes of Alaska and South America, those of Hawaii were not created by very explosive eruptions. Formed mostly by lava flows, they are great rounded mountain masses, rather than steep-sided cones. Mauna Kea, dormant for centuries, is the highest mountain in the state. It rises to 4,205 m (13,796 ft) above sea level, and its summit is dotted with cinder cones formed by fire fountains ejecting millions of small pieces of volcanic cinder and ash.
The state’s two main active volcanoes, Mauna Loa and Kilauea, are both on the island of Hawaii. Although they erupt periodically, large-scale volcanic explosions have not occurred in recent history. Eruptions are usually accompanied by minor earthquakes, but large and hazardous earthquakes are known to occur. Lava flows are generally not dangerous, but they have destroyed extensive areas of farmland. The molten lava sometimes reaches the sea and has, in places, created new land areas. Mauna Loa has a summit 4,170 m (13,680 ft) above sea level. Its chief crater, Mokuaweoweo, erupted several times during the 20th century, but the lava that sometimes cascades down its slopes issues from openings on the flanks of the mountain. Kilauea lies on the southeastern slope of Mauna Loa. Halemaumau, a crater in the summit caldera of Kilauea, erupts occasionally and fire fountains near the summit or along the rift zones sometimes eject volcanic particles far into the air.
Lava occurs on all the main islands in either of two basic forms, pahoehoe and aa. Pahoehoe is a smooth, ropelike form of lava with small holes formed by gas escaping as it cooled. Aa is a rougher and more pitted kind of lava, formed when the flow of escaping gas is less regular and of greater intensity. Among the lava features associated with volcanic eruptions are Pele’s hair and Pele’s tears, which are named for the Hawaiian goddess of volcanoes. Pele’s hair is formed when small particles of molten material are thrown into the air and spun out by the wind into long hair-like strands. Pele’s tears are formed when the particles fuse into tearlike drops of volcanic glass. "Hawaii" © Emmanuel BUCHOT, Encarta, Wikipedia
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