A blast from the past . . .
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 . . .
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 . . .
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.
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.
|Visual of the Beaverhead Impact Structure|
|*See other impact sites around the world|
|Shatter cones for sale|
|*A famous impact crater in Arizona|
|*The impact associated with extinction of the dinosaurs|