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The 2.25 billion-year-old Yarrabubba crater in Western Australia has recently been identified as the world's oldest impact crater. It is said to be 43 miles (69 km) in diameter, yet the photo which accompanies the news item shows a circular structure about 430 metres across, hardly more than quarter of a mile. I realise there has been massive erosion since the crater was formed, but erosion to this extent is hard to believe.

The Manicouagan crater in Quebec has a clearly visible diameter of 62 miles (100 km). which is its official diameter. Its age is 214 million years. Erosion to the same extent as is claimed for the Yarrabbuba crater would give Manicouagan an original diameter of about 10,000 miles (16,000 km)! Clearly this is impossible, so being only about a tenth the age of the Australian example it has eroded far less. But that is not the same as no erosion at all.

Can anyone give an approximate diameter of Manicouagan at the time it was formed? Another peculiarity of the Yarrabbuba crater is that in the photo it appears to be slightly domed rather than sunken. Is this an artefact of the photograph, a sort of optical illusion, or is it really raised?

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    $\begingroup$ What photo? What news item? $\endgroup$ – Jean-Marie Prival Jan 22 at 16:06
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    $\begingroup$ Nobody can reasonably answer the first part of your question as long as you don't tell us what kind of foto or news item you have seen and if it wasn't simply an artist's impression ... $\endgroup$ – user18607 Jan 22 at 16:40
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    $\begingroup$ And you think that a few hundred metre hill survived 2.2 billion years of weathering and erosion, while the rest of the crater has completely eroded ? $\endgroup$ – user18607 Jan 22 at 17:06
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    $\begingroup$ No, it did not. The original crater has eroded. The structure on wikipdeia is only part of an eroded sequence that formed the base of a central uplift. Estimations for the whole crater vary (190 to 300km). earthobservatory.nasa.gov/images/92689/vredefort-crater and Wiki. Erosion: en.wikipedia.org/wiki/Erosion $\endgroup$ – user18607 Jan 23 at 13:30
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    $\begingroup$ "The one at Yarrabubba", whatever that is, is a misinterpretation of either the makers of news item who posted a random image of an eroded rock or by @MichaelWalsby seeing a structure where there is none. $\endgroup$ – user18607 Jan 23 at 13:46
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There is a recent paper(open access) that i assume the news item refers to. That paper does not mention 430m. They use a magnetic anomaly to extrapolate the original crater size from the eroded central uplift. The crater itself is long since eroded.

Therefore, the ~20 km diameter magnetic anomaly has been interpreted to represent the remnant of the deeply buried central uplift of the structure, which is consistent with an original crater diameter of 70 km

and

The present day exposure represents a deep erosional level, as neither impact breccias nor topographic expressions of the over-turned rim or central uplift are preserved.

The samples for age determination were taken from a structure called "Barlangi rock":

The structure is centred on a large exposure of granophyre known locally as Barlangi Rock (Fig. 1; 118˚50′E, 27˚10′S). Barlangi granophyre is a sodic rhyolite[...] that has been interpreted as an impact-generated melt rock[...], radiating dyke-like apophyses of granophyre outcrop as far as 3 km from the centre of the structure

This is not the central uplift itself like from a crustal rebound, but

... has thus been interpreted to have intruded into the Yarrabubba monzogranite along faults rather than forming a flat-lying, crater-filling melt sheet ...

The filling at Barlangi has a complex age range (2.79 to 2.23Gy). 2.229 has been determined out of this as the most likely date of the impact. And of course, since then it has been exposed to weathering and erosion.

So there are two separate things here: the size inferred from magnetism, and the age determination from an intrusive melt.

Wikipedia estimates the Manicouagan structure's original diameter to ~100km. More on that event and it's proposed impact on the environment can be found here (open access). The introduction names an initial diameter of 90km for Manicouagan.

Hope that clarifies. Corrections and annotations welcome.

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  • $\begingroup$ The size of the crater is easily estimated by the size of the bushes on it. My question was about the original size of the Manicouagan crater in Quebec, again easily found on the internet. I f I can find it, anybody can. $\endgroup$ – Michael Walsby Jan 22 at 17:03
  • $\begingroup$ I was under the impression that 62 miles was the diameter of the clearly visible part of the Manicouagan crater. Have I misunderstood it? If so, what is the diameter of the crater shown in the photographs if 62 miles was the original size of the crater? $\endgroup$ – Michael Walsby Jan 22 at 19:04
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    $\begingroup$ I can't give you a well thought through answer if i do not know which photographs you're speaking of. $\endgroup$ – user18607 Jan 22 at 19:07
  • $\begingroup$ There are lots of them on the internet, all of them pretty much the same. Try Wiki. The outline of Manicouagan is really clear, which makes it a favourite example in articles about enormous, ancient impact craters. There must have been some erosion in 214 myr, three times the age of Chicxulub. As it is one tenth of the age of Yarrabubba, could it have undergone one tenth of the erosion? $\endgroup$ – Michael Walsby Jan 22 at 19:18
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    $\begingroup$ Erosion depends on many things, there is no metric. Chicxulub hasn't been exposed to erosion that much, mostly sedimentation. That's why much of it, including ejecta in form of breccia and shocked minerals as well as buried structures are still there, as bore cores show. Maniwhatever has undergone erosion by ice (clearly visible on the pictures), there isn't much left. The lake is only the inner structure i read. A mountain range like the eastern alps can mostly erode in ~60 million years. After 2.2Gy, nothing is left of Yarrabunga. But these are only examples, no rules ! $\endgroup$ – user18607 Jan 22 at 20:01
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When comparing the sizes of the two craters, you are confusing how erosion works.

Erosion did not shrink the original crater to from 43 miles to 430 meters. The crater was 43 miles across and was completely erased: Nothing of it remains, neither the rim nor the central uplift. In fact, erosion has even erased the rocks into which the crater was blasted. This is what erosion over 2 billion years does. The only thing that is left are remnants under the central uplift, and these are 430 meters across.

On the other hand, the Manicouagan is as large as it has always been. Erosion has simply not leveled its outer ring yet. But when it does, it will still have the same diameter. Erosion isn't going to shrink it, it's just going to level it.

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    $\begingroup$ Well, the 430m are an invention of @MichealWalsby derived from a mysterious foto he saw. It is not a structural remain from the impact, interpreted as just some melt that intruded into cracks. Erosion shrinks a crater if it erodes it upside down, like a section through a bowl from rim to bottom. The rim of the Manicouagan(M) is eroded and its original size (~90km) inferred from the partly eroded central uplift and the surrounding ring of softer material excavated by ice, that is today filled by an aritficial lake. $\endgroup$ – user18607 Jan 24 at 11:09

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