Scientists say they've settled a centuries-long debate about how a lake located in southern Sweden was formed, calling the rounded shoreline of Hummeln Lake the clear hallmark of an impact crater courtesy of a space rock.
The fairly regular rounded shape of the main body of the lake has fascinated scientists since the early 19th century, but the possibility of it being an impact crater wasn't considered until the early 1960s, says geologist Carl Alwmark, a geologist at Sweden's Lund University.
Before then, most scientists had believed the 0.7-mile-wide flooded feature was in an extinct volcano.
The key evidence in confirming the impact crater theory came from the discovery at Hummeln of shocked quartz grains that are created on Earth only as the result of the intense geological pressures generated by meteorite impacts, researchers reported in the journal Geology.
The shocked quartz was found in breccia, a kind of rock consisting of small fragments of other rocks bound together by a finer-grained medium.
While breccia is common in many Earth settings, only cosmic impacts can create the shocked quartz sometimes found within it, the researchers report.
Hummeln Lake has long been a popular tourist stop, and Alwmark described stopping there one day and picking up some rocks -- which yielded the smoking gun evidence of a cosmic impact.
"These shocked features are not very common, and we got lucky," he says.
The finding is more evidence of a sustained meteor bombardment during what is known as the Late Ordovician period from around 485 to 440 millions years ago, when 100 times as many meteorites are believed to have fallen on Earth as are seen today, Alwmark says.
Scientists believe the bombardment could have been the result of a catastrophic collision between two giant bodies in the solar system's asteroid belt, creating space debris that showered the Earth.
Around a dozen large impact craters on Earth have been dated to that period, including the Ames Crater in Oklahoma and the unique Lockne-Malingen double crater, also in Sweden.
Researchers say they believe the Hummeln crater was created by a space rock that was probably between 300 feet and 500 feet in diameter, probably just one of a number of large impacts during the Ordovician period.
"There are too many craters at this point for it just to be a coincidence," he says. "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."
