Plate Tectonics

Briefly, the huge Pacific plate (the ocean floor) is drifting slowly northeast. It collides with the North American plate, on which the continent rests, along an arc that stretches from the western Aleutians in the Gulf of Alaska to the Inside Passage—defining one section of the famous Pacific “Ring of Fire.” This meeting of plates jams the ocean floor under the continental landmass and gives rise to violent geological forces: upthrusting mountains, extensive and large earthquakes, volcanic rumblings and eruptions, and movement along fault lines.

Somewhere in the mists of early geological time, a particularly persistent and powerful collision between the two plates caused the Brooks Range to rise; erosion has whittled its highest peaks to 8,000 feet, half their original height. Later, a similar episode thrust the Alaska Range into shape. The Pacific plate even today continues to nose under the continental plate in the vicinity of Yakutat (near where Southeast meets Southcentral Alaska). The force pushes Mt. Logan—Canada’s highest peak—slowly upward. Learn more about the geological forces that shape Alaska at the University of Alaska Fairbanks Geophysical Institute website (www.gi.alaska.edu).

Earthquakes

One of the world’s most seismically active regions, Alaska has withstood some of the most violent earthquakes and largest tidal waves ever recorded. In the last century, more than 80 Alaskan earthquakes registered higher than 7 on the Richter scale. The most destructive occurred at 5:35 p.m. on Good Friday, March 27, 1964, when the most powerful earthquake ever recorded in North America rocked Southcentral Alaska.

This 9.2 magnitude quake (80 times bigger than the famous 1906 San Francisco earthquake, which is estimated at around 7.8) had a devastating impact on the region, flattening 75 homes and businesses in Anchorage, creating tsunamis that wiped out nearly every coastal village in Southcentral Alaska, and wreaking havoc all the way to California. The quake and its tsunamis killed 131 people. It was the second strongest in the 20th century; only a 9.5 magnitude quake in Chile in 1960 was larger.

Anyone who spends more than a few months in Southwest or Southcentral Alaska will probably feel at least one earthquake, and residents of the Aleutian Islands barely take notice of anything with a magnitude less than 6. The Alaska Earthquake Information Center’s website (www.aeic.alaska.edu) provides detailed information on Alaskan earthquakes, including today’s activity. For additional background on earthquakes in Alaska, visit the USGS Earthquake Hazards Program website (www.earthquake.usgs.gov).

Tsunamis

An earthquake deep below the ocean floor in the Gulf of Alaska or the open Pacific is especially dangerous to the coasts of Alaska and Hawaii, along with the west coast of Canada and the United States. The activity creates enormous tidal waves (tsunamis), which, although they are only 3–5 feet high in the open ocean, can travel at speeds exceeding 500 mph.

Contrary to popular fears, a tsunami does not slam into the coast with 20 or 30 feet of water, washing away everything in its path like a flash flood. Instead, the water slowly inundates the land to a depth of 4–5 feet. Then, after a brief and chilling calm, the wave is sucked back out to sea in one vast undertow. Most of the destruction caused by the great Good Friday earthquake was of this nature, attested to by hair-raising pictures that you’ll see in places like Valdez, Seward, and Kodiak.

If you hear a tsunami warning, get to higher ground immediately. The West Coast & Alaska Tsunami Warning Center is based in Palmer, and its website (http://wcatwc.arh.noaa.gov) provides information on recent and historical tsunamis.

Volcanoes

Like its earthquakes, Alaska’s major volcanoes are located along the Aleutian chain. In fact, 57 active volcanoes stretch along this arc, and most have been active in the last 300 years. The largest recorded eruption occurred when Novarupta blew its top in 1912, the most cataclysmic natural disaster since Krakatoa had cracked 30 years earlier.

There’s always an active volcano somewhere in the state, especially in the Aleutians and along the Alaska Peninsula. The University of Alaska Fairbanks Alaska Volcano Observatory keeps track of volcanic activity, and its website (www.avo.alaska.edu) has details on current and historic eruptions.

Several volcanoes get particular attention because of their recent activity and proximity to major population centers: Mt. Spurr (78 miles west of Anchorage) erupted in 1992, Mt. Augustine (171 miles southwest of Anchorage) in 2005, and Mt. Redoubt (103 miles southwest of Anchorage) in 1989 and 2009. Each of these dumped ash in varying amounts on the region, closing schools and businesses, filling the air with ash (hazardous for people, electronics, and car engines), and creating havoc for air travel.

The 1989 eruption of Mt. Redoubt nearly brought down a KLM jet when it flew into the ash cloud, causing all four engines to shut down. The plane dove two miles before the pilots were able to restart the engines and land safely in Anchorage. That same year, Redoubt generated a massive debris flow down the Drift River, inundating an oil terminal and threatening to spill oil into Cook Inlet.

© Don Pitcher from Moon Alaska, 10th Edition