Tuesday, February 8, 2011

Remotely India # 5: Lonar Crater

Move over Chicxulub Mexico of dinosaur extinction fame, move over Meteor Crater Arizona.. I've got my own meteor crater right here in my backyard.

Okay... okay.. this crater didn't transform the planet, didn't disrupt ecosystems or cause mass extinctions.. but there it stands, a beautiful round hole punched in the earth.. testimony to the fragility and exposure of our planet to bombardments from space.

Lonar Crater is a small crater formed by the impact of a meteorite. It forms a 1.88 km diameter depression in the Deccan Basalts in eastern Maharashtra near the town of Aurangabad about 8-9 hours drive from Pune. There is a relief of about 135 meters from the rim to the surface of the lake. The image below is a rendering of the relief of the region produced by draping a satellite image over a digital elevation model.



Source: Maloof et al 2009

For years there was some uncertainty on whether this depression represented a collapsed volcanic cone or whether it is of impact origin. Meteorite fragments have not been found at Lonar. But there always were doubts about the volcanic origin since geomorphic features of the crater such as its excellent preservation, a very thin sediment cover in the crater lake and immature river incision features surrounding the crater suggested a much younger origin than the ancient Deccan Basalts.

Now detailed studies of the deformation of basalt flows in the crater walls and evidence of impact shock from the presence of maskelynite a glassy phase formed by shock conversion of plagioclase feldspar has put that debate to rest. Several types of glassy spherules show that the crater formed due to a meteorite impact.  Below is a geological map of the lonar crater which shows the distribution of the ejecta blanket. Series Q are Quaternary features: Qb - beach, Qe - ejecta, Qf - forest, Qh - histosol, Qi - irrigated alluvial fan, and Qt - talus. Series T represent basalt flow in the crater walls. Coordinate are in Universal Transverse Mercator Projection.



Source: Maloof et al 2009

Dating the crater using various methods had previously suggested an age ranging from 12 thousand years to 65 thousand years. A recent study by Maloof et al 2009 carbon dated soils preserved underneath the ejecta blankets. The dates indicate a very young maximum age of about 12 thousand years for the impact. On the other hand another recent study using Argon-Argon dating method on impact melt rock has suggested an age of about six hundred and fifty thousand years. The consensus though seems to be leaning towards a much younger age for the crater.

The crater has  received a lot of attention from geologists. Its importance lies in its very recent origin. High erosion rates on earth wipe away bits and pieces of any feature, many times making a detailed reconstruction of events impossible. In Lonar crater though, impact features are well preserved and have proved useful in understanding cratering mechanisms. Its location within a basaltic terrain makes it a good analogue for small impact craters on the moon and other rocky planets. 

Ecologically too the crater is a unique place. The lake has high salinity levels and hosts a variety of extremophile bacteria.


The Geological Survey of India protects the crater and its rim but not all of the ejecta blanket. Urbanization and agriculture are slowly destroying it.

Digital Elevation Model of Lonar Crater derived from GPS measurments:



Source: Maloof et al 2009

6 comments:

  1. yeah.. it is a great one day field trip, some good deformation features in the basalt flows along the rim and crater wall and a hypersaline lake at the bottom..

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  2. This is an exciting discovery. First time I am reading about a crater in India.

    As per the age, we know our ancestors came to India from Africa. I am curious how the time of humans arrival lines with (rather) young age (12K to 65K years) of crater.

    Apart from that there is so much about Geology of India in your blog. It is exciting to know where we fit in the history of Earth.

    Thanks for writing
    Ramesh

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  3. Ramesh- there have been several migrations from Africa into India. The oldest were around 1.7 million years ago when Homo erectus migrated out of Africa. Our species Homo sapiens migrated into India around 50 -70 thousand years ago going by the types of stone tools. We don't have skeletal material to confirm that but the best guess is 50k to 70k.. so early humans might even have witnessed the meteorite crashing at Lonar!! :)

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  4. Is it meteroite crater or volcanic crater? Is there any data available?

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  5. Udayk.. it is meteoritic in origin.. i have mentioned the evidence in the post..

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