United States Meteorite Impact Craters

Shatter cones and high pressure mineral polymorphs such as coesite and stishovite emerged as indicators of meteorite impact craters only a few years before grain-scale petrographic indicators.  An understanding of microscopic changes in rocks subjected to shock metamorphism emerged from a combination of research on rocks from meteorite impact craters and rocks from nuclear test sites.

[review Short (1966), Bunch (1968), Carter (1968) and expand this page]

By 1966 two critically important pieces of microscopic evidence had been recognized in quartz, and to a lesser extent in other silicates.  These were distinct, parallel fractures of mineral grains along cleavage planes, now called planar fractures, or PFs, and more closely spaced, parallel fracture-like features in grains that are now called planar deformation features, or PDFs.  [when did the nomenclature shift, what other terms have been used for each, when were PDFs first used as a stand-alone line of evidence?]

These features were initially used as an indicator of hypervelocity impact related shock deformation, since they either don't occure at all (PDFs), or occur very rarely (PFs) in rocks subjected to ordinary geologic processes.  But their potential as a shock barometer, an indicator of the specific peak shock pressure to which a rock had been exposed, was soon recognized. [see Dence, 1968; Stoffler, 1966; Engelhardt and Butsch, 1969].

Recognizing PFs and PDFs in quartz grains involves using a petrographic microscope and universal stage to view and measure angles of the fractures and deformation planes in the crystal grains.  A universal stage is a special add-on component that allows a specimen to be tilted under a microscope, in order to view and measure features at different angles.  PFs and PDFs can be viewed and identified with an ordinary petrographic microscope, but they cannot be used as a shock barometer without a universal stage.  Universal stages are rare and expensive.  They haven't been manufactured for decades, and do not fit on most modern petrographic microscopes.

This is a petrographic microscope, which is also sometimes referred to as a polarizing microscope.  In addition to the various components of an ordinary microscope, it has a number of unique features that enable it to be used to analyze rocks and minerals.  The simplest of these are a pair of polarizing plates and a rotating stage.

Below is a thin section of a rock from a possible impact crater.  A thin section is a microscopic slide that is prepared for mineralogical and petrological analysis with the petrographic microscope.  Thin sections are prepared by gluing a cut and finely sanded surface of a rock to a thin piece of glass (microscope slide), and then grinding away the excess rock until there is only a thin, translucent layer remaining.  The thickness of the layer is optimally about 30 microns (micron is slang for micrometer), or about 30 one-thousandths of a millimeter.  This thickness allows light to pass through the slide in a way that produces useful color (and other) distinctions when viewed through the microscope.  Producing thin sections and viewing them through a petrographic microscope allows researchers to distinguish the individual microscopic mineral grains that make up a rock, and to distinguich changes to these grains that may tell the story of the origins of the rock, including exposure to high levels of shock associated with crater formation.

 

The petrographic microscope is not the only instrument that is used to view and analyze very small scale changes that are produced in rocks by hypervelocity impacts, but it is by far the simplest and most accessible.  With it, an investigator can identify planar fractures (PFs), planar deformation features (PDFs), microtwinning in calcite, kink banding in mica, and many other suggestive or diagnostic features.

Quality varies greatly in these instruments, however, and it requires a relatively decent system with a good quality camera to gather and communicate the necessary evidence to confirm an impact crater.

[add a picture or two of mineral grains viewed in a typical thin section, with labels and arrows pointing to PFs, PDFs, etc.]

Planar Fractures (PFs) and Planar Deformation Features in Quartz (PDFs)

Planar Fractures , or PFs, are basically repeating brittle fractures, or cleavage, of quartz grains along crystallographic planes conforming to crystallographic axes, meaning planes defined by the shape of the molecules that make up the crystal, and how these molecules stack. 

'Cleavage' is a term used to describe the neat fracturing of a mineral along meaningful crystallographic planes.  Some minerals, like calcite or fluorite, cleave nicely, producing beautiful flat surfaces when they fracture at any scale, from microscopic to hand specimens.  Quartz, however, is a tough mineral.  It does not normally cleave at all.  Instead, when subjected to stresses, it will typically crush or fracture irregularly or along a conchoidal trajectory.  If you are unfamiliar with conchoidal fracture and cleavage, you can do a google image search for the terms 'conchoidal fracture' and 'calcite cleavage,' to better understand how different rocks break.

When subjected to very high and instantaneous levels of stress associated with a shock front from a large meteorite impact, quartz breaks differently; it cleaves along crystallographic axes.  These cleavage fractures are called planar fractures (PFs).  At even higher levels of stress, planar deformation features (PDFs), will form along these same rational crystal planes.  PDFs aren't exactly fractures.  Instead of cleaving, the quartz changes to very thin layers of glass along would-be cleavage planes.  PDFs form very close together and can be difficult to resolve as individual lines at less than 100x magnification.  Quartz grains in thin section will also sometimes take on a slightly 'toasted' appearance when PDFs are present.  PFs are individually more distinct, wider, and smaller in number than PDFs.  They also generally extent to the edge of the quartz grain, while PDF do not necessarily do so.  The angles of the PFs and PDFs relative to the optic axes (meaning, more-or-less, the direction of growth) of a quartz crystal correspond to the shock pressure necessary to produce the structures.  The results of measurement of these angles, and of the crystallographic planes that the angles indicate, are generally communicated with a histogram (a type of bar graph) showing how often each angle occurs.  We call this a frequency histogram.

Name Changes

Understanding naming changes over time is necessary in order to make sense of earlier literature and avoid confusion.

PFs and PDFs are not always distinguished from one another in literature from the 1960s and earlier, and the names of both have changed and refined over time.  Both PFs and PDFs may be called planar elements in early work.

PFs are sometimes called cleavage or microfaulting [see Carter (1968)] in earlier papers.

PDFs are slso called shock lamellae, planar lamellae, or planar elements 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.

PFs and PDFs were defined in Carter (1968) as [add this].  Along with the names we use for these structures, modern definitions differ slightly. [add modern definition].

Above: 2 sets of planar fractures (PFs) in a quartz grain.  This grain also showed very strong mosaicism in extinction.  When working with research thin sections, bear in mind that most examples do not look as pretty as the ones that we choose for publications or for websites.

Above: PDFs are not as easy to visually resolve with a petrographic microscope.  The individual PDFs are both much thinner and much more closely spaced than planar fractures.  In aggregate, when viewed through a microscope at 100x to 400x magnification, they appear as a distinct criss-crossed brush pattern, most often in two or three directions (though 1 to 5 sets are possible).  It takes a pretty good microscope and camera to get a good picture of PDFs. 

How good are they as impact evidence?

PDFs - A published record of PDFs in quartz grains that are associated with a clearly identified structure or rock unit are typically considered to be adequate unambiguous evidence of a crater forming meteorite impact event.  Alongside shatter cones, they represent the current 'gold standard' of impact ecidence.  Reporting PDFs typically involves very clear (micro)photographic record of the grain-scale structures.  While not strictly necessary if they are otherwise clearly documented, most authors who report PDFs in a peer reviewed paper in recent years also 'index' the PDFs by taking measurements on a universal stage.  Even if they are not using the presence of the structures as a shock barometer in a general sense, reporting frequency and orientation within the grains and the corresponding peak shock pressures that these measurements suggest, provides an empyrical and quantified basis for the claim of impact origin.  Indexing also ensures that what is being measured and described conforms to rational crystallographic planes, meaning that what is being reported is consistent with published descriptions of PDFs. Mistakes have been made in the past, and small controversies have resulted.

PFs - When not accompanied by other impact evidence, PFs in quartz have not generally been considered to be adequate as stand-alone evidence of impact origin, since they may also be produced by some ordinary terrestrial processes.  However, when PFs are considered with due diligence to understanding their abundance within the context of surrounding rocks and soil, they may provide tentative to strong indication of a hypervelocity impact.  PFs are particularly compelling when present in more than one set within quartz grains, though one would hope for the presence of other indicators as well.  As with PDFs, measuring planar fractures increases their utility as a line of evidence, since it ensures that a mistake has not been made concerning the identity of the structures that are being reported, and defines a minimum peak shock pressure to which the mineral grains have been exposed.

Indexing of PDFs, and Shock Barometry

There are several crystallographic axes along which PFs and PDFs may form.  Some directions of cleavage in quartz are easier to 'break' than others.  PFs and PDFs will form in the easiest directions first, and will only develop parallel to successively more difficult planes with increasing shock pressure.  PFs and PDFs formed along different planes are called 'sets.'  By recording which sets have formed in quarz grains, a researcher may get a sense of specific shock pressure.  In this way, PDFs may serve as a shock barometer.  The process of cataloging and recording PDFs and their abundance along particular axes is called indexing.

This technique was first explored and described in the 1960s [see: Dence, 1968; Stoffler, 1966; Hörz, 1968; Robertson et al., 1968, 1975; Bunch, 1968;  Engelhardt and Butsch, 1969]

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.

It is probably preferable that PDFs be indexed in order to be published as a absolutely definitive evidence of an impact, but the vast majority of impacts thus far cataloged on earth have not had PDFs indexed for practical reasons.  Universal (spindel) microscope stages are uncommon, and most institutions do not have one, and indexing PDFs also requires knowledge and skill that is challenging to acquire.  Nevertheless, its a worthwhile thing to do if possible.  Indexing PDFs not only acts as a shock barometer, establishing a specific range of shock pressure to which the grain was subjected, it also confirms that the features that one is observing and reporting are oriented parallel to meaningful crystallographic planes, significantly reducing the possibility of misidentification.

The results of indexing PDFs in quartz are often reported in the form of a frequency histogram of the angles between the quartz c-axis, or optical axis, and the angle at which the PDFs occur in the quartz grain.  These angles may be equated to the various rational crystallographic planes at which quartz may cleave at increasing levels of shock pressure.

A discussion of the application of the universal stage to the specific task of indexing PDFs is found at the bottom of Langenhorst F., 2002, Shock metamorphism of some minerals: Basic introduction and microstructural observations, Bulletin of the Czech Geological Survey, Vol. 77, No. 4, 265–282.  The document is at:

Other key references associated with the process and meterials include:

Stöffler and Langenhorst, 1994; Grieve et al., 1996, Wilcox, 1959;

Grieve and Robertson, 1976, describes a classification scheme for progressively shocked rocks based on which sets of PDFs are present.

An excellent discussion of PFs, PDFs, and structures with which they might be confused, as well as pictures of each, can be found in Reimold et al., 2014.  The text of this article also points the researcher towards a significant body of related literature and introduces the relevance of each article: 

Reimold, W. U., Ferrière, L., Deutsch, A. and Koeberl, C. (2014), Impact controversies: Impact recognition criteria and related issues. Meteoritics & Planetary Science, 49: 723–731. doi: 10.1111/maps.12284

http://onlinelibrary.wiley.com/doi/10.1111/maps.12284/abstract

PDFs and PFs Can be Confused With

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.

Durability of Evidence

Quartz is one of the most stable minerals within earth's surface-normal temperature and pressure and chemical environment.  Quartz grains can easily preserve PFs and PDFs for a billion years or more.  Even when subject to substantial heat and pressure, shy of the destruction of the quartz grain itself, the signatures may remain - though they do become less distinct.  Within the few hundred million years range of time that encompasses most discussion of impact craters on earth's continental surfaces, PFs and PDFs provide excellent durability as impact indicators.

Toasting and Mosaicism

Most quartz grains that contain PFs or PDFs will also exhibit a feature called mosaicism.  When a quartz grain is rotated on the stage under a polarizing microscope, it will go from white to black (extinction) and back again.  You can see this here:    If a quartz grain has experienced strain at some point its history, it may go into extinction unevenly; the dark region sort of 'rolls' across the grain as it is rotated.  This is called undulose extinction, and it is not a sign of impact alteration.  You can see this here, particularly in the center of the grain:   In a shocked quartz grain that has developed PFs or PDFs, the extinction can become very uneven and patchy.  This is called mosaicism.  Some heavily shocked grains also develop a substantiall overall darkening, which has been described as a toasted appearance.  The grain shown earlier on this page to illustrate PDFs is also slightly 'toasted' in patches.  (Though not visible in the image, this grain also showed strong mosaicism.)  Toasting is discussed here:   and here

 

Above: Mosaicism - patchy, uneven extinction common in quartz grains that exhibit PFs or PDFs.

Other grain scale changes

In addition to fracturing in interpretable ways along meaningful crystallographic axes, mineral grains at impact sites may also be cracked, crushed, and shattered in more conventional ways.  It is not unusual to see crushed and fractured mineral grains in thin sections from impact sites, though such grains are also found at non-impact related geological structures.  A unique and extreme version of this is the 'rock flour' first reported at Barringer and subsequently described in (the remnants of) sandstone units from other sites.  Rock flour is finely powered flour-like sand produced when quartz grains in a sandstone are completely shattered.

[add pictures and explanations of others]

Further Reading

A nice discussion of PDFs, along with some remarkable photomicrographs, can be found in: 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 freely available variation of this paper, containing excellent discussion and photomicrographs of PDFs (beginning on p. 66) can be found at:  

Bunch T. E. (1968) Some characteristics of selected minerals from craters. in Shock Metamorphism of Natural Materials, B. M. French and N. M. Short, editors, pages 413-432, The Mono Book Corp., Baltimore.

Carter N. L. (1968) Dynamic deformation of quartz, in Shock Metamorphism of Natural Materials, B. M. French and N. M. Short, editors, pages 453-474, The Mono Book Corp., Baltimore.

Carter N. L. (1968) Meteoritic impact and deformation of quartz. Science, Volme 160, pages 526-528.

Dence M. R. (1968) Shock zoning at Canadian craters: Petrography and structural implications, in Shock Metamorphism of Natural Materials, B. M. French and N. M. Short, editors, pages 169-184, The Mono Book Corp., Baltimore.

Engelhardt W. V., Bertsch W. (1969) Shock induced planar deformation structures in quartz from the Ries crater, Germany. Contributions to Mineralogy and Petrology Volume 20, No. 3, pages 203-234.

Ferriere L., Morrow J. R., Amgaa T., Koeberl C. 2009. Systematic study of universal-stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results, Meteoritics & Planetary Science, Volume 44, No. 6, pages 925-940. DOI: 10.1111/j.1945-5100.2009.tb00778.x



French B. M. and Short N. M., editors. 1968. Shock metamorphism of natural materials. Baltimore: Mono Book Corporation. 644 p.

Grieve, R. A. F., Langenhorst, F. and Stöffler, D. (1996), Shock metamorphism of quartz in nature and experiment: II. Significance in geoscience. Meteoritics & Planetary Science, Volume 31, Issue 1, pages 6-35. doi: 10.1111/j.1945-5100.1996.tb02049.x  

 

or 



Hörz F. (1968) Statistical measurements of deformation structures and refractive indices in experimentally shock loaded quartz, in Shock Metamorphism of Natural Materials, B. M. French and N. M. Short, editors, pages 243-253, The Mono Book Corp., Baltimore.

Robertson P. B. (1975) Zones of shock metamorphism at the Charlevoix impact structure, Quebec. Bulletin of the Geological Society of America, Volume 86, pages 1630-1638.

Robertson P. B., Dence M. R., Vos M. A. (1968) Deformation in rock-forming minerals from Canadian craters, in Shock Metamorphism of Natural Materials, B. M. French and N. M. Short, editors, pages 433-452, The Mono Book Corp., Baltimore.

Short N. M. (1965) A comparison of features characteristic of nuclear explosion craters and astroblemes. Annals of the New York Academy of Sciences, Volume 132, pages 573-616.

Short N. M. (1970) Progressive shock metamorphism of quartzite ejected from the Sedan nuclear explosion crater. Journal of Geology, Volume 78, pages 705-732.

Stoffler D. (1966) Zones of impact metamorphism in the crystalline rocks of the Nordlinger Ries crater. Contributions to Mineralogy and Petrology, Volume 12, pages 15-24.

Stoffler, D. & Langenhorst, F. 1994. Shock metamorphism of quartz in nature and experiment: I. Basic observation and theory. Meteoritics, Volume 29, pages 155-181. doi: 10.1111/j.1945-5100.1994.tb00670.x 

 

or 



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

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IMPACT CRATERING: AN OVERVIEW OF MINERALOGICAL AND GEOCHEMICAL ASPECTS at
Koeberl, C., 1997, Impact cratering: The mineralogical and geochemical evidence. In: Proceedings, "The Ames Structure and Similar Features", ed. K. Johnson and J. Campbell, Oklahoma Geological Survey Circular 100, 30-54.