Lonar Remembers: How Scientists Read the Story of a Meteorite in Stone
Stand on the rim of Lonar and the first thing you see is the crater.
Walk down into it, and the landscape begins to tell a different story.
Walk down into it, and the landscape begins to tell a different story.
1
Remembers Past
The forest closes around you. Dark basalt rises along the crater walls. The lake appears through the trees.
Ancient temples sit quietly within the landscape.
But there is another story here — one that cannot be seen with the naked eye.
It is written inside the rock.
Scientists studying Lonar have discovered that some of the basalt around the crater still preserves a magnetic record of the ancient Earth, the meteorite impact that created the crater, and the thousands of years that followed.
In a sense, Lonar remembers.
Ancient temples sit quietly within the landscape.
But there is another story here — one that cannot be seen with the naked eye.
It is written inside the rock.
Scientists studying Lonar have discovered that some of the basalt around the crater still preserves a magnetic record of the ancient Earth, the meteorite impact that created the crater, and the thousands of years that followed.
In a sense, Lonar remembers.
2
A memory older than the crater
The rock beneath Lonar is far older than Lonar itself.
The crater formed in the Deccan Traps, the vast volcanic province created by enormous lava flows around 65 million years ago.
As those lava flows cooled, tiny magnetic minerals within the basalt aligned with Earth's magnetic field.
Some of that alignment became locked into the rock.
Scientists call this remanent magnetization.
For a traveller, the simplest way to imagine it is this:
the basalt carries a tiny magnetic memory of the Earth that existed when it cooled.
That was roughly 65 million years ago.
The meteorite that created Lonar arrived much later.
So when scientists examined the rocks around the crater, they were looking at something extraordinary:
rock that had already recorded one ancient chapter of Earth's history before another event dramatically changed the landscape.
The crater formed in the Deccan Traps, the vast volcanic province created by enormous lava flows around 65 million years ago.
As those lava flows cooled, tiny magnetic minerals within the basalt aligned with Earth's magnetic field.
Some of that alignment became locked into the rock.
Scientists call this remanent magnetization.
For a traveller, the simplest way to imagine it is this:
the basalt carries a tiny magnetic memory of the Earth that existed when it cooled.
That was roughly 65 million years ago.
The meteorite that created Lonar arrived much later.
So when scientists examined the rocks around the crater, they were looking at something extraordinary:
rock that had already recorded one ancient chapter of Earth's history before another event dramatically changed the landscape.
3
Then came the meteorite
Around 50,000 years ago, a meteorite struck this ancient basalt at tremendous speed.
The collision was over almost before the landscape could respond.
A shock wave travelled through the rock.
The ground was compressed and fractured.
Basalt layers were pushed upward, folded and overturned.
Huge amounts of material were excavated and thrown outward.
Some of the molten material cooled into tiny glassy droplets.
And the crater took shape.
Today, Lonar is about 1.88 kilometres across, but its present form is not simply the hole made in the first instant of impact. The crater walls continued to move and adjust after the collision, leaving behind folds, overturned layers and other remarkable evidence of the event.
The landscape itself became a record of what happened.
But scientists wanted to know even more.
Could the rocks tell them how they had moved?
The collision was over almost before the landscape could respond.
A shock wave travelled through the rock.
The ground was compressed and fractured.
Basalt layers were pushed upward, folded and overturned.
Huge amounts of material were excavated and thrown outward.
Some of the molten material cooled into tiny glassy droplets.
And the crater took shape.
Today, Lonar is about 1.88 kilometres across, but its present form is not simply the hole made in the first instant of impact. The crater walls continued to move and adjust after the collision, leaving behind folds, overturned layers and other remarkable evidence of the event.
The landscape itself became a record of what happened.
But scientists wanted to know even more.
Could the rocks tell them how they had moved?
When a rock becomes a compass
This is where paleomagnetism becomes fascinating.
Imagine that a tiny compass is permanently embedded inside a rock.
Now imagine that someone folds or rotates the rock.
The compass moves with the rock.
If you know the direction that the compass originally recorded, you can work backwards and reconstruct how the rock was moved.
That is broadly what scientists were able to do at Lonar.
They sampled rocks that had been folded and overturned by the impact and compared their magnetic directions. They also studied fragments within the ejecta using what is known as a conglomerate test.
Why?
Because the timing matters.
Scientists need to know whether the magnetic signal was acquired before the impact, during the impact or afterwards.
At Lonar, paleomagnetic tests helped establish that much of the magnetic memory existed before the crater-forming event.
The rock was therefore doing more than proving that an impact had occurred.
It was helping scientists reconstruct the movements caused by that impact.
Imagine that a tiny compass is permanently embedded inside a rock.
Now imagine that someone folds or rotates the rock.
The compass moves with the rock.
If you know the direction that the compass originally recorded, you can work backwards and reconstruct how the rock was moved.
That is broadly what scientists were able to do at Lonar.
They sampled rocks that had been folded and overturned by the impact and compared their magnetic directions. They also studied fragments within the ejecta using what is known as a conglomerate test.
Why?
Because the timing matters.
Scientists need to know whether the magnetic signal was acquired before the impact, during the impact or afterwards.
At Lonar, paleomagnetic tests helped establish that much of the magnetic memory existed before the crater-forming event.
The rock was therefore doing more than proving that an impact had occurred.
It was helping scientists reconstruct the movements caused by that impact.
Rebuilding the impact
The magnetic evidence was only one part of the investigation.
Scientists also mapped the crater in detail and used elevation data to understand its shape. They found upturned basalt flows, large folded sections of the crater rim and other signs of enormous ground movement.
Then they built computer models of the impact.
The models attempted to recreate what happens when a meteorite strikes basalt at enormous speed.
When the simulations produced features similar to those observed at Lonar — including an upturned crater wall, overturned folds and a deeply fractured zone beneath the crater — the pieces began to fit together.
The scientists were no longer simply looking at a crater and imagining what might have happened.
They were comparing several independent clues:
the shape of the crater,
the position of the rock layers,
the magnetic directions in the rocks,
the ejecta surrounding the crater,
and computer simulations of the collision.
Together, they told a remarkably consistent story.
Scientists also mapped the crater in detail and used elevation data to understand its shape. They found upturned basalt flows, large folded sections of the crater rim and other signs of enormous ground movement.
Then they built computer models of the impact.
The models attempted to recreate what happens when a meteorite strikes basalt at enormous speed.
When the simulations produced features similar to those observed at Lonar — including an upturned crater wall, overturned folds and a deeply fractured zone beneath the crater — the pieces began to fit together.
The scientists were no longer simply looking at a crater and imagining what might have happened.
They were comparing several independent clues:
the shape of the crater,
the position of the rock layers,
the magnetic directions in the rocks,
the ejecta surrounding the crater,
and computer simulations of the collision.
Together, they told a remarkably consistent story.
The impact did not erase the memory
Perhaps the most surprising discovery was what the meteorite didn't do.
You might expect an impact powerful enough to create a crater almost two kilometres wide to completely destroy the magnetic information stored in the basalt.
It didn't.
The ancient magnetic signal survived.
Researchers found only relatively subtle changes in the magnetic properties of the rocks they studied, with no evidence of wholesale shock demagnetization.
That survival gave scientists another clue.
Paleomagnetism could help them estimate how much the rock had actually been heated during the impact.
The result was surprisingly low: the relevant material was estimated to have experienced shock heating of less than approximately 187 ± 15°C.
The impact was violent enough to move enormous volumes of rock.
But in the material they examined, it did not simply erase the magnetic memory.
The rock kept changing after the impact
The story did not end when the crater formed.
Over the roughly 50,000 years that followed, the basalt gradually acquired another magnetic signal.
Some of this is called viscous remanent magnetization — a slow adjustment of magnetic minerals towards the direction of Earth's present magnetic field.
Chemical changes in the rock may also have created new magnetic signals.
So the basalt around Lonar contains several layers of history:
an ancient magnetic record from the cooling of the Deccan basalt,
the physical disturbance caused by the meteorite impact,
and a much younger magnetic signal acquired after the crater formed.
It is almost like several generations of handwriting on the same page.
The job of the paleomagnetist is to work out which line belongs to which chapter.
Did the impact create a giant magnetic field?
Then came an even stranger question.
Could the meteorite itself have created a huge magnetic field when it struck?
Some theoretical models suggested that the enormous amount of energy and electrically charged material produced during an impact might generate a very strong, short-lived magnetic field — potentially around 0.1 tesla under certain assumptions. That would be more than a thousand times stronger than Earth's present surface magnetic field.
If that happened at Lonar, perhaps the impact itself had left a magnetic signature.
There was only one way to find out.
Look at the glass.The tiny pieces that flew through the sky
When the meteorite struck, some basalt melted.
Tiny droplets of that molten rock were thrown into the atmosphere and cooled as they travelled through the air. They eventually landed around the crater as impact-glass spherules.
These tiny pieces offered scientists an unusual opportunity.
They had melted during the impact.
They then cooled after being thrown through the air.
If a powerful magnetic field had existed during that process, the cooling glass might have recorded it.
Researchers from MIT, Harvard and Princeton collected thousands of these basaltic glass samples around Lonar.
And they discovered something unexpected.
Some of the smallest, splash-shaped pieces had cooled while they were still flying through the air. Their magnetic minerals were acquiring magnetization as the particles moved, producing a distinctive behaviour the researchers identified as a form of motional thermoremanent magnetization.
The glass had captured more than the fact that it had once been molten.
It had preserved clues about how it was moving as it cooled.
And they didn't find the giant magnetic field
After all this searching, the answer was surprisingly quiet.
The researchers found natural magnetic signals in the impact glass.
But they found no evidence of the enormous transient magnetic field predicted by the strongest theoretical models.
Their results placed an upper limit of roughly 100 microtesla on a strong impact-generated field in the material they studied.
That negative result was important.
It showed that an apparently convincing magnetic signal does not automatically mean an impact generated a giant magnetic field.
Sometimes science advances because an experiment tells us what didn't happen.
Why Lonar matters to planetary science
This is why scientists at institutions such as Princeton and MIT became interested in a crater in Maharashtra.
Lonar is unusually useful because it is a relatively young, exceptionally well-preserved impact crater formed in basalt — the same broad type of volcanic rock found on other planetary bodies.
That makes Lonar a kind of natural field laboratory.
Here on Earth, scientists can walk around the crater, examine the folded basalt, study the ejecta, collect impact glass and measure the magnetic record.
Those observations help scientists understand what may have happened at impact craters elsewhere in the Solar System, including on the Moon and Mars.
A crater on another world may be millions or billions of years old.
Lonar lets us study a much younger one.
And that makes it extraordinarily valuable.
You can visit Lonar simply to see the lake.
You can come for the forest, the birds and the temples.
You can come because the crater itself is extraordinary.
But there is another way to experience it.
Look at the basalt.
The stone around you was once molten lava.
It recorded Earth's magnetic field when it cooled roughly 65 million years ago.
Then, tens of millions of years later, a meteorite arrived and violently rearranged that landscape.
The rocks were folded.
The crater was excavated.
Material flew through the sky.
Glass droplets cooled in mid-air.
And yet the ancient magnetic memory survived.
Even after all those events, the rocks continued changing as Earth's magnetic field and chemical processes slowly added new signals.
The crater you see today is therefore more than a scar in the landscape.
It is a record.
The shape remembers the impact.
The folded rock remembers the movement.
The ejecta remembers the explosion.
The glass remembers the journey through the air.
And the magnetic minerals remember the Earth that existed long before Lonar did.
Long before anyone stood on this rim and looked into the crater, the rocks were already keeping the story.
Lonar remembers.
And science is learning how to read it.
You might expect an impact powerful enough to create a crater almost two kilometres wide to completely destroy the magnetic information stored in the basalt.
It didn't.
The ancient magnetic signal survived.
Researchers found only relatively subtle changes in the magnetic properties of the rocks they studied, with no evidence of wholesale shock demagnetization.
That survival gave scientists another clue.
Paleomagnetism could help them estimate how much the rock had actually been heated during the impact.
The result was surprisingly low: the relevant material was estimated to have experienced shock heating of less than approximately 187 ± 15°C.
The impact was violent enough to move enormous volumes of rock.
But in the material they examined, it did not simply erase the magnetic memory.
The rock kept changing after the impact
The story did not end when the crater formed.
Over the roughly 50,000 years that followed, the basalt gradually acquired another magnetic signal.
Some of this is called viscous remanent magnetization — a slow adjustment of magnetic minerals towards the direction of Earth's present magnetic field.
Chemical changes in the rock may also have created new magnetic signals.
So the basalt around Lonar contains several layers of history:
an ancient magnetic record from the cooling of the Deccan basalt,
the physical disturbance caused by the meteorite impact,
and a much younger magnetic signal acquired after the crater formed.
It is almost like several generations of handwriting on the same page.
The job of the paleomagnetist is to work out which line belongs to which chapter.
Did the impact create a giant magnetic field?
Then came an even stranger question.
Could the meteorite itself have created a huge magnetic field when it struck?
Some theoretical models suggested that the enormous amount of energy and electrically charged material produced during an impact might generate a very strong, short-lived magnetic field — potentially around 0.1 tesla under certain assumptions. That would be more than a thousand times stronger than Earth's present surface magnetic field.
If that happened at Lonar, perhaps the impact itself had left a magnetic signature.
There was only one way to find out.
Look at the glass.The tiny pieces that flew through the sky
When the meteorite struck, some basalt melted.
Tiny droplets of that molten rock were thrown into the atmosphere and cooled as they travelled through the air. They eventually landed around the crater as impact-glass spherules.
These tiny pieces offered scientists an unusual opportunity.
They had melted during the impact.
They then cooled after being thrown through the air.
If a powerful magnetic field had existed during that process, the cooling glass might have recorded it.
Researchers from MIT, Harvard and Princeton collected thousands of these basaltic glass samples around Lonar.
And they discovered something unexpected.
Some of the smallest, splash-shaped pieces had cooled while they were still flying through the air. Their magnetic minerals were acquiring magnetization as the particles moved, producing a distinctive behaviour the researchers identified as a form of motional thermoremanent magnetization.
The glass had captured more than the fact that it had once been molten.
It had preserved clues about how it was moving as it cooled.
And they didn't find the giant magnetic field
After all this searching, the answer was surprisingly quiet.
The researchers found natural magnetic signals in the impact glass.
But they found no evidence of the enormous transient magnetic field predicted by the strongest theoretical models.
Their results placed an upper limit of roughly 100 microtesla on a strong impact-generated field in the material they studied.
That negative result was important.
It showed that an apparently convincing magnetic signal does not automatically mean an impact generated a giant magnetic field.
Sometimes science advances because an experiment tells us what didn't happen.
Why Lonar matters to planetary science
This is why scientists at institutions such as Princeton and MIT became interested in a crater in Maharashtra.
Lonar is unusually useful because it is a relatively young, exceptionally well-preserved impact crater formed in basalt — the same broad type of volcanic rock found on other planetary bodies.
That makes Lonar a kind of natural field laboratory.
Here on Earth, scientists can walk around the crater, examine the folded basalt, study the ejecta, collect impact glass and measure the magnetic record.
Those observations help scientists understand what may have happened at impact craters elsewhere in the Solar System, including on the Moon and Mars.
A crater on another world may be millions or billions of years old.
Lonar lets us study a much younger one.
And that makes it extraordinarily valuable.
You can visit Lonar simply to see the lake.
You can come for the forest, the birds and the temples.
You can come because the crater itself is extraordinary.
But there is another way to experience it.
Look at the basalt.
The stone around you was once molten lava.
It recorded Earth's magnetic field when it cooled roughly 65 million years ago.
Then, tens of millions of years later, a meteorite arrived and violently rearranged that landscape.
The rocks were folded.
The crater was excavated.
Material flew through the sky.
Glass droplets cooled in mid-air.
And yet the ancient magnetic memory survived.
Even after all those events, the rocks continued changing as Earth's magnetic field and chemical processes slowly added new signals.
The crater you see today is therefore more than a scar in the landscape.
It is a record.
The shape remembers the impact.
The folded rock remembers the movement.
The ejecta remembers the explosion.
The glass remembers the journey through the air.
And the magnetic minerals remember the Earth that existed long before Lonar did.
Long before anyone stood on this rim and looked into the crater, the rocks were already keeping the story.
Lonar remembers.
And science is learning how to read it.
