United States Meteorite Impact Craters

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, though these types of evidence are accepted for structures on other surfaces within the solar system. This is partially because earth offers a far wider range of other kinds of structures that might be easily mistaken for impact structures, but it also reflects the often 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 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.

His book, Traces of Catastrophe, is also freely available online, and is the logical primer for anyone beginning to seriously address this subject.

Diagnostic Evidence of Hypervelocity Impact

High pressure Quartz polymorphs - Coesite and Stishovie

Identified by X-Ray Powder Diffraction (XRD) in Stoffler (1971), also by Raman Spectroscopy (from a conversation).

Stöffler, D. (1971), Coesite and stishovite in shocked crystalline rocks, J. Geophys. Res., 76(23),5474–5488

Planar Deformation Features (PDFs) in Quartz

(Also called shock lamellae or planar lamellae in earlier papers.  More recent work typically uses Planar Deformation Features (PDF), and distinguishes these from Planar Fractures (PF), which can form at lower levels of stress. 

Planar Deformation Features are identifiable in petrographic thin section, but a researcher must have a somewhat less accessible universal stage and significant training and skill in order to accomplish indexing them.

Structures known as Bohm Lamellae or Boehm Lamellae can be confused for PDFs, but the distinction is pretty straightforward with the accumulation of modest experience and a good set of photos of each.  Boehm lamellae can be formed from static stresses to quartz grains.  They occur in only one set (direction) within a grain.

Izett, G.A., 1990, The Cretaceous/Tertiary boundary interval, Raton Basin, Colorado and New Mexico, and its content of shock-metamorphosed minerals—Evidence relevant to the K/T boundary impact-extinction theory: Geological Society of America Special Paper 249, 100 p.  A variation of this paper, containing excellent discussion and photomicrographs of PDFs can be found at: 

Wilcox, R.E., 1959, Use of the spindle stage for determination of principal indices of refraction of crystal fragments: American Mineralogist, v. 44, no. 11–12, p. 1272–1293.

Shatter Cones

(unique structures - visually identifiable to trained eye)

Suggestive, but Non-diagnostic Evidence

PDFs in Feldspar - could be diagnostic - I do not know.

Breccia, especially in massive quantities

Megabreccia

Mosaicism in Quartz

Platinum Group Element Enrichment

Planar Fractures in Quartz

Diaplectic Glass

Impact Melt Rocks

Intense 'unresolvable' faulting with irregular displacement (megabreccia)

Morphology (central uplift, annular basin, proximal ejecta rim)

Intense Calcite Twinning

A good understanding of the basis of what constitutes diagnostic evidence of impact can be gained from French 1998, French & Koeberl 2010, Reimold 2007.

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)

References and Further Reading:

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. 

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