DEFINITION OF ICELAND

The Mid-Atlantic Ridge rises above sea level on

the North Atlantic island of Iceland, lying 178 miles off the

coast of Greenland and 495 miles 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 that is interacting with

the mid-ocean ridge system beneath the island. The Mid-

Atlantic Ridge crosses the island from southwest to northeast

and has a spreading rate of 1.2 inches per year (3 cm/yr) with

the mean extension oriented toward an azimuth of 103°. 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

Tjörnes 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 that are deposited unconformably over

older tholeiitic basalts. Active spreading occurs along a series

of 5–60-mile (10–100-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 kilometers below the surface, then dikes

intrude the overlying crust and flow horizontally for tens of

miles to accommodate crustal extension of several feet 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.

ice sheet See ICE CAP.

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