21 December 2010

New York state’s hidden secret: A subterranean impact crater

Posted by mohi

I’ve heard of stealth bombers and stealthy ninjas and even a super-sneaky magnitude 8 earthquake, but until today I hadn’t heard about stealth craters: large features, more than three kilometers across and more than 300 meters deep– definitely big enough to be obvious, one would think.

But not if it’s buried. One such crater lurks 1.2 kilometers underground in New York’s Finger Lakes Region. Located near Syracuse and close to the Bear Swamp State Forest, the crater is completely invisible from the surface. The ground is flat, and the tree-covered vista is not unlike those that characterize upstate New York’s wine-growing region.

Syracuse crater

Location map showing the area of the 3D seismic survey and then newly discovered crater. The inset in the lower left shows the buried rock layer that holds the crater, and the inset at the lower right shows the crater in 3D. Image courtesy of AGU/Dan Leiphart

Dan Leiphart from Chesapeake Energy Corporation presented a poster describing the hidden crater during Fall Meeting, at Friday afternoon’s poster session P53C, Mineralogical Studies of Impact Craters: Exhumed Crust, Hydrothermal Processes, and Post-Impact Weather II.

He said finding the crater was totally unexpected, the result of a small-scale, 3-D seismic survey in the area looking for a different geological formation. The seismic survey used dynamite to blast sound waves into the Earth’s crust. The waves bounce around the dark, subterranean environment and return data that Leiphart, a geophysicist, analyzes.

“We saw a circular feature. It was not what we were looking for,” he said. “Kind of an accidental stumbling.”

About three times larger than Meteor Crater in Winslow, Arizona, the buried crater covers 10 square kilometers, is 3.5 kilometers in diameter, and 305 meters deep. The crater sports a 160-meter tall bump in its center, indicating that resulting shock waves were strong enough to bubble back up from the crater’s belly. But, “as far as impact structures go, it’s pretty small,” Leiphart said.

Past studies indicate that the area was likely a shallow marine environment when impact occurred around 444 million years ago, or at the end of the Ordovician period. Once the crater formed, it quickly filled as water and accompanying sediment rushed to fill the void.  The weight of this material began to slowly push the crater into the Earth’s crust. “You put sediment on it, it sinks a little bit. You put more sediment on, it sinks a little more,” Leiphart  said.

The last piece of proof scientists needed came from drilling down to the crater and recovering samples. One of the samples contained shocked quartz,  which is diagnostic of meteors striking rock and deforming quartz crystal structures. Noting that no other geological phenomenon produces shocked quartz, “That was pretty much the smoking gun we were looking for,” Leiphart said.

How many more stealth craters are waiting to be discovered?

– Nadia Drake is a science communication graduate student at UC Santa Cruz