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Baiju Mathew
Feb 168 min read

Lonar: The Meteor Impact Crater

reading time 11 minutes, 2,200 words



Pic from personal collection


In the first week of February 2024, my group of four friends, avid road trippers, embarked on a journey to Lonar. It is a small town in the Buldhana district of Maharashta, about 460 km from Mumbai on the road to Nagpur. The drive on the recently built Mumbai-Nagpur highway was filled with excitement, as we engaged in lively banter and enjoyed our favourite tunes playing in the background.. In the midst of our revelry, we didn’t notice that the fuel tank indicator was in red. By the time we noticed it, the dashboard indicator showed balance fuel only for 15 km while, GPS showed the nearest fuel station to be 17 km away. The nail biting situation was reminiscent of the suspense in a cricket match where our team needs to score 15 runs in the balance 10 balls. Fortunately, we made it to the fuel station about 4 km after the dashboard indicator had shown fuel tank to be fully empty. I suppose Toyota cars have built in idiot-proof calibration of fuel indicators for such situations.



At Lonar, we met Amol Sardar, our guide. Amol, native of Lonar is very knowledgeable. He has a wealth of information, thanks to his experience in assisting various scientific studies conducted on the crater. For the interested readers, a YouTube link featuring Amol guiding tourists to Lonar is provided at the end of this post.


Crater features:


a.      Physical


Because Lonar crater is located in the Deccan Plateau, it was initially assumed to be volcanic. Later, it was discovered to be an impact crater formed by a meteor strike. Shortly, we will delve into why its location in the Deccan Plateau suggested a volcanic origin. With an oval shape and a circumference of about 5 km, the crater is shallow on the eastern end and deeper towards the west. The entire crater is divided into 5 zones, as listed below from outer region to the inner region:


1. Ejecta Rim : Region where debris settled after the meteor impact.

2. Crater Rim : The ridge peaks of slopes originating from the crater floor. 

3. Slopes : Extending from the crater floor to the rim.

4. Crater basin : Excluding the lake.

5. Crater lake : The water body.




b.      Chemical:

 

The crater lake is both, saline and highly alkaline. The water has a pH value of about 11-13. The pH scale measures the acidity or alkalinity of a substance. A pH of 7 is neutral, values below 7 are acidic, whereas those above 7 are alkaline. This scale is logarithmic, implying a tenfold increase in the acidity or alkalinity for every difference of 1 in pH value. To put into perspective, hand-wash has a pH of about 9 and caustic soda used for clearing clogged drains has a pH of about 13. I have not been able to discover the reason for the salinity and the alkalinity; we can assume this to be due to the minerals that made up the meteor. At least that is the laziest assumption in the absence of firm knowledge.

 

It is worth noting that wikipedia mentions that the lake has two zones of pH. A pH of 7 towards the outer zone and of about 13 towards inside. However, during our visit, we measured the pH, at the shore, to be around 11. The following video shows us testing the pH.



c.      Biological:

 

The only biological presence in that water are of some microbes and algae including the Blue-green Algae. These algae, known for nitrogen fixing, form such a dense cover that it prevents sunlight from reaching the lakebed. Other than such algae and microbes, there are no insects or any other biological life form found in the lake. However, we witnessed many birds. Amol explained that these are migratory birds visiting the lake in winter specially to feast on the Blue-green algae.  In the evening, we witnessed the mesmerising phenomenon of murmuration, where a large group of birds, Rosy Starling in this case, move in a choreographed manner forming various shapes, as shown in figure 3 below:



The meteor impact and formation of the crater:


It is conjectured that the meteor struck at an angle of 35-40 degree. The crater’s eastern wall is shallow, while the western wall is much higher, indicating impact’s direction. Figure 5 below gives a schematic diagram of the crater:



It is estimated that the meteor hurtled at a speed of roughly 18 km/s i.e., 64,800 km/hr, and had a diameter of about 100 meters, with a mass of about 2 million tonnes. For comparison, the highest velocity achieved by humankind is that of the Apollo 10/11 missions to the moon, which had a velocity of about 40,000 km/hr. Thus, the meteor struck Earth at a velocity that was 1.62 times the velocity of Apollo 10/11.


The parameters like velocity, mass and dimensions are estimated by examining the features of the crater, including shape, size and material redistribution. This data is then correlated with information from other sites and model simulations. The impact energy is estimated to be equivalent to a 6-megaton bomb. To provide a context, ‘Little Boy’, the nuclear bomb dropped on Hiroshima, had an estimated yield of 15 kilotons. Therefore, the Lonar impact was 400 times larger than the explosion of ‘Little Boy’.


The Deccan Plateau, on which Lonar is located, is in peninsular India, beneath the Narmada River. This plateau forms the largest area of the southern portion of the peninsula, covering the states of Maharashtra, Karnataka, Andhra, Telangana and parts of Madhya Pradesh and Tamil Nadu The plateau, which commences from the high altitudes of the Western Ghats, slopes gently towards east. It was formed by volcanic activity some 65 million years ago. While there might have been large volcanic eruptions, the predominant volcanic activity is supposed to be effusive, with lava oozing out. The Deccan Plateau is composed of basalt since it is formed by lava. This crater being in basalt is considered a distinctive feature, one of the very few of its kind on earth. The rocks found here are similar to the lunar rocks we brought back from the moon. It should be noted that the moon has large regions made of basalt.


The very high temperature and the pressure, resulting from the impact, caused the basalt to melt and on cooling, form a glass like material known as Maskelynite. This material is formed only under very high temperature and pressure produced only by such impacts. This is a clinching evidence that this crater was formed by a meteor strike, rather than by a volcano. Our guide, Amol, showed us a piece of Maskelynite from his collection. The structure of the rocks and the fractures further support the conclusion that it is an impact crater.


Another intriguing feature is the presence of rocks having a magnetic orientation. When we placed a magnetic compass on these rocks and moved the compass around, its needle kept moving, indicating a magnetic field emanating from these rocks. Furthermore, in certain areas, the ground dirt has a high proportion of magnetic substance that can be picked up with a piece of magnet. Although literature on why some stones here are magnetic is limited, we can speculate that the sudden melting of the rocks on impact, followed by a fast cooling would facilitate the alignment of some of the magnetic material present in the rocks along Earth’s magnetic field. This will result in the rocks having a magnetic orientation post cooling. It may be noted that in the normal course, the lava or magma would cool over an elongated period, during which the magnetic material are all randomly oriented, and therefore, does not show any overall magnetic orientation.


Dating the Crater:


Readers would be wondering why I have not mentioned the age of this crater yet. I wanted to keep it to the end. There are various estimates for the age of this crater depending on the method used. I have come across three dates predominantly. Let me explain them from the youngest to the oldest


  1. 15,000 – 30,000 years ago: The effects of erosion on the crater is mild, indicating that this is a young formation. Further, radio carbon dating of organic material from the site at various depths adds credence to such a date range.

  2. 52,000 +/- 6000 years ago:  This estimate is arrived at by a method called as Thermoluminescence (TL). Certain rocks and crystals trap electrons when exposed to radiation, i.e. from the natural radiations that occurs on earth. When such material is heated to a sufficiently high temperature, the trapped electrons escape. To find the age of the rock, we have to heat it to the required temperature where all trapped electrons are released, the amount of electron released is measured, and using this the rock is dated. This method can be used at Lonar since all trapped electrons in the rocks would have escaped on the meteor strike when the rocks melted.  

  3. 560,000 +/- 47,000 years ago: Some sources indicate this date range using Cosmic Ray Exposure (CRE) dating. Other sources claim that this date range is from Argon-Argon (Ar-Ar) dating. I have not been able to gather indisputable information of which is the right version. The principle behind Ar-Ar dating is that rocks have a deposit of radioactive potassium, which over a period decays into argon. By measuring the portion of this isotope of argon, we can estimate the age of the rock. CRE is based on measuring the effects of cosmic rays on the rocks over time. Some cosmic rays from outer space reach the surface of the Earth, producing what is called as cosmogenic radionuclides in materials like, like beryllium, aluminium and chlorine, present in the rocks. The rocks can be dated by measuring the proportion of such cosmic ray induced changes. 

 

Thus, in conclusion we note that there are different methods of dating such craters. In the case of Lonar, different methods give different results and hence the precise age of the crater remains uncertain.



Mythologies and Temples around the Lonar crater:


I believe that every mythology or folklore has a piece of history woven into it. Such mythologies add a cultural and historical narrative to a place, helping us understand the people of that place better. If the meteor strike at Lonar had happened at the time when there were humans in the Indian peninsula, this impact would have been recorded in the folklore. Certainly, any life around the vicinity of the impact would have been destroyed, but we can envision a community that lived at a safe distance from Lonar. They would have witnessed the effects of the impact, and it might have, in some form, become a part of their folklore. I am not aware if any such folklore exists. Perhaps it is there and is known to some anthropologists, but I have not discovered them yet. Or perhaps the crater was formed much before human settlement reached this region. There is a mythological story of origin for the name of this place. According to local lore, this is the spot where Lord Vishnu killed the demon Lavanasur (also called Lonasur) in Lord Vishnu’s avatar as an infant. Interestingly, there is also a story that Lonasur was killed by Shatrughana, the brother of Lord Rama.




There are lots of temples and mythologies in the area. The figure 6 above show some of them, within the crater region itself. Most of these temples are of the 10th Century C.E. or later. I gather that Lonar finds mention in the Skanda Purana and the Padma Purana. Nevertheless, these all are much later to the youngest estimated date of the crater. I may consider putting down the description of these temples and mythologies around it at a later date.




In conclusion, I urge the readers to make a trip to this magical place if they haven’t done it already. This confluence of science and mythology coupled with the stunning beauty offers a charming and exhilarating experience.


I express my gratitude to my friends, Subash, Santosh and Lokesh, who travelled with me and to our guide, Amol, for allowing me to use the photograph and helping me out with some of the details that I had missed.

 

Reference:

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