Richard Roberts' Rock
Plowing his field at Salcha, south of Fairbanks, Richard Roberts' tiller came up hard upon a rock. As Mr. Roberts picked up the foot-long rock he realized that there were two peculiar things about it. For one thing it was the only rock around of its size, it had been found in fine-grained, windblown soil. Secondly, it was dark and pitted like a igneous rock, but Mr. Roberts knew that there was no evidence of volcanic activity in the area.
A stony meteorite? Mr. Roberts thought it might be, so he brought it to the Geophysical Institute for evaluation. There, through the mouths of several geologists, the rock told some of its history.
The rock is not a meteorite but is, in fact, what is called a vesicular basalt because of its swiss-cheese appearance. Petrologist Dr. Sam Swanson pointed out that the vesicles, remnants of gas bubbles left when the rock solidified from a liquid magma, were flattened into exaggerated egg shapes. This showed, he said, that the rock had formed in a gravity field, and furthermore that the direction of gravity was at right angles to the elongation. Were the vesicles formed by the melting at the edges of a meteorite in flight, they would have been tear-shaped and strung out in lines paralleling the direction of flight.
University of Alaska's Sir Edward Bullard and Dr. Swanson peered closely at Richard Roberts' rock through their geologist lenses and agreed that the rock had no olivine, a magnesium and iron silicate mineral commonly found in stony meteorites. The texture on one side of the rock differed from that on the other, indicating to Dr. Swanson that the rock had solidified near the edge of a lava flow or a subsurface dike.
All the evidence that the rock was not a meteorite left unanswered the mystery of how the rock got to where it was found. Geologist Florence Weber of the U.S. Geological Survey is familiar with the rocks of the Salcha area and is able to verify that Mr. Roberts' rock is unusual for that area. It is four times larger in diameter than the cobbles in the gravels below the silt in which the rock was found. These gravels form a terrace of outwash gravels laid down during a glaciation thousands of years ago. Like these gravels, Mr. Roberts' rock is well-rounded, indicating that it has been stream transported a distance of several tens of miles, most probably from the Alaska Range upriver to the southeast.
How did it get up into the fine loess from the gravels below?
The best guess is that seasonal freeze-and-thaw ground action preferentially jacked up this big rock away from the smaller gravel and into the soil above.
So it seems that there is a less dramatic, earth-origin explanation for the rock being found where it was--though maybe it sounds a bit too contrived. In any event, the rock is not a meteorite.
Aside from the clues to its own origin, Mr. Roberts' rock testifies to the alertness of its finder in recognizing that something was unusual. Furthermore, he was curious enough and persistent enough to take the trouble to try to learn the whys of his freak find. Having that kind of curiosity and persistence adds a bit to the joys of life.
