An asteroid as big as a city block smashed into what is now northern Iowa about 470 million years ago, says a Smithsonian geologist, supporting a theory that a giant space rock broke up and bombarded Earth just as early life began flourishing in the oceans.

The impact dug a crater nearly four miles wide that now lies beneath the town of Decorah, said Bevan French, one of the world’s foremost crater hunters and an adjunct scientist at the National Museum of Natural History.

The asteroid that carved it would have dwarfed the estimated 55-foot-wide space rock that exploded over southern Russia on Friday.

The Decorah object smashed into bedrock with such force that it shattered tiny grains of minerals. French found this “shock quartz” in gravel from beneath the town, he told two dozen colleagues during a seminar at the museum last week.

Finding impact craters is rare, as erosion and the shifting of tectonic plates tend to erase them. The Decorah crater, if accepted by other scientists, would be just the 184th known, according to an international database at the University of New Brunswick.

But spying the evidence of the Earth’s most dramatic explosions requires only humble equipment — a simple black microscope. As sun streamed into French’s office above Constitution Avenue one recent afternoon, he placed a glass slide under the microscope’s lens and invited a reporter to peer in. A thin slice of rock from beneath Decorah sat on the slide.

Three white circles — quartz crystals no bigger than mustard seeds — popped into view. Dozens of parallel lines striped each circle: evidence of a rock-crushing pulse.

“They’re shattered,” French said of the crystals. Geologists consider shock quartz near-definitive evidence of an extraterrestrial impact.

The Decorah crater lay undiscovered until now because almost none of it peeks above ground. Instead, it is filled by an unusual shale that formed after an ancient seaway sluiced into the crater, depositing sediment and an array of bizarre sea creatures that hardened into fossils, French said.

This shale was the first clue that certain Iowans may be unknowingly living in a crater.

Jean Young, an amateur geologist in northern Iowa, noticed the shale about a dozen years ago, when inspecting gravel pulled up by well-drilling machines. It looked like no other rock she had seen in the region.

Young sent samples to Robert McKay, a geologist at the Iowa Department of Natural Resources. McKay then pulled from his files “churn gravel” from dozens of other wells drilled in a four-township area centered on Decorah.

He found the same shale in some of the other samples, too. When plotted on a map, the shale-rich borings described a “nice circular basin” about 31 / 2-miles wide, neatly bisected by the Upper Iowa River and almost completely encompassing Decorah, he said.

McKay, French and other colleagues are preparing a scientific paper describing the discovery.

McKay and colleagues dubbed the black rock Winnesheik Shale and published a scientific paper describing it in 2006. But only after French recently identified the shock quartz — which was pulled from beneath the shale — did a giant impact seem more certain.

“They found what would be expected from an impact,” said Michael Velbel, a Michigan State University geologist, on sabbatical at the Smithsonian, who attended French’s talk last week. “It’s clear the shale fills a crater.”

Fossils in the shale — including eel-like conodonts, worms called verimforms, and shrimp-like creatures called eurypterids — date the crater to about 470 million years ago, McKay said. This geologic period, known as the Middle Ordovician, was marked by an explosion of early life in the oceans.

This period was also marred by an apparent uptick in the number of asteroid impacts on Earth. About a dozen of the planet’s known impact craters hail from that time.

In 2004, astronomers including Birger Schmitz of the University of Lund, Sweden, proposed a shocking explanation: A massive collision in the asteroid belt beyond Mars about 469 million years ago bombarded Earth with asteroid fragments. Supporting this idea: About 20 percent of all meteorites on Earth — known as “L-chondrites” — date from this period. They appear so similar as to have broken off from the same parent body.

The Decorah crater may have been formed by one such fragment, French said. Even more intriguingly, this newfound crater lies on a line between two other impacts of roughly the same age: the Rock Elm crater in Wisconsin and the Ames crater in Oklahoma.

A single giant asteroid may have flown in from the south, shattered and left a pockmarked trail of smoldering craters strung along about 500 miles.

French labeled the possibility “stimulating speculation.”

But because dating techniques render it impossible to narrow down the age of the three craters to a single century — let alone a single day — that notion probably will remain in the realm of speculation.

“It’s certainly possible,” McKay said. “But trying to prove all three came down at the same time would be tough. I’m not exactly sure how we could do that.”