Lonar crater in the Maharashtra province of India was long thought to be of volcanic origin. This was mostly due to its location on the Deccan Traps, one of the largest flood basalts (512,000 km2) on the planet. However, Lonar turned out to be the sole impact crater on earth preserved in basalt. The blanket of basalt that underlies the crater is about 400m thick.
So, what made scientists change their mind about the crater? First of all, the presence of maskelynite, a naturally occurring glass that forms during extremely high-velocity impacts. Also, deformed basalt layers and ejecta surrounding the crater, shatter cones, and shocked breccia inside the crater were indicators for a non-volcanic origin.
There is discussion about the timing of the impact. Traditional thermoluminescence dating has suggested a date between 50,000-35,000 years ago. However recent Argon-argon dating proposes a date of 656,000 years old. The latter seems most reliable, since erosion of the crater also indicates an older age.
The saltwater lake that fills Lonar crater is about 150m deep. In the middle the water has a pH of 10.5 and thus is highly alkaline. Here, we find unusual microorganisms that can only survive in these highly alkaline environments. Near the shores the water is less saline due to the presence of small streams of freshwater. In the dry season large quantities of soda can be retrieved when evaporation reduces the waterlevel. The blue-green color of the lake is caused by blue green algae. In addition, the crater rim is known for its of acacia, teak and tamarinds trees in the otherwise dry and barren landscape.
The crater is surrounded by 12 temples of which a large number lies in ruins and are now inhabited by bats and monkeys.
Image: Courtesy of George Thengummoottil. Satellite image of Lonar crater.
References: http://www.princeton.edu/geosciences/people/maloof/research/lonar.xml http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1661.pdf http://earthobservatory.nasa.gov/IOTD/view.php?id=8654
