The mid-Atlantic ridge rises above sea level on the North Atlantic island of Iceland, lying 178 miles (287 km) off the coast of Greenland and 495 miles (800 km) from the coast of Scotland. Iceland has an average elevation of more than 1,600 feet (500 m), and owes its elevation to a hot spot interacting with the midocean ridge system beneath the island. The mid-Atlantic ridge crosses the island from southwest to northeast and has a spreading rate 1.2 inches per year (3 cm/yr), with the mean extension oriented toward an azimuth (compass direction) of 103 degrees east of north. The oceanic Reykjanes ridge and sinistral transform south of the island rises to the surface and continues as the Western Rift zone. Active spreading is transferred to the Southern Volcanic zone across a transform fault called the South
Iceland Seismic zone, then continues north through the Eastern Rift zone. Spreading is offset from the oceanic Kolbeinsey ridge by the dextral Tjornes fracture zone off the island's northern coast.
During the past 6 million years the Iceland hot spot has drifted toward the southeast relative to the north Atlantic, and the oceanic ridge system has made a succession of small jumps so that active spreading has remained coincident with the plume of hottest, weakest mantle material. These ridge jumps have caused the active spreading to propagate into regions of older crust that have been remelted, forming alkalic and even silicic volcanic rocks deposited unconformably over older tholeiitic basalts. Active spreading occurs along a series of 5-60-mile (8-100 km) long zones of fissures, graben, and dike swarms, with basaltic and rhyolitic volcanoes rising from central parts of fissures. Hydrothermal activity is intense along the fracture zones, with diffuse faulting and volcanic activity merging into a narrow zone within a few kilometers depth beneath the surface. Detailed geophysical studies have shown that magma episodically rises from depth into magma chambers located a few miles (kilometers) below the surface, then dikes intrude the overlying crust and flow horizontally for tens of miles to accommodate crustal extension of several to several tens of feet over several hundred years.
Many Holocene volcanic events are known from Iceland, including 17 eruptions of Hekla from the Southern Volcanic zone. Iceland has an extensive system of glaciers and has experienced a number of eruptions beneath the glaciers that cause water to infiltrate the fracture zones. The mixture of water and magma induces explosive events including Plinian eruption clouds, phreo-magmatic, tephra-producing eruptions, and sudden floods known as jokulhlaups, induced when the glacier experiences rapid melting from contact with magma. Many Icelanders have learned to use the high geothermal gradients to extract geothermal energy for heating, and to enjoy the many hot springs on the island.
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