Aleutian Islands

The Aleutians are a chain of over 200 islands that arc from the Alaska Peninsula (163° W) east for 1700 miles across the International Date Line toward the Kamchatka Peninsula (172° E). With an area of 6821 square miles, the chain is composed of volcanic islands, called the Aleutian Ridge, that have been active for at least 55 million years. With over 80 major volcanic vents resulting from the tectonic collision of the southern Pacific and the North American plates, just south of the ridge is the deep Aleutian Trench. All along the chain, but especially the eastern end, earthquakes are frequent and sometimes quite severe—the Aleutians experienced two of the world's top ten earthquakes between 1904 and 1997.

The Aleutian chain can be segmented into five major links from west to east: the Near Islands in the west, the Rat Islands, the Delarof and Andreanof Islands, the Islands of the Four Mountains, and the Fox Islands in the east. Geologically, while the southern front of the chain is eroding into the Aleutian Trench, volcanoes are forming on the northern front, as evidenced by Bogoslov Island. In addition, the far western Near Islands are slowly heading for collision with the Kamchatka Peninsula. This geologic structure of the Aleutian Islands has contributed to the formation of ocean currents of the North Pacific and Bering Sea, the island chain acting as a sieve or porous boundary between these bodies of water, with 14 shallow Aleutian passes.

The climate is dominated by the barometric pressure phenomenon known as the Aleutian Low, a cyclonic center caused by the temperature differences between the cooler land and the warmer waters from the Alaska Current and North Pacific Drift. The Aleutian Low is characterized by the colliding frontal zones of warm Pacific air, contrasted with the cold, dense Arctic air. Thus, while in summer the islands are wrapped in seemingly endless fog, in winter they are wracked by continual storms. In the summer months the Aleutian Low is weak, but the sky is overcast 95% of the days, and fog forms 25-40% from June to August. Because land cools relatively faster than water, cold and dense air builds up along the windward slopes of the mountains, eventually falling rapidly on the leeward side due to gravity. These severe winds are called "williwaws," and they can be frequent; wind speeds of 44 m s-1 are common. Wind can also create considerable snowdrifts, even though the wet and heavy snow usually accumulates less than a foot at one time during winter.

However, because of the warm currents and the bow of the arc to a latitude nearly equivalent to the Canada-United States border, the Aleutians have a mild annual temperature. In summer, the average temperature is 65°F, while in winter it is barely below freezing. This relatively mild climate, along with a large amount of precipitation, encourages the success of a variety of flora and fauna. The Bering Sea as a whole, due to its extensive winter ice coverage and large continental shelf, is the year-round home of an extremely productive ecosystem, with some of the largest marine mammal populations in the world. The Bering Sea also contains some of the most productive fisheries, such as groundfish, salmon, and crab; out of approximately 450 species of fish and invertebrates, as many as 25 species are harvestable.

The Aleutians are home to tremendous populations of birds as well, for the lack of safe ship harbors, and high, inaccessible cliffs are ideal habitat. Fulmars, petrels, cormorants, gulls, kittiwakes, murres, auklets, and puffins congregate in the millions. Biologists have identified approximately 200 species that nest in, migrate through, or stray into the region. Unimak Island, in the east, also has a bear population; red foxes are also indigenous to the Fox Islands. The Aleutians do not naturally include trees, but instead tall grasses, wild rye, and alpine meadows with ferns, salmonber-ries, and wildflowers such as fireweed, lupine, saxifrage, and iris.

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