What this page contains and how to use these pages: This is the second page of a sub-group of web pages that are intended to provide the reader with an introduction to the basics of the science of meteorite impact crater recognition. If you are new to impact crater science, you might want to start by reading [Crater Identification] before returning to this page. This page provides a basic introduction to types of evidence used in evaluating possible hypervelocity impact craters, introduces the difference between diagnostic and suggestive evidence, suggests a few good things to read when beginning to engage the subject, and provides introductions and links to several sub-pages on more specific topics. Please note that this website is perpetually under construction in an ongoing effort to make it more understandable and more useful.
Introduction - Why Require Rigorous Proof
What we learn from impact craters today will determine how we spend future research time and money. Because sloppiness in the short term can lead to wastes of time and money in the future, the scientific community is constantly trying to refine and improve what gets published. In addition to striving to be good stewards of time, knowledge and money, there is also a great satisfaction in simply doing good science.
Following the introductory paragraphs below, this page will examine the nature of diagnostic and suggestive evidence of a large-scale hypervelocity impact event, and will provide references to at least some of the literature that can get a person moving towards building an understanding of collecting and evaluating such evidence.
A Reasonable Double Standard
Remote sensing data and ground-based investigation of morphology, alone, are not recognized as adequate evidence of impact origin for structures on earth. In other words, it is not enough to point out that a structure is round and is shaped like a crater. Though typical shape and topography are generally accepted as reasonable evidence that impact-like structures on the moon, Mars, and other surfaces in the solar system were caused by impacts, this is not adequate on earth. There are good reasons for this. Earth offers a far wider range of other kinds of structures that might be easily mistaken for impact craters. The demand for better evidence also reflects the obscured morphology of earth's weathered cratering record and of the simple fact that ground-truth can be achieved here, and cannot on other bodies.
How Many Undiscovered Craters?
Undiscovered impact craters far outnumber those that have been identified. French (1998) cites Trefil and Raup (1990) and Grieve (1991), as estimating the number of undiscovered impact structures at several hundred. Stewart (2011) calculates an estimate for undiscovered impacts in phanerozoic rocks of around 714 or more, and estimates that 228 of these will be 2.5 km or larger. Given the rate at which they have been identified within the last century and the density of craters in those regions that have been most thoroughly explored, I would be genuinely surprised if the number of known terrestrial impact structures does not eventually exceed 1000, counting those eventually found preserved as ejecta lenses and as remnants in older rocks. Only time and hard work will tell.
Regardless of how many preserved impact craters remain to be discovered, there are far more circular objects on the earth (and far more masses of breccia beneath its surface) that are not impact craters than that will ever prove to be. As a result, good science means listing confirmed impacts only in terms of reasonably diagnostic criteria.
Databases of Known Craters
Confirmed impact structures at which diagnostic impact signatures have been reported in peer reviewed literature are listed in the PASSC Earth Impact Database. A broader summary of both confirmed and refuted craters, which also reports on the many that have been incompletely investigated and fall between these extremes, can be found in the Impact Database produced by Rajmon (2009). An alternative list of confirmed, possible, and refuted impact structures, Impact Structures of the World, has been compiled by Jarmo Moilanen (2009). The Expert Database on Earth Impact Structures provides another list. Links to all of these are below, in references.
Great Things to Read When Getting Started
Bevan French's article beginning on page 3 of the Winter, 2005 (volume 2) newsletter of Impacts in the Field provides an excellent brief overview of the subject of impact evidence, both diagnostic and subjective. And best of all, its free. The link is below. (If you only read one thing, read this one.)
French, B. M., 2005, Stalking the Wily Shattercone: A Critical Guide for Impact-Crater Hunters. Impacts in the Field. vol. 2 (Winter), pp. 3-10.
His book, Traces of Catastrophe (French, 1998), is also freely available online, and is the logical primer for anyone beginning to seriously address this subject.
Another key reference is: Bevan M. French, Christian Koeberl, The convincing identification of terrestrial meteorite impact structures: What works, what doesn't, and why, Earth-Science Reviews, Volume 98, Issues 1–2, 1 January 2010, Pages 123-170.
It is also a very good idea to read the cogent summary written by Christian Koberl: Impact Cratering: An overview of mineralogical and geochemical aspects - Koeberl 1997 (and) Koberl, C. (2002) Mineralogical and geochemical aspects of impact craters. Mineralogical Magazine, October 2002, Vol. 66(5), pp. 745±768.
Additional useful insights and perspecitves can be gained from: Reimold, W. U. (2007), The Impact Crater Bandwagon (Some problems with the terrestrial impact cratering record). Meteoritics & Planetary Science, 42: 1467–1472.
Potentially Diagnostic Evidence of Hypervelocity Impact
Lets start with a discussion and tentative definition of the concepts that the title of this section presupposes. That is to say, 'Is there such a thing as a diagnostic indicator of a hypervelocity impact?' and if so, 'What does such a beastie look like?' For purposes of this writing, and in keeping with a fairly consistent trend in the peer-reviewed scientific literature of the last few decades, diagnostic indicators are presumed to be eye-visible or microscopic (grain-scale) geological structures that are only known to result from the levels of shock pressure that are present in large meteorite impact craters. Because several such indicators can be confused with non-shock-related structures that are similar in appearance, we should also consider it necessary that such indicators should be unambiguously demonstrated to be present. And because some such indicators are durable and portable, it is probably also reasonable to specify that the evidence should be demonstrably concentrated in a specific region or rock unit. Only a few types of evidence can be used to fit these conditions in building a compelling case that a geological structure is the result of a meteorite impact. The most commonly used examples (by far!) are shatter cones and planar deformation features. These and others will be discussed below and in linked pages of more detailed information.
The investigator should also look at the 'big picture.' Any proposed diagnostic evidence should make sense within its geological context. Coesite may indicate an impact, or it may indicate ultra-high pressure metamorphism of a deeply subducted rock unit. If it is found within a small region in a young, non-metamorphosed region, it certainly represents the former. Lechatelierite and other very high temperature melts may form from lightning strikes or impacts. Context can make this discernment. The same sorts of critical evaluation of alternative explanations should be used for any line of evidence, in order to build a coherent and reasonable story.
The existence of unambiguous diagnostic evidence of a large meteorite impact event makes one further assumption: that other classes of evidence are inadequate to reliably demonstrate the existence of an impact crater. Among those classes of evidence that are less compelling, we gather (among others) the existence of breccia, certain kinds of melted rocks, the presence of meteorites, planar fractures in quartz, and a crater-like morphology - circularity, a raised rim, bowl shaped features, and so on. Each of the lines of evidence may contribute to the argument that a structure has an impact origin, but because each of them exists more commonly in non-impact environments than in impact craters, none can prove an impact origin. Details of some crater-like terrestrial structures can be found here: [PSEUDO-IMPACTS AND CRATER-LIKE OBJECTS].
Links to Pages Focussing on Specific Types of Evidence
Planar Fractures (PFs) and Planar Deformation Features (PDFs) in Quartz
High pressure quartz polymorphs - Coesite and Stishovite
Diaplectic Glass / Mosaicism in quartz
Other high pressure mineral polymorphs - diamonds, etc.
Suggestive, but Non-diagnostic Evidence
Breccia, Megabreccia, and Melt - especially in massive quantities
[BRECCIAS AND MELTS]
Intense 'unresolvable' faulting with irregular displacement (megabreccia)
Intense Calcite Twinning
impact microspherules or spherules - (also called spherical lapilli, impact melt microspherules, and so on )are easily confused with micrometeorites and the now extremely common cenospheres, and yet these are a major feature at several impact sites, such as Canyon Diablo, where they areabundant. Larger spherical 'bombs' or 'carbonate lapilli' have also been occasionally reported (ie: at Alamo and Chicxulub). Some of these could be nucleated concretions formed during lithification of soft sediments or alteration of cemented sediments in a post impact environment. They have received little study.
Chemical traces of an impactor - elemental enrichment in rock units that suggest a contribution from an impactor. This can mean something similar to the famous irridium anomaly associated with the K/T boundary signature found around the world, or it can mean the more widely used trace element ratios analyzed near the centers of a number of specific craters in order to characterize specific impactors. This kind of investigation typically involves neutron activation analysis. Chemical anomalies must be considered in context.
Impactor Components - meteorites, zircons or other durable impactor grains.
Spinel grains and other durable remnants of meteorites. The presence of a portion of an impactor component, even if it is a meteorite itself, is not, independent of other criteria, proof that a structure is an impact crater.
Tsunamite Deposits - Impact generated tsunamite deposits are rock units emplaced or altered by tsunami waves. They are dificult to definitively prove in the geological record, and can be easily confused for several other kinds of deposists, including those generated by earthquakes, volcanic eruptions and storms. Even when identified correctly, tsunamite deposits are not, independent of other evidence, definitive proof of an impact event. Just as the waves we see today, the tsunami waves that produced tsunamite deposits in the geological record originate more frequently from volcanism, undersea landslides and earthquakes than from meteorite impacts. Even so, they are worth considering in some contexts - particularly in the interpretation of altered or emplaced rock units surrounding impact craters that have been identified as marine events.
The Problem With Small Impact Craters
Researchers may encounter significant problems when attempting to confirm very small impact craters in the 100 meter to 1 kilometer range, as such impacts do not consistently produce PDFs, high pressure quartz polymorphs, or shatter cones, and are very vulnerable to morphological softening or complete destruction by weathering. This might be considered one of the greater unresolved challenges facing the impact crater research community. (see Odessa)
Small impact craters of 1 km or less, even if young and extremely well preserved, often do not provide any widely accepted diagnostic evidence of hypervelocity impact. Shock pressures are inadequate to produce some types of evidence, and the volume of material in which other types of evidence is preserved is very small. This is a real problem for the impact crater researcher. In such cases, an impact origin for a structure can only be argued based on a preponderance of suggestive evidence. The Odessa, Texas, crater is an excellent example of this.
Among potentially diagnostic evidence found at small craters, one might consider, in addition to the previously noted materials, the scoriaceous glass found at Henbury and Monteraqui, both of which contain metal blebs from the impactor, and the iron rich impact spherules easily gather with a magnet at Barringer.
Database - Earth Impact Database
Database - Impact Database (ref: Rajmon, D. (2009) Impact database 2010.1.)
Database - Impact Structures of the World - Jarmo Moilanen
Database - Expert Database on Earth Impact Structures
References and Further Reading:
Schreyer, W. (1995), Ultradeep metamorphic rocks: The retrospective viewpoint, J. Geophys. Res., 100(B5), 8353–8366.
Ferriere, L. and Osinski, G. R. (2012) Shock Metamorphism, in Impact Cratering: Processes and Products (eds G. R. Osinski and E. Pierazzo), John Wiley & Sons, Ltd, Chichester, UK.
French B. M. 1998. Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. LPI Contribution No. 954, Lunar and Planetary Institute, Houston. 120 pp.
Stewart, 2011, Estimates of yet-to-find impact crater population on earth, Journal of the Geological Society, London, Vol. 168, pp. 1-14.