ElEmEnts, Vol. 8, pp. 19–24 February 201219

1811-5209/12/0008-0019$2.50    DOI: 10.2113/gselements.8.1.19

IMPACT! – BOLIDES, CRATERS, AND CATASTROPHES

IMPACT CRATERING IN THE SOLAR SYSTEMSome 50 years of space exploration have demonstrated that all solid surfaces of planetary bodies in the Solar System—the terrestrial planets (Fig. 1), the moons, the minor planets—and even the cores of comets are peppered with the results of cosmic bombardments. A range of crater geometries, such as simple bowl shapes (Fig. 1a, d), complex geometries with a central uplift (Fig. 1a, b) or central peak ring (Fig. 1c), and multi-ring impact structures, is preserved on these surfaces (e.g. Reimold and Koeberl 2008). The largest, very old impact structures known in the Solar System measure thousands of kilometers in diameter (e.g. the 3000 km diameter Valhalla basin on Callisto, a moon of Jupiter). At the other end of the spectrum, the smallest impact craters known are found on the surfaces of glass beads in the lunar regolith and on microtektites.

When fragments of comet Shoemaker-Levy 9 impacted the atmosphere of planet Jupiter in July 1994, the world took note that catastrophic impact events in the Solar System are not just a phenomenon of the distant past. Recently, two small impact craters were discovered at Carancas, Peru, and Kamil, Egypt. The Peruvian crater was formed on 15 September 2007, and the Egyptian crater was also created

in the recent past, judging by its youthful appearance and the excellent preservation of iron meteorite fragments (Earth Impact Database 2011). On Earth, only very young impact structures that have not yet fallen prey to erosion or are covered with younger sedi-ment have fresh geometries; older structures are variably degraded (see also Turtle et al. 2005; Collins et al. 2012 this issue).

We now know that impact cratering is a fundamental process in the Solar System (box 1). The accretion of tiny particles, then of planetesimals, and finally of planets involved repeated impacts. Impact events contributed to the primordial kinetic energy required for the internal differentiation of

planetesimals and later of planet-sized bodies; they shaped the earliest crustal fragments, and they were fundamental in the formation of early crustal lithologies. Time and time again, impact events led to the destruction or formation of planetary bodies through gigantic collisions (the most famous example being the impact event related to the genesis of the Moon; Canup and Asphaug 2001). The role of impact cratering during the earliest stages of life devel-opment on our planet is widely debated: Did the giant impacts of comets bring life-sustaining water to the planet? Did the seemingly hostile interiors of impact craters harbor beneficial microclimates and chemistries suitable for spawning primitive organisms? Did the intense impact activity during the first half billion or so years of Earth’s history frustrate the development of any primitive forms of life prior to the formation of the oldest-known terrestrial life about 3.8 billion years (Ga) ago.

Large-scale impact events were prominent in the early stages of Solar System development, as amply demonstrated by the cratered highlands of the Moon (Stöffler et al. 2006), Mars, and Mercury (Fig. 1a) (see reviews in Koeberl 2006a, b; Grieve et al. 2006). In contrast, evidence for early impact events is rare on Earth. Only a small number of distal impact deposits, with ages of ~3.5 and ~2.5 Ga (Glass and Simonson 2012 this issue), are known from the first half of the history of our planet, and the oldest-known impact structure is Vredefort (South Africa) at 2.02 Ga. This lack of evidence for early impacts is clearly the result of plate tectonics on our dynamic planet, which has caused the obliteration of numerous impact structures and their deposits through erosion, deposition of a sedimentary cover, tectonic dismemberment, or subduction.

It is now universally accepted that the impact of planetesimals, asteroids, and comets has been a fundamental process throughout the Solar System. Catastrophic impact events have been instrumental in developing the

early history of the planets and have caused environmental disasters throughout Earth history. A major mass extinction at the Cretaceous–Paleogene boundary has been confidently related to an impact event (Chicxulub, Mexico). While the study of impact cratering is a multidisciplinary field, mineralogical and geochemical investigations have been central since the beginning, focusing on the nature of impact-generated rocks and of the extraterrestrial projectiles as well as their interaction with geological mate-rials. Chemical and isotopic techniques have allowed the dating of impact events and the identification of traces of meteoritic projectiles in impact-formed rocks on Earth and the Moon.

Keywords: impact cratering, impactites, shock metamorphism, shatter cones, projectile identification, mass extinctions

W. Uwe Reimold1 and Fred Jourdan2

1 Museum für Naturkunde – Leibniz Institute at Humboldt University Berlin Invalidenstrasse 43, 10115 Berlin, Germany E-mail: uwe.reimold@mfn-berlin.de

2 Western Australian Argon Isotope Facility Department of Applied Geology & JdL Centre, Curtin University GPO Box U1987, Perth, WA 6845, Australia E-mail: f.jourdan@curtin.edu.au

Impacts everywhere! Even the bolides them-

selves are cratered! Galileo image of asteroid Gaspra

(ca 18 km from end to end). Image P-40450-C

Courtesy of Nasa/Jet ProPulsIoN laboratory

ElEmEnts February 201220

Of the 180 or so confirmed impact structures known on Earth (Fig. 2), only three are suspected to be of the multi-ring-basin type (Grieve et al. 2008): Vredefort, at 250 km diameter the largest-known terrestrial impact feature (Gibson and Reimold 2008); Sudbury, Canada (200–250 km, ~1.85 Ga); and Chicxulub, Mexico (180 km, ~66 million years, Ma). Only a few structures have Precambrian ages, and the impact age population is strongly biased towards the last 250 million years (Jourdan et al. 2012 this issue). The spatial distribution of known impact structures is also uneven, with most known structures located in North America, northern Europe, and Australia. In heavily vegetated regions, such as the rainforest belt where remote

sensing and field work operations are hampered, the search for impact structures has been seriously hindered. On the other hand, there still is much potential for discovering impact structures in the desert regions of the Middle East and Africa, in Asia, and in Antarctica.

A single impact structure (Mjølnir) is known on oceanic crust, which covers about two-thirds of Earth’s surface. As the entire oceanic crust can be recycled in only 150 to 250 million years, its earlier impact record has been lost.

The study of impact structures on the ground can contribute excellent subsurface stratigraphic information. A notable example is the deeply eroded Vredefort structure in South Africa, which has provided insight into the nature of the mid-crust and the Kaapvaal craton’s geological evolution over nearly 3.4 billion years. The high-resolution imagery of Martian impact structures has also provided important information on the subsurface of that planet.

Based on the early studies of impact structures, mostly in North America and Russia, much has been learned about the highly dynamic impact process (Melosh 1989; Collins et al. 2012). Extraterrestrial projectiles approach their targets at hypervelocity speeds (on average, 15–20 km/s for asteroids and 30–35 km/s for comets). The considerable

Figure 1 (A) High-resolution image mosaic of a densely cratered surface on Mercury (Messenger spacecraft,

October 2008, flyby2_20081007) showing simple bowl-shaped impact structures, complex structures with central uplifts, and peak ring structures. The largest crater structure at the top of the image is 133 km in diameter. (B) The complex impact crater Verdi (150 km diameter) on Mercury. CredIt for (a) aNd (b): Nasa/JohNs hoPkINs uNIversIty aPPlIed PhysICs laboratory/CarNegIe INstItutIoN of WashINgtoN (C) Mosaic of Clementine UV-VIS images of the Schrödinger impact basin (312 km diameter) on the Moon. sourCe: WWW.lroC.asu.edu; CredIt: Nasa/gsfC/arIzoNa state uNIversIty (D) Aerial photograph of a simple, bowl-shaped crater (Wolfe Creek, Australia, 0.9 km diam-eter). Courtesy of a. bevaN

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kinetic energy of such bolides is transferred via a shock front into the target rock and causes irreversible deforma-tion and transformation (Langenhorst and Deutsch 2012 this issue). Three stages of impact cratering are distin-guished: (1) the contact and shock compression stage; (2) the excavation stage, leading to the formation of the tran-sient crater; and (3) the collapse or modification stage, whereby the transient crater succumbs to gravity (Melosh 1989; French 1998).

IMPACTITES AND SHOCK METAMORPHISMA series of distinct rock types is generated by the destruc-tive forces of impact, beginning with compression and followed by decompression from peak shock pressures, with concomitant heat generation. These impactite types (nomenclature recommended by Stöffler and Grieve 2007) range from purely clastic breccias composed of fragments of target rock to impact melt rocks composed of a ground-mass of melt containing clasts of target rock and minerals. The transitional rock type is known as suevite. Note that the recommendations have not remained unchallenged (e.g. the nature of suevite groundmass and the genesis of pseudotachylitic breccia; Reimold et al. 2008).

The irreversible effects on rocks and minerals that are caused by the progression of a shock front through the target rock (i.e. compression) and by decompression imme-diately thereafter are generally referred to as shock meta-morphism. In the 1960s, a number of impact structures were studied in great detail, and in cases where it was possible to investigate comprehensive profiles from the center of a structure outward, it was recognized that the degree of shock deformation decreases with radial distance from the center. During this period, the principles of progressive shock deformation were established. Specific petrographic indicators can be related to different shock pressure levels (Fig. 3). In order of increasing shock pres-sure, these include planar fractures, planar deformation features (PDFs), diaplectic mineral glasses (produced without fusion), fused mineral glasses, and whole rock melts. For many rock-forming minerals, shock experiments in the <5 to >100 gigapascal (GPa) pressure range have calibrated the pressures at which specific shock effects occur.

Distinctive shock effects occur in a P–T regime entirely separate from that of normal crustal metamorphism (Fig. 3), which explains why PDFs and other features are unique characteristics of impact (e.g. Stöffler and Langenhorst 1994; Grieve et al. 1996; French and Koeberl 2010). Some of these shock features can be recognized with an optical microscope, but often more sophisticated approaches, such as transmission electron microscopy or microRaman spectrometry, are required (Langenhorst and

Figure 2 Distribution of the approximately 180 confirmed impact structures known on Earth as of February 2011

(Earth Impact Database 2011).

Box 1 IMPACT CRATERING: A FUNDAMENTAL PLANETARY PROCESS

� Impact cratering has been an accretionary process throughout planetesimal/planet evolution.

� It is a fundamental agent shaping the surfaces of all solid planetary bodies (planets, asteroids, comet nuclei).

� Impact events had a decisive, yet poorly understood, role in Archean geological evolution.

� Impact crater counting provides a means for relative surface chronology on planetary bodies accessible only to remote sensing.

� Impact craters provide important targets for the study of properties (e.g. presence of water, near-surface stratigraphy, sedimentary processes) on otherwise inaccessible bodies.

� Asteroids and comets can be studied through surface analysis (chronology, strength analysis) or artificial impact experiments. These studies yield information on target composition and behavior.

� Catastrophic impact events have had a role throughout Earth history in the development of life and have been implicated widely in the debates about the causes of mass extinctions and major radiation events.

� The risk of future impacts on Earth should be assessed.

� Economic resources have been accumulated as a result of impacts (ore deposits, fossil fuel reserves, water reservoirs), and many impact structures represent sites of high educational, and sometimes also recreational, value.

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Deutsch 2012). There is also a series of shock metamorphic calibrations for felsic and mafic target rocks, into which the experimental findings for several target-rock minerals have been integrated (Stöffler and Grieve 2007).

RECOGNITION CRITERIA FOR IMPACT STRUCTURES AND DEPOSITSIn the interest of the continuing improvement of the terres-trial impact-cratering record, and particularly of the ancient impact record, recognition criteria for the products of impact are required (box 2) (French and Koeberl 2010; Koeberl et al. 2012 this issue). The geometry and morphology of suspected impact structures observed by remote sensing, circular and annular geophysical anoma-lies, and seismic information can all provide valuable first hints for the presence of an impact structure; however, in each case, confirmation by ground truthing is required (Reimold and Koeberl 2008). Where a structure is known from remote sensing data only and is not accessible at surface, drilling is mandatory to obtain rock samples and evidence of shock metamorphism. Field work is required to search for shatter cones (see inset in Fig. 3), which have been described only in impact structures and in man-made nuclear explosions. Rock breccias, especially those with melt, are important, as they may contain shock-metamor-phosed lithic or mineral inclusions covering a wide range of shock pressures and may be suitable for dating (Jourdan et al. 2012).

Shock deformation in quartz (e.g. PDFs) is most important, because this mineral occurs in numerous crustal rock types. Zircon, which is particularly resistant to weathering and metamorphic overprint, is very useful for the investi-gation of the presence of shock deformation and for deter-mining age (Jourdan et al. 2012), especially of old, strongly overprinted rocks.

Box 2 RECOGNITION CRITERIA FOR IMPACT STRUCTURES AND DEPOSITS

First indications may be a circular structure that is anomalous in the regional geology, or a circular or annular geophysical anomaly; neither is diagnostic and an impact association must be confirmed by the presence of the following criteria:

� Shatter cones – To date they have only been described from impact structures (and nuclear explosions), where they are formed in the shock pressure range of ~2 to 30 GPa.

� Diagnostic shock-metamorphic effects – planar deformation features; planar fractures in zircon; diaplectic glass; high-pressure mineral polymorphs such as diamond, coesite (in upper crustal rocks), stishovite, and ringwoodite (Langenhorst and Deutsch 2012; Collins et al. 2012)

� Chemical or isotopic traces of an extraterrestrial projectile (Koeberl et al. 2012)

Impact melt rock has the added potential to contain traces of the meteoritic projectile (Koeberl et al. 2012). Upon impact, most of the mass of the projectile is vaporized and only a small amount may be incorporated into impact melt rock, which is generated close to the point of projectile disintegration. Sophisticated analysis, at the highest possible resolution, of the abundances of elements that might distinguish projectile and target rock is required. Much of the past work has focused on the analysis of sidero-phile elements such as Ni and Co, but lately, the use of the

platinum-group elements (PGEs) and of several isotopic methods (Re–Os and Cr isotope systematics) has become prominent. These methods have allowed the detection of very small amounts (<0.2%) of components derived from the extraterrestrial impactor and, in some cases, the iden-tification of the nature of the impactor.

THE TOOLS OF IMPACT-CRATERING STUDIESOver the last 50 years, we have made great progress in understanding the physical and chemical processes related to this complex, catastrophic, and highly dynamic (ultra-high shock pressures and temperatures, very high strain rates) phenomenon. However, there are still significant gaps in our knowledge. For example, the generation of the common impact breccia known as suevite is not sufficiently understood.

At the very least, five vital aspects of impact cratering require further exploration:

1. Our understanding of the basic process must be improved to have a better comprehension of impact energy generation and distribution.

2. The existing data do not adequately allow the identifica-tion of rocks that were impact-deformed in the low-shock-pressure regime, below the onset of PDF formation (ca 8–10 GPa). Notably, rocks subjected to such low shock pressure represent the major part of the entire affected target-rock volume.

3. The interaction of shock waves (i.e. shock metamor-phism) with some geological materials is not well under-stood (e.g. basaltic rocks, despite their ubiquity in the Solar System).

4. We need to know if, besides shatter cones, there are other macro- or mesoscopic criteria that could unam-biguously confirm the presence of impact structures in the field.

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Figure 3 Pressure (P)–temperature (T ) diagram, showing the different regimes for “normal” crustal metamorphism

and shock metamorphism. Various P–T regions characterized by the development of specific shock-metamorphic effects are indicated. Also shown are the stability fields for quartz (Qz), coesite (Coe), and stishovite (Stv), typical Raman spectra for these minerals, and liquidi for the various SiO2 polymorphs. SC = shatter cone; PDF = planar deformation feature. dIagram by Jörg frItz, museum für NaturkuNde berlIN

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REFERENCESCanup RM, Asphaug E (2001) Origin of

the Moon in a giant impact near the end of the Earth’s formation. Nature 412: 708-712

Collins GS, Melosh HI, Osinski GS (2012) The impact-cratering process. Elements 8: 25-30

Earth Impact Database (2011) www.unb.ca/passc/ImpactDatabase

French BM (1998) Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures. Lunar and Planetary Institute, Houston, TX, Contribution CB-954, 120 pp

French BM, Koeberl C (2010) The convincing identification of terrestrial meteorite impact structures: What works, what doesn’t, and why. Earth-Science Reviews 98: 123-170

Gibson RL, Reimold WU (2008) The Geology of the Vredefort Impact Structure. Council for Geoscience, Pretoria, Memoir 97, 181 pp

Glass BP, Simonson BM (2012) Distal impact ejecta layers: Spherules and more. Elements 8: 43-48

Grieve RAF, Stöffler D, Langenhorst F (1996) Shock metamorphism in nature and experiment. II. Significance in geoscience. Meteoritics & Planetary Science 31: 6-35

Grieve RAF, Cintala MJ, Therriault AM (2006) Large-scale impacts and the evolution of the Earth’s crust: The early years. In: Reimold WU, Gibson RL (eds) Processes on the Early Earth. Geological Society of America, Boulder, Special Paper 405, pp 23-31

Grieve RAF, Reimold WU, Morgan J, Riller U, Pilkington M (2008) Observations and interpretations at Vredefort, Sudbury and Chicxulub: Towards an empirical model of terrestrial impact basin formation. Meteoritics & Planetary Science 43: 855-882

Jourdan F, Reimold WU, Deutsch A (2012) Dating terrestrial impact structures. Elements 8: 49-53

Koeberl C (2006a) Impact processes on the early Earth. Elements 2: 211-216

Koeberl C (2006b) The record of impact processes on the early Earth: A review of the first 2.5 billion years. In: Reimold WU, Gibson RL (eds) Processes on the Early Earth. Geological Society of America, Boulder, Special Paper 405, pp 1-22

Koeberl C, Claeys P, Hecht L, McDonald I (2012) Geochemistry of impactites. Elements 8: 37-42

Langenhorst F, Deutsch A (2012) Shock metamorphism of minerals. Elements 8: 31-36

Melosh HJ (1989) Impact Cratering: A Geologic Process. Oxford University Press, New York, NY, 245 pp

Pierazzo E, Artemieva NA (2012) Local and global environmental effects of impacts on Earth. Elements 8: 55-60

Reimold WU, Koeberl C (2008) Catastrophes, extinctions and evolu-tion: 50 years of impact cratering studies. In: Gupta H, Fareeduddin F

5. The Earth’s impact record through time must be further improved to fully understand the role of impact catas-trophes, in particular with respect to the evolution of the planet and of life (e.g. Pierazzo and Artemieva 2012 this issue).

Impact-cratering and impactite studies have been conducted using four methodologies: (1) multidisciplinary analysis of impact structures; (2) laboratory studies of impact-affected rocks and minerals; (3) experimental calibration of shock-metamorphic stages and experimental cratering; and (4) numerical-modeling approaches designed to investigate, in great detail, the influence of key parameters on the cratering process, but also to constrain the environmental effects of large-scale, catastrophic impact (Pierazzo and Artemieva 2012).

Impact-cratering studies are a vibrant, integrated, multi-disciplinary branch of science. They have brought together scientists from astronomy and planetary exploration, geology, geophysics, remote sensing, mineralogy, geochem-istry, geochronology, life sciences, and other fields. At the same time, other disciplines have now taken note of the

importance of impact events and, in recent years, chapters about impact cratering have regularly appeared in new textbooks. In IMPACT! we share with readers of Elements our enthusiasm for this burgeoning branch of Earth and planetary sciences.

ACKNOWLEDGMENTSThis article benefited from insightful reviews by Richard Grieve and Sandro Montanari. We are particularly grateful to the authors of the articles in this issue for their expert work and their great support during the various phases of preparation of the issue. Elements editors Pierrette Tremblay and Hap McSween were instrumental in keeping us on our toes and made valuable input throughout the process. The following reviewers enhanced the standard of the papers: Gareth Collins, Richard Grieve, Peter Haines, Friedrich Hörz, Christian Koeberl, Sandro Montanari, Marc Norman, Birger Rasmussen, Birger Schmitz, Claudia Trepmann, Mario Trieloff, Axel Wittmann, Kai Wünnemann, and an anonymous referee. We also thank all colleagues who made their material available for inclusion in these articles.

Dedication of this issue to Elisabetta (Betty) Pierazzo On 15 May 2011, during the finalization of the papers in this issue, one of our contributors and a close friend, Dr. Elisabetta (Betty) Pierazzo, passed away at the age of only 47 after a short but serious illness. The authors and editors of this issue would like to honor Betty by dedicating IMPACT! to her and her scientific achieve-ments. We all have lost a very close friend and esteemed colleague in the impact-cratering field.

Betty was a senior scientist at the Planetary Science Institute, Tucson (USA), and an expert in the fields of impact modeling and planetology, work that certainly took her around the Solar System. She made major contributions to the understanding of the Chicxulub impact and its environmental consequences and to cratering processes during the formation of Meteor Crater. She placed constraints on the thickness of ice on Jupiter’s satellite Europa, and she investigated the possible delivery of organics to planets and Europa by comets, as well as hydrothermal systems related to large impacts on Mars. She led a benchmark effort to cross-validate the main numerical-modeling software and methods, was passionate about science education and public outreach, and loved playing soccer. Betty is sorely missed by us— a cherished friend and dear colleague.

Betty Pierazzo in her element – doing fieldwork at Wetumpka impact crater (Alabama, USA)

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GLOSSARY

Asteroid – a rocky or metallic body, ranging in size from meters to hundreds of kilometers, in orbit around the Sun. Many asteroids are found in the asteroid belt between the orbits of Mars and Jupiter.

Comet – a minor body that is composed mainly of water ice and some silicate (rocky) material. Comets originate in the Kuiper-Edgeworth Belt and the Oort Cloud, both far outside the Solar System.

Impact breccia – a type of impactite formed entirely of fragments of rock, minerals, or melt

Impact crater/structure – the geological structure resulting from the impact of an extraterrestrial body. Degradation of a crater feature due to erosion, sedimen-tary cover, or tectonic dismemberment can seriously change the original crater geometry, and one should then use the term impact structure.

Impactite – a rock formed from the impact of a large extra-terrestrial projectile

Impact melt rock – Following target penetration by the projectile, shock pressures are so high that associated postshock temperatures may exceed the melting temper-ature of the target rock, producing bulk or partial melts.

Lithic impact breccia – an impact breccia that is composed entirely of mineral and rock (lithic) fragments derived from the target rock. One distinguishes monomict (only one rock type) and polymict varieties.

Diaplectite glass (including maskelynite) – a glass-like (i.e. isotropic) material produced by shock metamor-phism. The precursor mineral has not been fused but has attained a glass-like state in which the original crystal shape and defects (inclusions, fractures) are preserved; maskelynite is the diaplectic glass phase after plagioclase.

Mass extinction – the demise of a large number of biolog-ical species in a relatively short geological time, thereby greatly exceeding the normal “background” extinction rate

Planar deformation features (PDFs) – microscopic features in rock-forming minerals that originate from elevated shock metamorphism. PDFs are parallel, extremely narrow, lamellar, isotropic features that have crystallographically controlled orientations.

Platinum-group elements (PGEs) – the elements ruthe-nium, rhodium, palladium, rhenium, osmium, iridium, and platinum. They are all strongly siderophile and are highly enriched in meteorites in comparison to rocks of the Earth’s crust.

Shatter cone – a conical fracture phenomenon in rock, with narrow ridges (striae, strahlen) emanating from a small apex area (see inset to Fig. 3). Shatter cones occur at scales from centimeters to meters; they are only known from impact structures and, thus, are considered to be the only known mesoscopic impact-diagnostic criterion. The shatter cone–forming process is still largely unknown.

Shock front – the instance of change from material prop-erties just before to material properties under compression. The duration of this change is ultrashort for relatively small-scale laboratory-based shock experiments, but is significantly longer in natural impact events.

Shock metamorphism (also known as impact meta-morphism) – the irreversible mineral and rock deforma-tion resulting from material compression caused by a shock wave. Whereas conditions of normal crustal meta-morphism range from ambient temperature to about 1000 °C and from atmospheric pressure to perhaps 10 kbar, shock metamorphism ranges up to hundreds of gigapascals (1 GPa = 10 kbar ≈ 10,000 atm).

Shock wave – a large-amplitude plastic compression wave. The amplitude of the shock wave in a given material is in excess of the elastic limit of a material; therefore, deformation effects resulting from shock wave deforma-tion are, at relatively low shock pressures, limited to mechanical deformation (fracturing, cataclasis), but beyond the elastic limit, they involve plastic deformation or even phase transformations.

Suevite – an impact breccia formed from rock and mineral fragments derived from the target rock(s) plus cogenetic particles of impact melt. The type locality for suevite is the Ries crater (Germany).

Target rock – the rock type(s) affected by the impact of an extraterrestrial projectile. Fragments of the target material are found in impact breccia, and the relative abundance of different rock fragments can possibly be used to infer the pre-impact geological composition of the impact-affected rock volume.

Tektites – small (millimeters to a few centimeters) glassy bodies of ballistically transported impact melt formed from uppermost target material. Some regional-scale tektite strewn fields are unambiguously linked to specific impact structures (e.g. moldavites in central Europe are related to the Ries Crater, and the Ivory Coast tektites to the Bosumtwi crater in Ghana). Microtektites are less than 1 mm in size.

(eds) Recent Advances in Earth System Science. Geological Society of India, Bangalore, Memoir 66, pp 69-110

Reimold WU, Horton JW Jr, Schmitt RT (2008) Debate about impactite nomen-clature – recent problems. In: Gibson RL, Reimold WU (compilers) Large Meteorite Impacts and Planetary Evolution IV, Vredefort, August 17–21, 2008, Vredefort Dome, South Africa. Lunar and Planetary Institute Contribution No 1423, 2 pp

Stöffler D, Grieve RAF (2007) Impactites. In: Fettes D, Desmons J (eds) Metamorphic Rocks: A Classification and Glossary of Terms.

Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks. Cambridge University Press, Cambridge, UK, pp 82-92, 111-125, and 126-242

Stöffler D, Langenhorst F (1994) Shock metamorphism in nature and experi-ment. I. Basic observations and theory. Meteoritics 29: 155-181

Stöffler D, Ryder G, Artemieva NA, Cintala MJ, Grieve RAF, Ivanov BA (2006) Cratering history and lunar chronology. Reviews in Mineralogy & Geochemistry 60: 519-596

Turtle ED, Pierazzo E, Collins GS, Osinski GR, Melosh HJ, Morgan JV, Reimold WU (2005) Impact structures: What does crater diameter mean? In: Kenkmann T, Hörz F, Deutsch A (eds) Large Meteorite Impacts III. Geological Society of America, Boulder, CO, Special Paper 384, pp 1-24