How many volcanoes are there in Alaska?

From the Alaska Volcano Observatory website:
Alaska contains over 130 volcanoes and volcanic fields which have been active within the last two million years.These volcanoes are catalogued at their website: http://www.avo.alaska.edu/volcanoes.

Of these volcanoes, about 90 have been active within the last 10,000 years (and might be expected to erupt again), and more than 50 have been active within historical time (since about 1760, for Alaska).

The volcanoes in Alaska make up well over three-quarters of U.S. volcanoes that have erupted in the last two hundred years.

Alaska's volcanoes are potentially hazardous to passenger and freight aircraft as jet engines sometimes fail after ingesting volcanic ash.

It is estimated that more than 80,000 large aircraft per year, and 30,000 people per day, are in the skies over and potentially downwind of Aleutian volcanoes, mostly on the heavily traveled great-circle routes between Europe, North America, and Asia. Volcanic eruptions from Cook Inlet volcanoes (Spurr, Redoubt, Iliamna, and Augustine) can have severe impacts, as these volcanoes are nearest to Anchorage, Alaska's largest population center.

The series of 1989-1990 eruptions from Mt. Redoubt were 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.

On the Kenai Peninsula, during periods of continuous ash fallout, schools were closed and some individuals experienced respiratory problems. At the Drift River oil terminal, lahars and lahar run-out flows threatened the facility and partially inundated the terminal on January 2, 1990. The Redoubt eruption also damaged five commercial jetliners, and caused several days worth of airport closures and airline cancellations in Anchorage and on the Kenai Peninsula. Drifting ash clouds disrupted air traffic as far away as Texas.

The three eruptions of Mt. Spurr's Crater Peak in 1992 deposited ash on Anchorage and surrounding communities, closed airports, made ground transportation difficult, and disrupted air traffic as far east as Cleveland, Ohio. More information about this eruption is available here. Many older Alaskans also remember ash falling on Anchorage during the 1953 eruption of Mt. Spurr's Crater Peak.

The 1912 eruption of Novarupta and Katmai, which formed the Valley of Ten Thousand Smokes on the Alaska Peninsula, was the largest 20th-century eruption on earth, and the largest historical eruption in Alaska. Ash from Novarupta spread worldwide, and is often still remobilized by strong winds. Roofs in Kodiak collapsed due to the weight of the ash; six villages close to Katmai and Novarupta were permanently abandoned. More information about this eruption is available here.

How often do Alaskan volcanoes erupt?
Although the historical record in Alaska goes back to about 1760 (there are a few earlier eruption accounts), the task of counting known eruptions and calculating an eruption frequency is complicated by the sparse and often inaccurate older accounts.

Many times, a volcano is reported as "smoking" without further clarification of what that smoke may have been - a real eruption, normal fumarolic activity, or even atypically tall clouds rising above a summit because of unstable weather conditions. The term "eruption" as used here includes vigorous explosions which may not contain fresh (juvenile) magma, as well as magmatic explosions and the effusion of lava as flows and domes.

Since 1760, 27 Alaskan volcanoes have had more than 230 confirmed eruptions. This averages to nearly one eruption per year. If we add in those volcanoes and eruptions that are suspected but unconfirmed (and often, unconfirmable), then we have 54 volcanoes with about 424 possible eruptions, yielding an average of 1.7 eruptions per year.

However, these figures do not consider the large discrepancy in observation and reporting of Alaskan eruptions. For the past 40 years - a period in which we have fairly good records -- Alaska has averaged more than two eruptions per year.