New study ranks Nevada crater among world’s largest

If it’s a super bowl you want, try this one on for size.

About 382 million years before Las Vegas sports books perfected the prop bet, a meteor crashed down on what is now Nevada, leaving behind a bowl-shaped crater nearly large enough to swallow Clark County.

A study published Jan. 14 estimates the crater at 69 to 93 miles in diameter, more than twice as big as previously thought.

If the projections are correct, the so-called Alamo impact would rank as one of the largest meteor strikes the Earth has seen in the past 500 million years.

“It was a pretty catastrophic event,” said Andrew Retzler, a geologist with the Minnesota Geologic Survey and the study’s lead author.

Had you been around to witness it, you would have needed a boat, since all of Nevada was a shallow sea that stretched east into present-day Utah.

Retzler said the Alamo crater was formed when the meteor smashed into the sea floor, triggering massive tsunamis that rippled out and then bounced back after colliding with the shore.

Geologic upheaval over the ensuing 382,000 millenniums has erased or buried most evidence of the impact, but traces of the cataclysm can be found high in the mountains surrounding Alamo in Lincoln County, where fractured grains of quartz and displaced fossils of eel-like sea creatures hint at the sudden violent event.

Retzler and two other researchers documented about a dozen of those sites during four trips to the area from 2011 to 2013.

“Basically we looked at all the impact deposits in the region and mapped them out,” said Retzler, then a master’s student at Idaho State University.

The resulting paper by Retzler and four other authors appears in the current issue of Geosphere, the online journal of the Geological Society of America.

Their findings suggest that only about half of the impacted area is actually exposed in the region, which explains why the crater was previously thought to be no more than 40 miles across. What researchers thought was the crater’s diameter is really more like its radius.

As large and violent as the Alamo impact was, Retzler said it apparently “didn’t wipe anything out.” So far, he said, fellow researchers have been unable to link any extinction events to the meteor, he said.

By contrast, the much larger Chicxulub impact on Mexico’s Yucatan Peninsula is widely blamed for killing off the dinosaurs about 65 million years ago.

At the time of the Alamo strike, all life on Earth was confined to its oceans, which were populated by corals, sponges, bivalve mollusks and other primitive organisms. The rise of the dinosaurs was still 150 million years away. The evolutionary arrival of humans was 380 million years out.

The new size estimate by Retzler and company would make the Alamo impact the largest crater in the United States by a fairly wide margin — if the impact event was actually recognized by the scientific community at large. About 20 years after scientists first spotted signs of an ancient meteor strike in the mountains 100 miles north of Las Vegas, the impact remains unconfirmed. It does not appear in the official Earth Impact Database, an online catalog of 185 confirmed craters around the globe.

Retzler can understand why. The site is so old and has seen so much geologic activity since then — from being buried in ocean sediment and showered in volcanic ash to being pulled apart by faults — that its story is hard to follow, even for experts.

“It does get pretty hectic,” he said. “There are definitely some gaps in the record, but that’s how geology works.”

Researchers might get a more complete picture of the crater if a portion of their potential study area wasn’t locked away within the boundaries of Area 51, the classified Defense Department installation not far from the Extra­terrestrial Highway and the rural town of Rachel.

As Colorado School of Mines professor emeritus John E. Warme, one of the first geologists to discover evidence of the Alamo impact, told the Las Vegas Review-Journal in 2013: “There are key rock outcrops (in) Area 51 that we would like to investigate but are ‘forbidden.’ ”

If the Alamo impact ever gains widespread scientific recognition, Retzler said it should land pretty high on the list of known craters. It won’t be at the top, he said, but Nevada’s super bowl should easily crack the top 10.

Contact Henry Brean at hbrean@reviewjournal.com or 702-383-0350. Follow @RefriedBrean on Twitter.