Alaska contains over 100 volcanoes and volcanic fields which have been active within the last one and a half million years. Over 40 of these have been active in historic time. These make up about 80% of all active volcanoes in the United States and 8% of all active above-water volcanoes on earth.
Most of these volcanoes are located along the 2,500 km-long (1,550 mile-long) Aleutian Arc , which extends westward to Kamchatka and forms the northern portion of the Pacific "ring of fire". Other volcanoes which have been active within the last few thousand years exist in southeastern Alaska and in the Wrangell Mountains . Smaller volcanoes, some active within the last 10,000 years, exist in interior Alaska and in western Alaska as far north as the Seward Peninsula.
Hardly a year goes by without a major eruption from a volcano in the Aleutian Arc. Those in the largely unpopulated western arc often go unremarked by all but volcanologists. The remote volcanoes are potentially hazardous, however - jet airplanes which enter eruption clouds often are severely damaged, and sometimes lose all engines temporarily. There are nearly 50,000 large aircraft per year, and 10,000 people per day, in the skies over Aleutian volcanoes, mostly on the heavily travelled great-circle routes between Europe and the North America and Asia. Volcanoes in the eastern arc, especially those from Cook Inlet volcanoes, can have severe impacts. The series of 1989-1990 eruptions from Mt. Redoubt was the second-most costly in the history of the United States, and had significant impact on the aviation and oil industries, as well as the people of the Kenai Peninsula. The three eruptions of Mt. Spurr's Crater Peak in 1992 deposited ash on Anchorage and surrounding communities, closing airports and making even ground transportation difficult, and disrupted air traffic as far east as Cleveland, Ohio. The 1912 Katmai eruption, which formed the Valley of Ten Thousand Smokes on the Alaska Peninsula was the largest 20th century eruption on earth.
Almost all of Alaska's active volcanoes are in the Aleutian arc. This map (86k GIF or 90k PDF) shows the locations of the volcanoes and this table shows their sequence. Many of the most active are labeled by name and with the date of their last major eruption. Major in this sense is not precisely defined, but generally means a large explosive eruption or emplacement of lava as domes or flows. Because of the imprecise way "major" is defined there is some room for argument about the years associated with some of the volcanoes. The following summary illustrates several years of volcanic activity along the Aleutian arc.