Montana's Earth Science Picture of the Week

A blast from the past . . .
In 1990 geologists discovered evidence of an ancient asteroid impact in the mountains of southwestern Montana. The team found rocks with distinct cone-shaped fracture patterns resembling horsetails. They recognized that these were “shatter cones”; the unique fracture patterns formed as intense shock waves generated by an asteroid impact travel through bedrock.

Right: This photo, which was provided by Mike Plautz, Science teacher at Hellgate Elementary in Missoula, shows a classic shatter cone at a location scientists refer to as the "Beaverhead Site”. The presence of shatter cones here puzzled scientists because the area lacked visible evidence and other characteristics typically associated with impact craters.

Solving the mystery . . .
Scientists have identified over 160 impact craters on Earth. Unlike the surface of the Moon where impact craters dominate the surface, most of Earth’s craters are much more difficult to identify. On Earth there are many processes that wear away or hide the craters, including erosion, vegetation, seafloor sediments, lava, and plate tectonics. So, in order to find impact craters scientists search for the following clues.

1. Evidence of rocks that have been changed by shock waves (shatter cones)

2. Craters, or geology that indicates the presence of a crater

3. Geophysical anomalies: variations in gravity and/or magnetism that stand out as “unusual”

4. The presence of meteorites: fragments from the asteroid (or comet)

The Beaverhead Impact Structure . . .
Although the shatter cones found in southwestern Montana were the first clue that there had been an impact, scientists eventually determined that the actual crater is centered about 50 miles southwest of the Beaverhead Site around the Challis, Idaho area. The mystery of the crater’s location was solved when scientists found that some of the rocks in the area around Challis had different properties when it came to magnetism and gravity readings. As it turns out these geophysical anomalies revealed a crater-shaped pattern indicating that the crater was probably about 100 km in diameter (65 miles). Radiometric dating done in 1999 suggested that the impact happened about 900 million years ago. . . . No wonder it was so hard to find! The crater, which was named the Beaverhead Impact Structure, is one of only eight known impact craters over 50 km in (32 miles) diameter. To find out where other craters are located, click on the Hot Link below.

Below: These diagrams are from a masters thesis paper by A. E. McCafferty, Colorado School of Mines. The top one shows a (proposed) cross-section of what the geology of the crater area might have been like soon after the impact 900 million years ago. The bottom one shows a cross-section of the area today. Compare the location of the points labeled “Grouse Peak” and “Beaverhead Site” on the two diagrams. Geologists believe that, since the impact happened, the top part of the crust moved several miles to the east causing these two points to be offset from their original position on oposite sides of the crater. This type of "thrust faulting" was common in the area during the formation of the Rocky Mountains. The yellow area indicates location of bedrock that provided the unusual magnetic and gravity readings.

Sources

McCafferty. A.E. , 1995, Assessing the presence of a buried meteor impact crater using geophysical data, south-central Idaho: Masters Thesis, Colorado School of Mines, 88p.

Carr, J., and Link, P.K., 1999, Neoproterozoic conglomerate and breccia in the formation of Leaton Gulch, Grouse Peak, northern Lost River Range, Idaho: Relation to Beaverhead Impact Structure, in Hughes, S.S., and Thackray, G.D., eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of natural History, p. 21-29.

Term: anomaly