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The Island of Kauai
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A volcanic island, Kauai county, Hawaii, U.S. lies 72 miles (116 km) northwest of Oahu
Island across the Kauai Channel. The northernmost and geologically the oldest of the major
Hawaiian Islands, it is also the most verdant and is known as the Garden Isle. The first
Polynesians to reach the Hawaiian Islands are said to have landed there a millennium ago
and settled at the mouth of the Wailua River. With an area of 552 square miles (1,430
square km), the nearly circular isle, whose place-name is of uncertain meaning, consists
mainly of Mount Waialeale (5,243 feet [1,598 m]) and marginal lowlands dissected by
fertile valleys and deep fissures. Principal towns are Lihue (the county seat), Kapaa, and
Hanapepe. Sugar is the major agricultural product, and there is diversified manufacturing,
especially of tourist-oriented goods. Kauai county embraces nearby Niihau Island (73
square miles) and the tiny uninhabited islets of Kaula and Lehua. The county suffered
considerable property damage when it was hit by Hurricane Iniki on Sept. 11, 1992. Pop.
(1990) Kauai county, 51,177; (1995 est.) Kauai county, 56,131. The extinct volcano that formed the island of Kauai is about 5,000,000 years old. Assuming that the hot spot that generates Hawaiian volcanoes is relatively fixed in place--and there is good evidence to support this assumption--then the movement of the Pacific Plate during the past several million years has been northwestward at a rate of about 10 centimetres per year. Topographic maps show a major submarine continuation of the Hawaiian Ridge to the northwest of the Hawaiian Islands, and then a dogleg bend into the Emperor Seamounts, which comprise an entirely submarine ridge continuing northward to the edge of the Pacific Plate. Ages of rocks obtained by dredging and drilling the Emperor Seamounts indicate that the Hawaiian-Emperor Ridge is a progressively older volcanic chain formed by volcanism at the Hawaiian hot spot. This major centre of volcanism in the Pacific has been active for at least 80,000,000 years, and the Pacific Plate has moved over it, first northward and later northwestward, at a rate of approximately eight to 10 centimetres per year. The bend between the Emperor Seamounts and the Hawaiian Ridge occurred about 40,000,000 years ago and indicates a significant shift in the direction of movement of the Pacific Plate. It is not known how a volcanic hot spot maintains its position for millions of years while a plate passes over it. One concept suggests that a hot spot is a deep-mantle plume, which is caused by very slow convection of highly viscous mantle material. As hot but solid mantle rock moves upward, partial melting may occur from the lowering of its pressure-dependent melting temperature. Detailed seismic sounding of the mantle over the next decade or two should help to resolve this problem of the mechanism of hot spots. Small and isolated intraplate volcanoes may simply be the result of deep fractures within the plates that allow pockets of partial melt in the low-velocity layer below to leak to the surface. The low-velocity layer begins about 50 to 150 kilometres below the surface and extends to a depth of roughly 300 kilometres; it is so named because earthquake waves travel more slowly in this hot, low-strength layer than in the overlying rigid plates. Some studies indicate that there may be a few percent partial melt within the low-velocity layer. Once a sufficient volume of magma forms in the subsurface, it tends to rise from its own buoyancy. Any fracture system at the plate margins or within the plates will facilitate this process. |
Pictures |
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Here is a picture of Waimea Canyon |
