It Lurks Beneath Barrow
Far under Alaska's northernmost town, something strange lies buried. It's big. It's mysterious. And the only reason anyone knows it's there is because an atomic bomb was detonated last year in China.
All right, so I'm having a little fun here. Though every word of the foregoing paragraph is true, the situation isn't as creepy as it's been made to sound. Here's the story.
In May 1992, a nuclear device was detonated in China. This underground test generated a one-second seismic punch that penetrated thousands of miles into the earth. Nearly on the other side of the globe, chains of seismometers in Canada and the United States picked up the pulse. In effect, it was the same thing oil-seeking prospectors do by setting off dynamite blasts or monitoring sonic pingers, only on a gigantic scale. Light doesn't penetrate into solid earth, so it's necessary to use some other sort of waves to provide images of what's within. Seismic waves are the best researchers have been able to use so far.
This time, with 14 chains of seismometers (effectively spanning North America and containing 1062 separate instruments) to pick up the waves and an A-bomb to provide the flashbulb, scientists were able to capture an image of something truly hard to picture. They now have what amounts to a snapshot of a portion of the boundary between Earth's molten metallic core and the overlying mantle.
The region that their wave traces delineated lay in the western Arctic, roughly from the area under Thule in northern Greenland to below Wrangell Island. The lower mantle in most of this region seemed to be homogeneous, but not in the area near Point Barrow. According to John E. Vidale and Harley M. Bent of the U.S. Geological Survey, a kind of subsurface island lies at the core-mantle juncture 2700 kilometers straight down under northern Alaska. This structure---they can't put a more exact name on it---is about 300 kilometers across and 130 kilometers thick. That sounds like a rather large lump, but it's the smallest anomaly anyone yet has been able to identify in that deep and virtually invisible realm.
The USGS researchers are proud of what they think is a very high-resolution picture of a very small area. Before the data from the Chinese test were collected and analyzed, earthquakes had provided the source for any seismic waves reaching the core-mantle boundary and returning to seismometers. Studies of such earthquake waves had indicated that a layer about 250 to 300 kilometers thick at the boundary region held significant variations, or heterogeneities. The earthquakes drew a fairly vague picture, but sufficient for the scientists to tell that the boundary was not uniform worldwide.
Like the odd structure under Alaska, these heterogeneities could be detected because seismic waves passed through them a bit more swiftly than they did through the neighboring rock. That change of pace is what delineates oddities deep in the earth.
Seismic waves move faster when they pass through material of a different temperature or a different chemical composition than the adjacent mantle. Thus, even though the explosion in China was a veritable strobe light compared to an earthquake's flashlight, all that the seismologists can see of the lump beneath Barrow tells them only that it's different. But that's a start, and a true advance in deciphering the nature of a place that no one has ever seen, and probably no one ever will. The USGS researchers hope that identifying one singular structure will be a first step toward understanding the interactions between Earth's mantle and its core.
There's something very appropriate about this discovery. Barrow at the surface is different from anyplace else on the surface. Why shouldn't it be different underneath?
