October 11, 2005
Tuesday PM

In the early 1950s, workers for the U.S. Navy drilled test wells in an area of the North Slope known as the Naval Petroleum Reserve. The drillers sent core samples of rock to Fairbanks, where Florence Weber and Florence Collins, both geologists with the U.S. Geological Survey, noticed something odd. The samples, taken from an area where the surrounding rock was lying flat, were tilted upright. Some of the rocks were shattered.

The strange rocks seemed vaguely familiar to Weber and Collins, two of the first women geologists in Alaska. Both recently had attended a field trip to Indiana to see an impact crater, the massive divot left behind after a meteorite hit the ground. Looking at the pulverized rocks from the petroleum reserve, they thought the Navy diggers may have tapped into an impact crater on the North Slope. Weber and Collins followed their hunch and wrote a USGS paper on what has become known as Avak, the only impact crater confirmed in Alaska.

The Avak impact crater, located east of Barrow, measures about six miles from rim to rim. Don't look for it from an airplane window, though. Several hundred feet of sediment covered Avak in the last million years, hiding the crater from view. Geologists know the crater exists because it's revealed in the core samples and seismic and other geophysical surveys.

Avak was born when a meteorite or comet the diameter of downtown Fairbanks crashed into northern Alaska millions of years ago. Buck Sharpton, a University of Alaska professor who studies impact craters, said the speeding celestial body struck the shallow ocean that covered the North Slope with a shock 10,000 times as powerful as an atomic bomb. The jolt triggered earthquakes, a tsunami, and sent earth flying in all directions. Animals unfortunate enough to be grazing near ground zero were vaporized. Sharpton said Avak was "extremely energetic," but it didn't have anywhere near the effect of the impact in Chicxulub, Mexico that probably caused the extinction of dinosaurs.

Avak provided a bit of energy for the people of Barrow. The concussion that made the crater created folds in nearby rock that trapped natural gas beneath a ceiling of impermeable rock. The Navy tapped one of the gas traps to provide natural gas to heat buildings in Barrow.

Avak is one of just 139 discovered impact craters in the world. Despite the rarity of known craters, Sharpton says Earth has absorbed thousands of meteorites and comets over the millennia. He estimates Alaska should be pocked with the indentations of about 250 meteorites and comets. Some may show themselves as circular lakes, or chains of lakes that make a circle. Most have been disguised by erosion, protruding mountains, or the movement of Earth's plates. But the craters are out there, waiting to be found by curious people like Florence Weber and Florence Collins.

This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. This article first appeared in 1999. Ned Rozell [nrozell@gi.alaska.edu] is a science writer at the institute.

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