Meteorites and Geology: big holes in the ground

Many thanks to Chris and Anne for such a great opportunity to step into the geoblogosphere. Here goes…

One of the few advantages of having being on the Earth for a while (0.00004 Ma in my case) is that I’ve had time to see new things come along in Science. One of the big themes in Geology in the last 20-30 years has been the realisation of the importance of asteroid impacts in Earth History. A lot of attention has rightly been focused on the role of massive impacts in causing mass extinctions (K-T boundary, Chicxulub)  but since meteorite impacts follow a power-law relationship of size to frequency alongside a few ‘earth-shattering’ impacts there should be lots of *smaller* ones. Even a ‘small’ impact will make a trace in the Geological record. So where are they?

How many craters?

The classic trace of an impact is a crater, a circular hole in the ground with a distinctive rim and maybe a central uplift.

Other things make circular structures of course, such as volcanic activity or salt diapirs, so to unequivocally attribute a structure to an impact, other evidence such as shocked quartz or shatter cones is required.

Many craters are on the Earth’s surface, which either means they are recent or have managed to survive erosion, perhaps by being very large. Sedimentary basins are made up of a whole series of fossil surfaces, which have been preserved by a covering of later sediment as the basin fills and/or subsides. So if a basin was hit by a significant impact during its history, we can expect to see it preserved in the pattern of its layers of sediment. Oil companies spend a lot of money collecting seismic reflection data precisely to see patterns in layers of sediment.

A fascinating recent paper “Estimates of yet-to-find impact crater population on Earth” by Professor Stewart of Heriot-Watt University in Scotland discusses this matter in detail. Firstly, he discusses the number of craters currently recognised (178) and the rate at which they are being found (2 a year, which is why the paper says 176 and the database today has 178). Next, he heroically attempts to quantify how many there should be yet to find. Numbers are necessarily imprecise, but he produces three sets of estimates.

  1. The first is based on the number of objects we can see in space, the number of craters we can see on Mars or the Moon and the number of small meteorites we can hear or see burning up in the Earth’s atmosphere. This evidence is used to estimate the rate of impacts on Earth per size of meteorite (that power-law again).
  2. The second number is the ‘area-timespan’ preserved  in Phanerozoic sedimentary basins (e.g. those of interest to oil companies where seismic data is collected; he is “Professor of Petroleum Geoscience”, after all). As a simplistic example (the paper is *not* simplistic) a basin of 10km2 that was accumulating sediment for 10Ma has an ‘area-timespan’ of 100 million km2 a. His estimate for the whole world comes to pleasingly large 9 quadrillion km2 a (9×1015).
  3. Combining those two numbers gives the third: there are over 700 craters larger than 1km diameter yet to be found.

Putting it another way, on average over the last 542 million years, over 700 big asteroids would have hit a part of the earth with a subsiding basin covered by a shallow sea. This is rather cool, I suggest, particularly the idea that many of these craters may already have been surveyed, but the seismic data has yet to be appropriately analysed. It’s a nice thought that most examples of what happens when something very big falls out of the sky are to be found deep underground.

Professor Stewart mentions a number of examples of possible impact craters known from seismic data which are not part of the 178 ‘known’ total. This is because evidence from the shape alone is not sufficient to make the ‘official’ list. The craters are buried under kilometres of rock so it would cost millions to gather rock samples containing the other types of evidence.

Silverpit crater

To speculate about the human side of this research (without knowing any of the
people involved), I think the writing of this paper must have been motivated in part by Prof. Stewart’s experience with the Silverpit crater. This is a beautiful circular structure, visible only in seismic data, deep under the North Sea. In a 2002 paper Prof. Stewart and a co-author interpreted it as a meteorite impact. Over the next few years a number of papers appeared suggesting alternative interpretations, all of terrestrial origin. The tone of the discussion became rather bad-tempered, by the standards of academic debate. Prof. Stewart would not be human if the experience of being accused (in the journal of world’s oldest Geological Society) of suggesting an impact origin “without a shred of scientific evidence” (in doi:10.1144/0016-764904-070) didn’t motivate him to find yet more scientific evidence for it.

This paper does just that: of the craters estimated yet to be discovered, based on its area-timespan, three would be in the North Sea. This further strengthens the argument that the Silverpit structure (the only candidate yet identified) is a buried impact crater. Given the difficulty of getting more direct evidence, this may be the best that is possible.

If the debate over Silverpit got emotional, then that makes sense to me. My experience of academia is that nothing is disliked more than people from one discipline ‘moving into’ another area. My emotional response in the 1990s to astrophysicists arguing for the importance of impacts in Geological history was of scepticism and dislike. A training in Geology is all about the Earth: we spend our time looking at the ground. Things coming in from Space just didn’t seem relevant or necessary.

Of course, science at its best works across artificial human distinctions and makes connections between seemingly separate things. It turns out there is plenty of Geological evidence for ancient meteorite impacts and that is what I hope to talk about in future posts.

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About Metageologist

Simon Wellings trained as a Geologist but professionally has metamorphosed into something else. He retains a keen interest in Geology, facilitated mainly by the wonders of the Internet. Simon now blogs at Metageologist.
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Comments (8)

  1. Matt says:

    Interesting post and a great start to the Erratics. Silverpit is an interesting case study, mainly because of the controversy. I expect you’re aware that there was an open debate on the subject at the Geol Soc of London in 2009… and a vote on the contending theories. Salt-related tectonism was thought to be the most likely cause, not a bolide. Doesn’t prove much, but there was clear consensus. Personally, I find the cross section you show here compelling enough!

    But what I really wanted to comment on was the logic in the argument that because we might expect three astroblemes in the North Sea, and because Silverpit looks a bit like an astrobleme… therefore Silverpit is probably an astrobleme. This does not seem like evidence to me.

    Looking forward to future posts!

  2. Chris Rowan says:

    A great first post, Simon.

    Does the paper attempt to compensate for the fact that, due to plate tectonics, some of these shallow shelf areas are probably now in the middle of mountain belts? Perhaps these effects are the ‘not simplistic’ bits…

  3. Thanks for the feedback.
    @Matt, a fair point about the evidence, or rather lack of it; I was (over)-stretching a point, which was in the absence of any direct physical evidence, the argument that one *ought* to be seeing bolide craters in the North Sea is the best case that can be made that Silverpit is one. At least I can’t think of a better one. Perhaps once the ‘missing 700′ are found we will have a better handle on the problem ;->

    @Chris, the estimates of area are based on AAPG and USGS ‘world petroleum’ databases, so I would guess this would exclude sediments that are no longer in ‘basins’ but have had interesting things done to them. I wonder whether that without seismic data, a crater would be rather hard to spot, especially in folded sediments. Of course there are other traces that might be visible; speaking of which, I should start writing my next post…

  4. Brian Romans says:

    While a subsiding basin is a great place to accumulate/preserve sediment overall, there is still quite a bit of local erosion and sediment redistribution going on. My gut feeling is that, even in a subsiding basin, it would take some special circumstances to preserve a crater so well that it’s recognizable in seismic-reflection data. Not saying it couldn’t happen — it surely could — but perhaps the estimate of how many impacts are out there would need a factor that took into account the preservation potential too.

    (note: I haven’t read that paper so I may be bringing up something the authors do already)

    Great post, I hope you continue to blog.

  5. Andrew Alden says:

    “Oil companies spend a lot of money collecting seismic reflection data precisely to see patterns in layers of sediment.”

    There’s another one of those “artificial human distinctions” that will severely limit the number of new craters reported from the basin record. Example from the latest AGU Fall Meeting: a small crater, securely identified by shocked quartz, in the Ordovician of New York. A poster presentation isn’t really a peer-reviewed publication, and the energy company whose data revealed the crater will probably never publish it formally. Does it go on the database? How many other craters are sitting in proprietary files? How much of the basin record is covered by commercial surveys?

  6. @Brian. Your’s is a fair point and the paper does address it. He has a ‘crater depth factor’ that basically assumes most of the record is lost, to erosion and so on. He also excludes deep water sediments, as a really deep water column might prevent a crater being formed on the sea-bed.

  7. @Andrew.
    I think you’re dead right, perhaps the paper is an attempt to encourage more data to be released.

    The crater you mention doesn’t seem to in the database, but it seems to meet the criteria.
    Submission is via email: passc at unb.ca, do you fancy emailing them? Or maybe pass the email onto the author, if you still have his contact details.

  8. Greetings Metageologist:
    Please continue this discussion about the challenges which accompany the interdisciplinary nature of impacts on Earth. Seemingly the sole provenience of astronomers, impact research gets hairy when the evidence is easily considered gradualistic deposits – which belong (rightly) to the gradualistic geologists. If it were not for the exploratory drilling off the Yucatan, the Chicxulub crater would have remained a figment of Dr. Alvarez’s imagination. He knew it was there somewhere, but could not PROVE it. A dispassionate observer might suggest that the parties become more amicable towards each other, especially as the # of verified – but once obscured – impacts continues to grow. This should not be a turf war.
    I’d also like to note that Dr. Michael Rampino has identified several areas of impact ejecta debris, which were originally “comfortably” designated as glacial till by Earth-centric geologists.

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