After two centuries of arguing about its origin, scientists have finally confirmed that Hummeln Lake in southern Sweden is an impact crater.
Hummeln Lake's rounded shoreline first drew interest from scientists as far back as the 1820s, but it wasn't identified as a possible impact crater until the 1960s, said Carl Alwmark, lead author of the new study and a geologist at Lund University in Sweden. Until then, geoscientists thought the circular structure, which is 0.7 miles (1.2 kilometers) wide and 525 feet (160 meters) deep, was an extinct volcano. Now researchers think the crater resulted from a space rock that was likely about 325 feet to 490 feet (100 to 150 m) in diameter, Alwmark said.
Alwmark and his colleagues recently found the telltale clues that confirm an impact carved out the Hummeln crater. Their findings were published Feb. 18 in the journal Geology. [Crash! 10 Biggest Impact Craters on Earth]
The key evidence includes shocked quartz from a layer of breccia at the lake. A breccia is a type of rock made up of angular fragments of other rocks held together by a finer-grained medium, similar to natural cement. Breccia forms in many settings on Earth, but the shocked features in the quartz minerals are created only under the intense pressures caused by meteorite impacts.
Others have searched before for similar features, but Alwmark hit the jackpot while working at the nearby Siljan crater, one of the largest on Earth. Hummeln Lake is a popular tourist stop, with rental cabins surrounded by silver birch trees, and Alwmark said he popped in and picked up some rocks on the drive between craters.
"These shocked features are not very common, and we got lucky," he told Live Science.
The crash site adds to the growing body of evidence that meteorites bombarded Earth during the Late Ordovician Period, Alwmark said. Scientists think that a wave of space debris slammed into the Earth after a huge smashup between two large bodies out in the asteroid belt some 470 million years ago. (One of the crash victims was the source of all L-chondrite meteorites.)
Researchers think about 100 times as many meteorites fell on Earth during the Ordovician compared with today. However, although many small meteorites and micrometeorites dated to the Late Ordovician have been found, scientists have only discovered about a dozen large craters. These include the unusual Lockne-Malingen double crater in northern Sweden and the Ames Crater in Oklahoma. The growing list of craters supports models that suggest larger rocks also pummeled the planet.
"There are too many craters at this point for it just to be a coincidence," Alwmark said. "If we start finding even more of these larger craters, then you should start speculating about whether this [bombardment] could have had a profound impact on the evolution of Earth's biology," he said.
Shallow seas covered much of the planet in the Late Ordovician, and the seafloor sediments that buried these craters protected the formations for millennia. Fossils from fish that patrolled the seas have helped researchers precisely date the ancient structures.