Alaska is home to approximately 8% of the earth's active volcanoes. Within the United States, 80% of the active volcanoes are located in Alaska, with the majority located in the Aleutian Islands and along the Alaska Peninsula known as the Aleutian Arc, extending 1,550 miles from central Alaska towards Russia. The Aleutian Arc forms the northern section of the well-known Ring of Fire which reaches as far as Japan.
Volcanoes result from a process called subduction in which one crustal plate slides under another. Volcanoes in the Aleutian Islands are a result of the Pacific Plate sliding beneath the Northern American Plate -- at a rate of about 3 inches per year. This movement also plays a major role in Alaska's many earthquakes. As the plates move, they build up energy that is released in the form of quakes capable of causing extreme devastation. Three of the ten largest earthquakes recorded since the year 1900 have occurred in Alaska as a direct result of these plate movements.
As these plates descend into the dense rock under the earth's crust, called the mantle, they undergo a series of physical and chemical changes caused by pressure and temperature imbalances. Water is released from the crust and, along with other impurities, rises into the mantle which lowers the mantle's melting point. Magma is formed and rises towards the earth's surface. When it reaches what is called the continental crust (less dense than the mantle), it forms into pools and begins to change. It heats up then melts again, mixing with rock and other minerals around it. As the magma cools, it crystallizes and the crystals that are formed, which differ in their composition from the magma, become separated from the magma. As it continues to undergo changes, the magma rises towards the surface and eventually erupts, either spewing forth non-explosive lava flows or volcanic ash, depending on the amount of dissolved gases in the magma. Volcanic ash can reach up to 45,000 feet in the air, before falling back to earth.
One to two volcanoes erupt every year in Alaska, though not all make headlines. Some of the most notable of Alaskan Volcanic eruptions include the 1992 Mount Spurr eruption which, in just 4 hours, landed 3mm of volcanic ash on Anchorage--Alaska's largest city.
The 1989 Redoubt eruption, which was Redoubt's fourth eruption in the 20th century, caused severe power outages, disabled aircraft, closed schools and halted oil production on 10 oil platforms in the Cook Inlet. The total loss incurred from the eruption totaled approximately $160 million, making it the 2nd most costly volcanic eruption in American history.
In 1912, the largest eruption in U.S. history and the largest rhyolite eruption (a combination of volcanic rock and magma with more than 72% silica) in recorded history took place in the Katmai Region of Southern Alaska, when Novarupta erupted. The eruption was especially devastating to those on Kodiak Island, which was covered in 12 inches of volcanic material. It formed major ashflows, fouling water supplies and damaging buildings. The Novarupta eruption also created the Valley of Ten Thousand Smokes -- named such because the cooling sheet of ash contained tens of thousands of fumaroles which vented hot steam and gas into the late 1920's.
Alaska's volcanoes are monitored year-round and scientists are often able to predict an eruption before it occurs, giving occupants in the endangered areas time to evacuate. To address the hazards, the AVO (Alaska Volcano Observatory), is continually upgrading its seismic network to watch all of Alaska's active volcanoes.
Because of Alaska's unique geographic location, there is a large amount of air travel over Alaska each day. The Anchorage International Airport transports more international airfreight (in dollars), than any other American airport. Over 80,000 aircraft and 10,000 people fly over Alaska's volcanoes every day. Contact between aircraft and volcanic ash can cause serious damage, as engines and other vital parts of an aircraft can be severely damaged by the abrasive ash. Since the year 2000, more than 80 jet planes have been damaged by volcanic ash worldwide. Seven of the encounters resulted in engine failure, though they were able to restart in time and no lives were lost. But before you decide never to fly over Alaska, you should know that aircraft over Alaska are at risk of encountering lethal ash clouds for only 4 days per year, on average.
Although volcanoes can be dangerous and very costly, they also have positive effects as well. The magma moving towards the earth's surface produces geothermal resources which can be used to produce low-cost electricity. Likewise, many extremely fertile agricultural soils are directly linked to volcanic activity such as in Hawaii--known for some of the best coffee in the world.