Asteroid impact crater found in Ireland?

As I was preparing some calibrated maps for OpenEir recently, it suddenly occured to me that  the surrounding hills of west Limerick, which I've been viewing all my life from the ground, have an extraordinarily circular formation when viewed from space!

This image from MapProxy demonstrates what I saw. To the north is the Shannon Estuary and the runways of Shannon Airport can clearly be seen on the Clare side. The smudge to the top right is Limerick city (hey, its been called alot worse!).

Towards the center of the image are the hills of Knocaderry, Ballingarry and Knockfierna (which is a known ancient volcano). Might these not be the remnants of the central ring of a complex crater as explained here? The circular outer rim is clearly visible to the left of the image on a segment about 12 or more miles in length. If you're viewing on a 1024 x 768 pixels screen hold a CD up to the image and see how the crest of those hills follow perfectly along the perimeter!

But what about the right hand side and the rest of the outer rim? Where is that gone? Well according to this map the glaciers wore it down during the last ice age. However, after processing the Mapproxy satellite map in GIMP using a simple NEON Edge-Detect filter we were left with this view which appears to show traces of the eastern crater wall. Convinced? Or is this one for the Fortean Times ;-)

PS. With the greatest of modesty I've decided to call this crater after my good self. Behold Corbett's Crater.

Comments

[Tony Mulqueen]

Well you have me convinced anyway. As a native of Limerick (and hillwalker) I've always had a subliminal sense of the county topology as bowl-like. And I've always been struck at the dramatic change at the hills of Lough Gur, on the eastern rim of Corbett's Crater.
Just one niggle - has any evidence of the rock types produced by asteroid impact / ultra hot fusion been found in the field?
Keep up the good work ...
/tonym

[Martin]

Hello to green Ireland,

I see what you mean. Personally, I would rather call it "Newcastle structure" as it is some kind of "geoscientific law" not to name geologic structures after yourself (but someone else could do that for you).

There are indeed several geological structures that resemble real impact structures. However, they have to show some clear and evident features that make them plausible. One good example seems to be the "Richat Ring" in Mauritania, which really looks like an impact structure. As far as we know today, it is a simple anticlinic structure laid bare by erosion (I would like to prove that, however).

According to the simplified Geological Map of Ireland it rather seems to be a usual stratigraphic basin succession of Carboniferous rocks with some Devonian remnants in the plain towards the East. This might be similar to the basins of Paris/France or Muenster/Germany, just to mention two examples. Anyway, the Nördlinger Ries crater looks quite similar as regards its topography, but completely different as to its rock distribution. But as the Irish rocks are quite old, the infill of an "alleged" impact crater could have been deeply eroded.

That´s the first point at which we meet here: It is, of course, possible to have an impact crater in Ireland - why not ??!. There are many craters confirmed even in the small country of Estonia. The size of ~25 km in diameter would make a great addition to the European impact inventory. We could lead it back to an impactor of about 1000 m in diameter. In addition, it would be great for Ireland as the UK has only one single impact structure so far (Silverpit) - and that´s under water in the North Sea ;))

Now the second point: facts!!!

Check out for shocked quartz grains in (polished) thin section. As they usually do not show any natural cleavage, you should observe straight lines that run through some Qz grains. Usually shocked quartz is clearly visible when the PDFs ("planar deformation features") are decorated by tiny fluid inclusions. PDFs only form under conditions of shock metamorphism (that only occurs during impact events and nuclear tests). Be aware of not mixing PDFs up with tectonically induced deformation features such as "Boehm lamellae". They can look very similar. Real shocked quartz is the most striking argument in favour of an impact structure (you can ask for a thin section at your next geological institute - maybe they are also interested in that).

Try to find some more arguments. Look for "shatter cones", maybe you can find some near the actual "crater floor". Shatter cones form just a few metres below the crater floor as the "target rock" is severly shocked by the impactor.

Maybe you can find some "breccia dykes" in the crater floor, as well. Real impactites like Suevites would be great, but I don´t expect to find them there. Suevites contain a reletively high amount of impact melt (glass) that are characteristic for impacts of larger size (see e.g. the Ries and Rochechouart craters in Germany and France; I have been to both of them).
Pseudotachylites are friction melts that form in faults in the shattered area below the crater floor.

For more information you can get a really great Impact Geology book here: Bevan M. FRENCH (1998): Traces of Catastrophe.
[http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html]

Good luck in your impact field work!

Best wishes from Stuttgart to Ireland,
Martin
(impact geologist)

[Tsetsuush]

I have one request. Could you advice me what I shall do. I am studying about impact shock metamorphism. My main aim is to find any shock significant that crater formed by impact or not. Now I want to prove by petrography and geochemical. I found some significant I mean I found some PDFs. But I don’t know how I will distinguish volcanic or impact crater and PDF’s. Could you tell me how I will do. Also I wrote brief information of study area.
This area is still not clear idea of impact. Impact structure first has been identified from satellite image. In satellite image, impact structure displays pronounced rim and almost completely filled by vegetated aeolian sand, which is 1.7km in diameter. The crater is surrounded by a slight, near-circular depression and outer ring of minor topographic highs. Although the structural and morphology of the crater strongly
suggest that the crater was made by an impact. No solid evidence to support the idea has been proposed.