Catastrophes, Extinctions and Evolution: 50 Years of ... · Catastrophes, Extinctions and Evolution: 50 Years of Impact Cratering Studies ... “New Catastrophism”, with main areas

GOLDEN JUBILEE MEMOIR OF THE GEOLOGICAL SOCIETY OF INDIANo.66, 2008, pp.69-110

Catastrophes, Extinctions and Evolution:50 Years of Impact Cratering Studies

W. UWE REIMOLD1 and CHRISTIAN KOEBERL

2

1Museum for Natural History (Mineralogy), Humboldt University Berlin,Invalidenstrasse 43, 10115 Berlin, Germany

2Department of Lithospheric Research, University of Vienna,Althanstrasse 14, A-1090Vienna, Austria

Email: [email protected]; [email protected]

AbstractFollowing 50 years of space exploration and the concomitant recognition

that all surfaces of solid planetary bodies in the solar system as well as of cometnuclei have been decisively structured by impact bombardment over the 4.56 Gaevolution of this planetary system, impact cratering has grown into a fullydeveloped, multidisciplinary research discipline within the earth and planetarysciences. No sub-discipline of the geosciences has remained untouched by this“New Catastrophism”, with main areas of research including, inter alia, formationof early crust and atmosphere on Earth, transfer of water onto Earth by cometsand transfer of life between planets (“lithopanspermia”), impact flux throughoutEarth evolution, and associated mass extinctions, recognition of distal impactejecta throughout the stratigraphic record, and the potential that impact structuresettings might have for the development of new – and then also ancient – life. Inthis contribution the main principles of impact cratering are sketched, the mainfindings of the past decades are reviewed, and we discuss the major currentresearch thrusts. From both scientific interests and geohazard demands, thisdiscipline requires further major research efforts – and the need to educate everygeoscience graduate about the salient facts concerning this high-energy geologicalprocess that has the potential to be destructive on a local to global scale.

INTRODUCTION

The year 2008 approximately coincides with the 50th anniversary of spaceexploration, which has gone hand in hand with the second major 20th centuryrevolution of the earth sciences (the first one having been the eventual victoryof continental drift theory and then plate tectonics) transformed into planetaryscience (e.g., Reimold, 2007a). It has been demonstrated that all solid surfaces

70 W. UWE REIMOLD AND CHRISTIAN KOEBERL

of planetary bodies in the solar system – from the terrestrial planets to theminor planets, also known as asteroids, and even comet nuclei have beenbombarded with cosmic projectiles (e.g., Fig. 1). Eugene M. Shoemaker, thefamous pioneer of planetary geology and impact cratering studies, already in1977 declared that impact was the fundamental process forming planetarysurfaces.

In July 1994, the fragments of comet Shoemaker-Levy 9 crashed into theatmosphere of planet Jupiter, producing impact plumes the size of our ownplanet, and brought home to billions of people all over the world that impact

Fig.1. Messenger spacecraft image of a portion of the surface of planet Mercury (14 January2008). This area near Mercury’s terminator shows a range of different impact cratergeometries, from small simple bowl-shape craters to complex crater structures with centraluplift features, and a very large crater with a peak ring structure (upper right). This verylarge crater measures about 135 km in diameter. Note the smooth interior of many largecraters, which is interpreted as being due to flooding with lava. Clearly these smoothterranes are much younger than the regions between large impact structures, as they carrymuch less impact structures than these. NASA, Messenger image 210544main_geo_hist2.

50 YEARS OF IMPACT CRATERING STUDIES 71

catastrophes are not only a matter of the distant past but that they could happenany time, any place in the solar system – also on Earth. And this article iswritten just about 100 years after the only well documented impact catastropheof the 20th century, namely the Tunguska explosion of 30 June 1908 thatdevastated some 2000 km2 of Siberian forests, and caused environmental effectsas far as London, and an air pressure signal around the world (Cohen, 2008;Steel, 2008). Having been debated as the result of impact as much as of theexplosion of a nuclear UFO, it is now widely accepted that this explosion wascaused by the explosive disruption of a small (maybe 30 to 50 m wide) asteroidwithin the atmosphere, with an explosive energy of about 5 to 10 megatonsTNT-equivalent.

Even more recently, a small cosmic projectile impacted in Peru. TheCarancas meteorite created a ca. 14-m-wide impact crater (Kenkmann et al.2008; Schultz et al. 2008; Tancredi et al. 2008), in a rather remote area of thatcountry, and besides several people sustaining a significant scare, no casualtiesare known. However, even though this impact event was of insignificant sizeon a planetary scale (the largest impact structure known in the solar system,the South Pole-Aitken basin in the south polar region of the Moon, measuressome 2500 km in diameter), there would have been hundreds of victims if thisevent had occurred in a densely populated area of our planet. Clearly theunderstanding of impact processes is an important issue for mankind – especiallyif one considers what happened to the dinosaurs and their contemporaries 65million years ago. The impact of a 5-15 km asteroid at Chicxulub on the Yucatánpeninsula generated then a 180 km diameter impact structure with globalcatastrophic effects (e.g., Sharpton et al. 1993, and papers in Ryder et al. 1996;Koeberl and MacLeod, 2002).

In this contribution we draw attention to the importance of impact crateringas a planetary process, as a catastrophic process that may severely affectbiodiversity on our planet, as well as how the investigation of this process andits effects has permeated all disciplines of the geological sciences.

The Impact Crater Record: General Considerations

A very detailed introduction to this topic has recently been published byStöffler et al. (2006), who reviewed the lunar and terrestrial impact record. Interms of nomenclature, we need to clarify right at the beginning that an impact“crater” refers to a fresh, uneroded crater-shaped, impact structure. Where,however, extensive degradation has taken place, or a very complex feature isdiscussed, the term “impact structure” is more appropriate. Generally, fourdifferent impact structure types are distinguished according to geometry, andrelated to increasing size of a crater structure: simple, bowl-shaped structures,complex impact structures with modified crater rims (terraces) and a central


uplift feature, and multi-ring impact structures. Amongst the complex impactstructures, the central uplift formations can be peak-shaped or, when the peakhas collapsed, peak ring geometries can be developed (see Pati and Reimold,2006). Several such geometries are shown in Figure 1, an image of a small partof the surface of planet Mercury, taken in fly-by in January 2008 by NASA’sMessenger space probe. General geometric/morphological consideration ofand nomenclature for impact structures have been discussed by Turtle et al.(2005).

In the terrestrial impact record, simple bowl-shaped craters are formed insedimentary and crystalline rocks when the crater diameter is smaller thanabout 2 or 4 km, respectively. Complex peak or peak-ring structures are knownfrom 2 to 4 km upward to more than 100 km diameters. Fresh simple cratershave an apparent depth (i.e., the distance from top of crater rim to present-daycrater floor) of about one-third of the apparent crater diameter (clearly boththese apparent values depend on the degree of erosion). Apparent crater depthfor complex structures is more like one-fifth to one-sixth of a diameter. Andcrater geometry depends strongly on planetary gravity and atmosphericconditions. The lunar gravitational acceleration, which is approximately one-sixth of the terrestrial value, leads to the formation of comparatively deeperimpact craters on the Moon, and a larger changeover diameter from simple tocomplex craters compared to the terrestrial situation. The reason for this is thatgravity counteracts excavation of material and the development of topography.

Finally, only three very large impact structures of suspected “multi-ringbasin” type (Spudis, 1993) are known on Earth: Chicxulub in Mexico of 65Ma age and 180 km diameter, Sudbury in Canada – 1.85 Gyr old and ca. 200-250 km original diameter, and Vredefort in South Africa (Fig. 2), at 2.02 Gyrage and 250-300 km original diameter widely held the largest and oldest knownimpact structure on Earth. Stöffler et al. (2006) reviewed some structuralparameters of lunar and terrestrial impact structures. Melosh (1989) is still theonly dedicated treatise of the impact cratering process, with regard to the variousstages of development of impact structures, and ejecta distribution. Otherreviews providing an entry into impact cratering studies have been publishedby, e.g., French (1998), Grieve et al. (1996; 2007), Reimold (2007a), Koeberl(2002).

The enormous kinetic energy supplanted by hypervelocity extraterrestrialprojectiles (at ca.11 to 72 km velocity, with average asteroidal and cometaryvelocities of 15 and 30 km per second, respectively) onto target rock triggers acatastrophic deformation and disruption process. The target region is partiallyexcavated, and partially the target rocks are vaporized, melted, or subjected toplastic and elastic deformation. For example, French (1989), Stöffler andLangenhorst (1994), and Grieve et al. (1996) have provided some introductory


works relating to some aspects of the formation of an impact crater andassociated impact metamorphism of the target materials. Also known as “shockmetamorphism”, this process involves those mineral and rock deformationsthat demand extreme pressures and temperatures, as well as ultra-high strainrates, far in excess of the conditions related to internally driven metamorphism– especially in near-surface environments.

The Impact Cratering Process

Very detailed accounts of the impact cratering process and the threemain stages (compression, excavation, and modification) are provided by e.g.,Melosh (1989), Melosh and Ivanov (1999), and Stöffler et al. (2006). Uponcontact of an extraterrestrial projectile with the target surface, shock wavesare generated and travel both into the target and into the impactor. Behind ashock wave, a release wave leads to material flowing out of the forming craterwith enormous velocity. Obviously the material properties of target and impactorare relevant with regard to the development of the flow field, but also projectilevelocity and angle of impact are important parameters.

Fig.2. Vredefort Dome – the deeply eroded central uplift of the Vredefort impact structure. Thecentral part (core) of the circular dome is ca. 45 km in diameter, and the prominent ridgesof the metasedimentary/-volcanic collar extend the diameter of the dome to ca. 90 km.Source: GoogleEarth


The compression phase involves the time between the initial contact ofthe projectile with the target until the moment when the entire impactor hasbeen affected by the shock compression wave. At impact velocities of tens ofkilometers per second, the compression phase involves extreme stresses, hightemperatures, and very high strain rates in the affected materials. Most materialof the impactor and of the vicinity of the impact point is vaporized or melted.Pieces of the impactor may be spalled off, and some of its material may bemixed with similarly affected target material. When the shock front travellingthrough the projectile reaches the free surface at its end, it is reflected forwardas a so-called “rarefaction wave”. At small and young craters that have formedby iron meteorites (e.g., Meteor Crater, Arizona; Henbury, Australia), fragmentsamounting to a few percent of the original mass of the impactor are commonlyfound (e.g., Schnabel et al. 1999; Melosh and Collins, 2005; Bevan, 1996).After decades of investigation of terrestrial impact structures, only very littleand extremely rare fragments of meteoritic matter have been recovered fromimpact melt rock or ejecta. One exception is a meteoritic fragment a few 10s ofcentimeters in size that was recovered from the Morokweng impact melt rockby Meier et al. (2006).

The excavation stage begins once the kinetic energy of the projectile hasbeen completely transferred to the target. The shock front travelling into thetarget – radially away from the contact point – is gradually weakened. Energyis converted into heat and used up in irreversible change of material states. Inthe words of Stöffler et al. (2006), “passage of the shock front imparts a velocityto the target material in a direction parallel to the movement of the front itself… Because the shock is hemispherical in a reasonably homogeneous target,the initial motion of the target material is radial from the impact site.” Once therarefaction front moves through the shocked material, the overall movementof target material will be at first directed downward and outward but thengradually turn upward toward the surface – as a consequence of thedecompression process. One distinguishes two volumes within the transientcrater – the ejected material and the displaced material (see also French, 1989).The ejected material seems to come primarily from the upper part of the transientcrater, whereas the remaining – about half – volume is pushed (displaced)downward/outward.

The modification stage begins at that time when the maximum depth ofthe transient cavity has been reached. The crater floor then is raised – in smallcraters just a bit but in larger ones significantly. Material strength is of greatimportance, as is gravity on the impacted body. The enormous amount of impactdeposited energy in the compressed target likely plays an important role aswell. It has been suggested that the rising material can be compared with aBingham fluid, and that its motion be regarded as a process known as “viscous


fluidization” (Melosh, 1989; Melosh and Ivanov, 1999). Stratigraphic uplifthas been scaled, based on natural observations (e.g., Melosh, 1989). In thecase of the very large Vredefort impact structure, the amount of stratigraphicuplift recorded in the central uplift known as the Vredefort Dome is consideredsome 20 km, against a 250-300 km diameter of the original impact structure.

Shock Metamorphism

Shock pressure and related post-shock temperature decrease radiallyaway from the point of explosion of the projectile. Where entire impactstructures are accessible for detailed mineralogical analysis, the concept ofprogressive shock metamorphism (i.e., progressive increase of shock pressureand temperature towards the epicenter of the event) could be developed. Withincreasing grade of deformation, specific rock mineral deformations andtransformations have been recognized. These, in turn, have been calibrated innumerous shock recovery experiments with constrained shock pressure andpre-shock temperature of the target rock. To date, by far the majority of shockrecovery experiments has been conducted with targets held at room temperature,but the limited number of experiments with pre-heated targets has shownunequivocally that equivalent shock degrees are reached in pre-heatedspecimens at significantly lower shock pressures compared to room temperatureexperiments. The effect of pre-cooling of targets on shock degree has not beenevaluated yet, but is important for those bodies in the solar system that havecold surfaces (asteroids, Mars).

Further important parameters are porosity and water content of targets,so that a shock pressure calibration for sedimentary rock is different from thatfor crystalline rocks. Upper crustal, relatively cold rock reacts differently froma hot, mid-crustal material. French (1989), Stöffler and Langenhorst (1994),Grieve et al. (1996), Huffman and Reimold (1996), and Stöffler and Grieve(2007) provide extensive information on this topic. Osinski et al. (2008) recentlyinvestigated the conditions of impact melting in sedimentary rock. An examplefor the progressive change of deformation with increasing shock pressure isprovided by the mineral quartz. Shocked at room temperature, quartz crystalsare elastically deformed only below pressures of ca. 8 GPa. So-called shockextension fracturing and irregular or (sub)planar fractures are induced.Above this shock level, planar deformation features, so-called PDFs (Fig.3)are induced, first as single sets per host grain and with increasing pressureas two or more sets. Up to pressures of ca. 30 GPa PDFs are the dominantshock deformation effect, which involves partial transformation of crystallineto amorphous silica, as well as the production of a whole host of differentlattice defects (e.g., Gratz et al. 1992; Stöffler and Langenhorst, 1994). Inaddition, transformation of quartz to coesite can occur from ca 15 GPa on, and


Fig.3. Planar deformation features: (a-c) Three examples of planar deformation features in quartzand feldspar in a sample of alkali granite from the Schurwedraai complex in the VredefortDome (courtesy Claudia Crasselt, Museum for Natural History): (a) Two sets of PDF ina plagioclase crystal. Crossed polarizers. Width of field of view 0.52 mm. (b) DecoratedPDF in a partially “toasted” quartz grain. Plane polarized light, width of field of view1.03 mm. (c) PDF sets trending NE-SW and NE-SE, as well as WNW-ESE in microcline.Crossed polarizers, width of field of view 0.52 mm. (d-e) Two images of planar deformationfeatures obtained at the TEM (transmission electron microscope) scale – courtesy ThomasKenkmann (Museum for Natural History Berlin): (d) Transmission electron micrographof an experimentally shock-deformed quartzite-dunite compound specimen: visible areamorphous PDF lamellae in olivine (length of the short edge is 1.8 µm). Original workby Kenkmann et al. (2000). (e) Bright field transmission electron micrograph of a quartzgrain from the Kayenta Formation of the Upheaval Dome impact structure (for moredetails see Buchner and Kenkmann, 2008). The lamellae along rhomboidal quartz planesare straight and parallel and have variable thickness. The original glassy lamellae arehealed and show extremely high defect densities, and are decorated by fluid inclusions.The short edge of a lamella corresponds to a length of 5 µm.

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above ca. 30 GPa, also transformation to stishovite. From this shock degreeon, amorphization of quartz has progressed so strongly that entire crystals areconverted to so-called diaplectic glass (formed through solid-statetransformation not involving a liquid phase). Once the shock temperature ofminerals begins to exceed the liquidus temperatures of individual minerals,shock melting takes place, from about 45 GPa onward; above 55-60 GPa, bulkrock melting is initiated. For a large number of important rock-forming minerals,Pati and Reimold (2006) provided a summary diagram detailing the importanttransitions of progressive shock pressure/temperature increase. The optical andelectron microscopic (e.g., Goltrant et al.1991; Gratz et al. 1996) confirmationof shock metamorphic effects in rock represents the best recognition criterionfor an impact deformed rock or the presence of an impact structure. Only onemacroscopic rock deformation structure, so-called shatter cones (Fig.4; e.g.,Wieland et al. 2006, for a recent review of literature about this phenomenon),are generally considered useful for unambiguous recognition of impactstructures.

Impact-derived rocks, so-called impactites, are generated in differentsettings of and around an impact structure (Stöffler and Grieve 2007; Stöfflerand Reimold, 2006). One distinguishes proximal and distal impactites, accordingto a very detailed schematic proposed recently by Stöffler and Grieve (2007).This classification of impactites involves crater facies, especially cataclasites

Fig.4. Shatter cones: (a) Large shatter cone from the newly discovered Santa Fe site in NewMexico (Fackelman et al. 2008). Note the prominent horse-tailing on the surface of thelarge cone. Scale bar in centimeter intervals. (b) Shatter cones in limestone from theWaqf as Suwwan impact structure in Jordan (Salameh et al. 2006, 2008).

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(so-called monomict or polymict lithic breccias), suevites (clastic matrixbreccias with a melt fragment component, and impact melt rock composed ofmelt matrix and a more or less abundant clast component. Proximal ejectainclude suevitic and lithic impact breccia ejecta, and distal ejecta includetektites and spherule beds such as the one known from the Cretaceous-Tertiaryboundary (Smit, 1999). In addition, one observes impact breccia injectionsinto the crater floor and those may involve lithic, suevitic, and impact meltveins, as well as a range of rocks (Fig. 5) that in the past have been summarilytermed “pseudotachylite” but that recently have been classified (e.g., Reimold

Fig.5. Bedding-parallel generation plane with several cm thick pseudotachylitic breccia. Aninjection perpendicular to bedding extends off the generation vein. Hammer for scaleca. 30 cm long. Thwane Hill, Hospital Hill Quartzite, western collar of the VredefortDome (Photo: W.U. Reimold).

and Gibson, 2005, 2006, and references therein) as “pseudotachylitic breccias”in order to distinguish tectonic friction melt (pseudotachylite sensu strictu)from similar looking impact breccias of possibly diverse origin (impact/shockmelting, friction melting, decompression melting, possibly pre- or post-impacttectonic breccias in the target area/impact structure). Distal impact ejecta inthe terrestrial stratigraphic record have been reviewed by Simonson and Glass(2004). Detailed accounts of impact breccias, also dealing with lunar andmeteorite breccias, include Stöffler et al. (1980, 1991), French (1989), andDressler and Reimold (2001, 2004).

Impact Cratering - A New, Integrated Planetological/Geoscientific Discipline

Historically, the first impact structures have been recognized by the


presence of remnants of the projectile (remnants of iron meteorites, as atMeteor Crater or Henbury, among others). It was not before the mid-20th centurythat detailed geological studies of so-called cryptoexplosion structures werecarried out, and only in the early 1960s the first recognition criteria for impactstructures were established (see e.g., reviews in Mark, 1987; Koeberl 2001;Reimold, 2007a, b). To date, impact cratering studies have become a multi-disciplinary research field, involving astronomy and planetary exploration,geology, geophysics, remote sensing, mineralogy, geochemistry, biologicalsciences, and chronology, inter alia. The four legs that this new discipline isbuild on include geological site studies, laboratory-based mineralogical andchemical analysis, shock experimentation for the calibration of shock pressureand temperature levels and associated microdeformation, and the relativelynew but very successful and still promising method of numerical modelling.With this approach it is not only possible to compare natural and artificial –under controlled laboratory conditions generated – rock and mineraldeformation, but it is also possible to isolate individual parameters, such assize of projectile, velocity of projectile, impact angle, mineral compositionand porosity of the target, water content, etc., to get down to the basicsdetermining the physical and chemical response of a material to shock wavecompression and decompression. In this way, impact cratering has come a longway from its infancy just 50 years ago.

The Terr estrial Cratering Record – With Special Reference to Asia

The Earth Impact Database (2008) of the University of New Brunswickprovides detailed information on ca. 175 known impact structures. While thisincludes at least 3 unconfirmed structures (the two Arkenu structures in Libyaand the so-called Suavjärvi structure in Russia, for which no unambiguousimpact evidence has been provided - see e.g., Reimold, 2007), and does not yetinclude three recently confirmed impact structures for which publications arein press (Dhala in India, Santa Fe in the USA, and Waqf as Suwwan in Jordan;see below for details), this database is an invaluable compilation of imageryand literature about the world’s impact structures. Figure 6a illustrates thedistribution of the impact structures listed in the Earth Impact Database (2008).It is immediately obvious that the spatial distribution is skewed, and that thenumber of impact structures known in parts of Asia, in Africa, and SouthAmerica is inferior in terms of landmass in comparison with the other parts ofthe world. Reasons for this are multifold: on the one hand, impact structures inNorth America and Russia have been investigated extensively in the context ofspace exploration, once the predominance of crater structures on the Moonhad become known. The Australian impact record is partly the result of twopeople’s lifework – Carolyn and Gene Shoemaker – and some of their friends.


North African impact structures are known from the extensive oil explorationphase of the 1960s and 1970s, and some dedicated impact work has been donein South Africa in more recent decades (Reimold, 2006). In South America,only a handful interested geologists have been studying impact, and the vastrain-forest terrane has hampered recognition of circular features from space.

Fig.6. (a) World map of impact structures – modified after the compilation accessible on theEarth Impact Database (2008). Two recently discovered impact structures in Asia –Waqf as Suwwan in Jordan and Dhala in India have been added to the web-based image.(b) Enlargement of the Asia part of the Earth Impact Database compilation of impactstructures. Note the locations of Waqf as Suwwan (WaS) and Dhala (D) in Jordan andIndia, respectively. Both these structures have only recently been discovered – the formerthrough reassessment of older published reports of a cryptoexplosion structure, followedby field work (Salameh et al. 2006, 2008), the latter through remote sensing and subsequentground-based studies (Pati et al. 2008).

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The same holds for central Africa and parts of Asia. In Africa in particular,civil war has been a further adversary to ground-based geoscientific studies.

Figure 6b highlights the impact structures known in Asia, and Table 1provides a summary of the Asian structures and some detail about them. OnlyRussia and some other countries derived from the former USSR show somesites of known impact structures. Quite strangely only one impact structure isknown from the vast territory of Mongolia and none from China. Regardingthe latter, only one feature, a so-called Duolon structure (Wu, 1987), was everreferred in this huge country – and despite some detailed analytical work couldnot be confirmed. And in the whole of southern Asia and the Middle East, onlythree impact structures have been confirmed, the existence of which has beendetermined through the presence of impact melt rock and extensive evidenceof shock metamorphism, to date. Whereas major parts of southern and southeastAsia are covered with dense rain forest or are subject to intensive agricultural

Table 1. Confirmed impact structures in Asia, based on the Earth Impact Database (2008), and Pati et al.(2008) and Salameh et al. (2006, 2008). Target composition is denoted S – Sedimentary,C – Crystalline, M – Mixed.

Crater Name Location Latitude LongitudeDia. Age

Exposed Drilled Target(km) (Ma)

Beyenchime-Salaatin Russia N 71°0' E 121°40' 8 40 ± 20 Y N S

Bigach Kazakhstan N 48°34' E 82°1' 8 5±3 Y Y M

Chiyli Kazakhstan N 49°10' E 57°51' 5.5 46±7 Y Y S

Chukcha Russia N 75° 42' E 97° 48' 6 <70 Y Y M

Dhala India N 25° 18' E 78° 8' 11 1600 - 2500 Y (Y) C

El’gygytgyn Russia N 67° 30' E 172° 5' 18 3.5 ± 0.5 Y N C

Gusev Russia N 48° 26' E 40° 32' 3 49.0 ± 0.2 N Y S

Jänisjärvi Russia N 61° 58' E 30° 55' 14 700 ± 5 Y N C-M

Kaluga Russia N 54° 30' E 36° 12' 15 380 ± 5 N Y M

Kamensk Russia N 48° 21' E 40° 30' 25 49.0 ± 0.2 N Y S

Kara Russia N 69° 6' E 64° 9' 65 70.3 ± 2.2 N Y M

Kara-Kul Tajikistan N 39° 1' E 73° 27' 52 <5 Y N C

Karla Russia N 54° 55' E 48° 2' 10 5 ± 1 Y Y S

Kursk Russia N 51° 42' E 36° 0' 6 250 ± 80 N Y M

Longancha Russia N 65° 31' E 95° 56' 204 0 ± 20 N N M

Lonar India N 19° 58' E 76° 31' 1.83 0.05 ± 0.01 Y Y C

Macha Russia N 60° 6' E 117° 35' 0.3 <0.007 Y N S

Mishina Gora Russia N 58° 43' E 28° 3' 2.5 300 ± 50 Y Y M

Popigai Russia N 71° 39' E 111° 11' 100 35.7 ± 0.2 Y Y M

Puchezh-Katunki Russia N 56° 58' E 43° 43' 80 167 ± 3 N Y M

Ragozinka Russia N 58° 44' E 61° 48' 9 46 ± 3 N Y M

Shunak Kazakhstan N 47° 12' E 72° 42' 2.8 45 ± 10 Y Y C

Sikhote Alin Russia N 46° 7' E 134° 40' 0.02 0.000059 Y N C

Sobolev Russia N 46° 18' E 137° 52' 0.05 <0.001 Y Y M

Tabun-Khara-Obo Mongolia N 44° 7' E 109° 39' 1.3 150 ± 20 Y N C

Wabar Saudi Arabia N 21° 30' E 50° 28' 0.11 0.00014 Y N S

Waqf as Suwwan Jordan N 31° 3' E 36° 48' 5.5 <20 Y N S

Zhamanshin Kazakhstan N 48° 24' E 60° 58' 14 0.9 ± 0.1 Y Y M

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Fig.7. Panoramic view of the 1.8-km-diameter Lonar impact crater in Maharashtra Province, India (view from the East) (Photo: C. Koeberl).


use – both not conducive to remote sensing investigations, much of the MiddleEast lacks dense vegetation. There is only the 1.8 km wide Lonar impactcrater (Fig.7) in India, at 19°59’N, 76°31’E, which has been known andconfirmed for many years (e.g., Fredriksson et al. 1979; Fudali et al. 1980;Hagerty and Newsom, 2003; Kumar, 2005; Osae et al. 2005; Chakrabarti andBasu, 2006; Maloof et al. 2008).

But in addition, and not yet listed in the Earth Impact Database (2008),there are three new, confirmed impact structures in India, Jordan, and the USA.As first reported by Pati and Reimold (2006), Dhala (Fig. 8) is a ca. 11-15 kmwide ancient impact structure, the existence of which has been confirmed bythe presence of impact melt rock and extensive evidence of shock metamorphismin quartz and feldspar, including PDF, and of ballen quartz (Pati et al. 2008).The age of the structure is not well constrained, but must range between ca.1600 Ma, the age of the overlying Vindhayan metasediments, and 2500 Ma,which is the well constrained age of the impacted Archean basement consistingof a coarse-grained, pink granitic gneiss. This circular structure is centeredat 25º17’59.7"; 78º8'3.1" on the Central Indian Bundelkhand craton, Shivpuridistrict, Madhra Pradesh. It is a complex structure with a well-defined central

Fig.8. GoogleEarth scene centered on the centrally elevated area (CEA) in the central part of the11 km Dhala impact structure, India. The expansive monomict impact breccia exposuresare obvious in the form of a ring around the CEA. For more detail, refer to Pati et al.(2008).

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Fig.9. Waqf as Suwwan impact structure in Jordan (Salameh et al. 2006, 2008): (a) A panoramic view of the entire impact structure, from the outer rim composedof massive chert (which has given the structure its name: Mountain of Chert). Diameter of the entire structure, as shown, is 5.5 km. (b) Enlargement of theeroded yet still very well exposed central uplift. Note the strong deformation of the strata. Dark rocks are chert, the lighter strata comprise limestone andminor sandstone. The width of the central uplift is about 0.9 km.

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area comprising post-impact sediments. Without any drilling evidence it hasnot been possible yet to establish whether this central elevation covers relicsof the central uplift structure.

Waqf as Suwwan (Fig. 9a and b), located at 31°3’N / 36°48’E in the desertof eastern Jordan, has been known as a so-called cryptovolcanic structure(thought to have an unconstrained volcanic origin) since the 1960s. However,recent field work by Salameh et al. (2006, 2008) revealed a large number ofoccurrences of shatter cones, proof for the definite existence of an impactstructure. Waqf as Suwwan is about 5.5 km in diameter and despite some erosionsince its formation at estimated 10-15 Myr ago, still shows a well exposedcentral uplift structure. Fieldwork in 2008 has shown that exposure is excellent,and this impact structure must be ranked as one of the best exposed of its kindin sedimentary rock. Detailed multidisciplinary analysis (remote sensing,geology, geophysics) is underway. Jordanian scientists and some authoritiesare considering the possibility of making this site a protected conservationarea and possible geopark.

Similarly, the existence of shatter cones confirmed the presence of a large,deeply eroded impact structure near Santa Fe in New Mexico, USA (Fackelmanet al. 2008). The cones occur as a penetrative feature in intrusive igneous andsupracrustal metamorphic rocks; they are unusually large (up to 2 m long and0.5 m wide at the base; Fig. 4a). In thin section, quartz grains within the conesurfaces show optical mosaicism, as well as decorated planar fractures (PFs)and rare planar deformation features (PDFs). The PFs and PDFs are dominatedby basal (0001) crystallographic orientation, which indicates a peak shockpressure of up to 10 GPa that is consistent with shatter cone formation (French,1998). Regional structural and exhumation models, together with anomalousbreccia units that overlie and crosscut the shatter cone-bearing rocks, shouldprovide size and age constraints for the impact event. The observed shattercone outcrop area suggests that the minimum final crater diameter of the SantaFe impact structure was about 6–13 km and the age is so far only vaguelyconstrained between about 500 Ma and 1.2 Ga (Fackelman et al. 2008).

A small, 4 km wide crater-like feature known as Ramghar (Fig. 10), locatedat N25°20’, E76°37’ in India, has been repeatedly related to impact (e.g., Masterand Pandit, 1999), most recently by Sisodia et al. (2006a,b). These authorsclaimed to have recorded impact-diagnostic shock deformation, includingalleged planar deformation features (PDF) in quartz, diaplectic glass, andimpact spherules. This claim was, however, rejected by Reimold et al. (2006),who discussed point by point that none of the evidence presented constitutedbona fide impact diagnostic findings. And in the vicinity of Lonar impactcrater, a ca. 300 m wide lake-filled feature been known as Little Lonar(19°58’N, 76°31’E) or Ambar Lake (Master, 1999). Fredriksson et al. (1973a)


and Fudali et al. (1980) suggested that Little Lonar is a second impact craterformed by a fragment of the larger bolide that was responsible for Lonar crater.Master (1999) claimed the presence of breccias and alleges shatter cones atLittle Lonar. This evidence, however, remains to be confirmed – shatter conesare unknown to form from such very small craters, and the ejecta found in thearea are simply part of the close-by Lonar crater (e.g., Osae et al. 2005). Detailedobservations by Maloof et al. (2008) also make it very unlikely that LittleLonar is an impact feature.

Karanth (2006) and Karanth et al. (2006) reported on an unusual, 1.2 kmwide feature in Kachhh, western India, and related some unusual observationsto an impact origin. As in the case of Little Lonar and Ramghar, the evidence atthis site needs to be examined carefully to evaluate this claim.

When talking about the terrestrial impact record, the question of ageconstraints on the known impact structures and on distal impact ejecta, andwhat it tells us about the flux of large extraterrestrial projectiles onto this planetand their catastrophic effects on Earth and evolution of life on it, must beconsidered. Careful checking of the age data available for the known terrestrialimpact structures (Jourdan et al. 2007a; Renne et al. 2007) reveals that only agood handful (in fact, only 11) impact events have been dated with uncertaintiesbetter than 1%. In many cases, no absolute age data are available at all, or error

Fig.10. Ramghar “crater” structure (modified, courtesy GoogleEarth). The “crater” structure isabout 4 km wide.


limits as high as 10-20% have been reported. Major problems related to theprecise dating of impact melt rocks are inherent to the nature of the compleximpact breccias. Impact melt rocks are mixtures of impact generated meltplus clastic debris from the target rock. The latter may not be fully reset withrespect to the isotopic system used in the dating experiment. A preferred methodis U-Pb single zircon dating, whereby success depends largely on the time ofcrystallization of the impact melt. In some cases, a newly grown zircon phasecould be dated (e.g., in the case of Vredefort, South Africa: Kamo et al. 1996;Gibson et al. 1997), but in others only zircon crystals inherited from targetrock have become available (e.g., Dhala – unpublished work by our group).The use of argon dating techniques may also be obstructed by insufficientdegassing of target rock clasts – especially in those cases where the agedifference between the impact event and the formation of the target rocks isvery large (Jourdan et al. 2007b).

Recently new analytical improvements in zircon dating allowed agedeterminations with a precision of ~ ±0.2 Ma for the Precambrian (Davis, 2008).In a test of the new method, Davis (2008) reported that zircon from a noriticphase of the Sudbury impact melt gives 1849.53 ± 0.21 Ma, whereas a phasefrom several hundred meters higher in the noritic layer is resolvably youngerat 1849.11±0.19 Ma (95% confidence errors). This technique should help toimprove the dating record of impact craters if appropriate samples are available.

However, it is obvious that such a limited impact chronological data basecauses severe problems when investigating the projectile flux onto our planetthrough time. What is worse, by far the majority of impact structures known todate is younger than 350 Ma – which means that >90% of Earth evolution arenot represented in this database. Only two impact structures have been positivelyidentified as being of early Proterozoic origin – Sudbury and Vredefort. Inaddition, there are three (possibly 4) known distal ejecta layers, so-calledspherule layers (Fig. 11), of ca. 3.4 Ga age in the Barberton Mountain Land inSouth Africa, as well as one spherule layer in the Archean of Western Australia(e.g., Simonson and Glass, 2004; Kohl et al. 2006; Hofmann et al. 2006). Theseancient ejecta layers represent the entire record of Archean impact on this planet(Koeberl, 2006a,b; Grieve et al. 2006). There are also three spherule layersknown in the stratigraphy of the Transvaal Supergroup in South Africa, andalso three in the Hamersley Basin succession of Western Australia (e.g.,Simonson et al. 2004, 2008). The spherules in these strata have similar petro-graphic appearance, which has been studied and described in great detail byBruce Simonson and his co-workers. Simonson et al. (2008) propose that twoof these respective layers in South Africa and Western Australia seem to becorrelatives, whereas the respective third cases may not be related to eachother.


Two other spherule layer occurrences have been reported in the literature:one in South Greeenland and that has been, on very poor chronologicalconstraints, tentatively related to either the Sudbury or the Vredefort event(Chadwick et al. 2005), but the impact origin of that layer has not yet beenconfirmed. Secondly, actual Sudbury ejecta have been detected in Minnesotaand Michigan (Addison et al., 2005; Pufahl et al., 2007). It is certain that thereare remnants of further impact events that so far have gone undetected, buttraces of which could be identified in the extensive records of Phanerozoicrocks that have been drilled and studied in outcrop widely. Therefore, it ismandatory that every geology student be informed about the existence andcharacteristics of these spherule horizons.

In the Phanerozoic, a number of other stratigraphic horizons are associatedwith mass extinctions, and these have been repeatedly linked with impact.However, to date there is no conclusive evidence that mass extinctions in theLate Ordovician, Late Devonian, at the Triassic/Jurassic or at the Jurassic/Cretaceous boundaries, and the strong environmental change at the Paleocene/Eocene thermal maximum should be related to impact. Particularly extensivedebate has centered on a possible link between the mass extinction at thePermian/Triassic boundary (252 Ma ago) and impact (e.g., Chapman, 2005),and a number of possible impact sites have been promoted for this allegedevent. It was suggested that an alleged 120 km sized impact structure in WesternAustralia, known as Woodleigh, could be the smoking-gun for the P/Trcatastrophe, but this impact structure turned out to be too small at 60 km diameterto have caused a global catastrophe capable of more or less instantaneouslydestroying about 90% of all species at that time (Reimold et al. 2003). Also the

Fig.11. Dense aggregate of impact spherules from the Princeton Mine near Barberton town,Barberton Mountain Belt. The sample is from the BA-1 locality described by Koeberlet al. (1993) and Koeberl and Reimold (1995).


age of Woodleigh, for which at first a P/Tr boundary age was favored, wasquickly changed to a Late Devonian age (conveniently also at a time associatedwith a major mass extinction (for discussion of this problem see Renne et al.,2002).

This debate was followed by the controversial report of a so-called Bedoutimpact structure northwest of Australia (Becker et al. 2004). However, thevery existence of a large circular structure there and a fortuitous age for anassociated melt rock (impact or volcanic in origin) of allegedly 250 Ma haveboth been rejected since as evidence for impact (Renne et al. 2004; Müller etal. 2005). Also, reports of shock metamorphosed quartz from Antarctic andAustralian P/Tr boundary sites have been refuted (Langenhorst et al. 2005).Coney et al. (2007) evaluated the pro and cons of the various lines of evidencefor impact activity at the Permian/Triassic boundary and found no evidencefor any impact event. This agrees with geochemical data by Koeberl et al.(2004).

Spherules are, of course, also a feature of the K/T (more recently termedthe K/P – Cretaceous/Paleogene) boundary (Smit, 1999; Simonson and Glass,2004). In contrast to the Archean ejecta layers, in which only very rare shockedmaterial (Rasmussen and Koeberl, 2004) has been detected so far and the originof which by impact is solely constrained by trace chemical and isotopicsignatures, there are shock deformed and impact generated particles (shockedquartz, glass, microtektites, high-Ni spinel), besides PGE enrichments and Crand Os isotopic impact signatures (see below), in this world-wide stratigraphicmarker. In addition, the impact site for the K/T catastrophe has been discoveredand extensively studied. Two special volumes (MAPS, 2004a) have beendedicated to a major drilling project in this large impact structure, and thesecontributions provide a useful entry into the Chicxulub and K/T impactliterature. Interestingly, while some groups took a long time to becomecomfortable with the idea of a K/T impact catastrophe, some of these workershave since switched camps and are now favoring two, related closely in time,impact events – with only one of them linked to the Chicxulub impact (Kelleret al. 2003, 2007; Stüben et al. 2005). This idea has, however, found sternopposition by both impact and stratigraphic workers (e.g., Arenillas et al., 2006;Schulte et al., 2006). In a succinct but highly analytical discussion, Morrow(2006) placed “impacts and mass extinctions” under the spotlight.

Finally, there are four distal ejecta occurrences known as tektite (Fig. 12)strewn fields, as well as a so-called micro-krystite layer, which will be describedin more detail below. However, for completion of this section it must be notedthat three of these tektite strewn fields have been unequivocally related tosource impact craters: the Ivory Coast tektites to the 1.07 Ma Bosumtwi craterin Ghana, the moldavites of central Europe to the 14.5 Ma Ries crater in southern


Fig.12. Australasian tektites: (a) Examples of different tektite forms and shapes (Australasiantektites). Top left: drop-shaped indochinite. Bottom left: button-shaped thailandite withflow textures. Right: Fragment of a Muong Nong-type (layered) indochinite. (b) Map ofthe Australasian tektite/microtektite strewn field, after Glass and Koeberl (2006). Tektiteoccurrences on land are indicated schematically by Xs. Microtektite-bearing sites in theocean are indicated with open circles and black dots. The black dots indicate sites withthe highest concentrations of microtektites and associated unmelted ejecta (the twoconcentric lines indicate sites that have microtektite abundances between 20 and200/cm2, and beyond the outer semicircle all the sites have low microtektite abundances,<20/cm2). Locations marked with X in the ocean did not yield microtektites. The knownoccurrences of tektites on land and microtektites in marine sites are enclosed with adashed line that indicates the approximate limits of the strewn field as presently known.The plus sign within the dark grey oval in northern Indochina indicates the location thatbest explains the geographic variation in abundance of unmelted ejecta and microtektites.However, any location within the dark grey oval, which extends from southern Chinathrough northern Vietnam into the Gulf of Tonkin, explains the geographic variation inabundance of impact ejecta and microtektites equally well. The still undiscoveredAustralasian tektite/microtektite source crater is expected to lie within this region.

a

b


Germany, and the North American tektite strewn field to the 35.4 MaChesapeake Bay impact structure in Virginia (USA).

Impor tant Impact Cratering Research Issues and Thrusts

An extremely significant issue is the question of “how to recognize impactstructures”! From remote sensing and morphological observations, andgeophysical data important initial observations can be made. For example,circular drainage patterns or surface forms, and circular positive or negativepotential field anomalies may hint at the possible location of an impact structure.However, one should never forget that many other geological phenomena (e.g.,near-circular kimberlite pipes or other types of igneous intrusions may showdistinct magnetic anomalies; sink-holes and roundish pans derived from othergeologic or human activity may indicate negative gravity anomalies or shapessimilar to those of impact craters). In large, complex impact structures thecentral uplift of raised, relatively denser, originally deeper seated rocks mayindicate a slightly positive circular anomaly, surrounded by the negative annularanomaly related to low-density impact breccia fill of the ring basin around thecentral uplift. In the case of very old, deeply eroded impact structures, this maywell be the only remaining feature. Potential field anomalies for impactstructures have been reviewed by, for example, Pilkington and Grieve (1992)and Grieve and Pilkington (1996). Magnetic anomalies of impact structures, inparticular, may have quite variable signatures, due to the combination of inducedmagnetic character of the various target rocks and the impact-induced remanentmagnetization superposed on target rocks and impact breccias (see, e.g., thereview by Henkel and Reimold, 2002). Seismic studies may show loss of seismiccoherence due to the impact-induced rock deformation. In general, remotesensing, morphology, and geophysics provide useful first-order indications ofthe possible presence of an impact structure. Indeed, in the last decades, anumber of impact structures have initially been suggested by such data.Especially some observations from ore exploration have proven useful inthis regard. However, none of these observations constitutes unambiguousevidence for an impact origin.

Definite confirmation of an impact origin can only be obtained frommineralogical or chemical observations; consequently, ground truthing orinvestigating a candidate structure by drilling and subsequent rock analyticalstudies are required. The impact specific high shock pressures and temperaturesinduce – in upper crustal rocks – the distinct deformation features alreadyintroduced as “shock metamorphism”. Whereas normal crustal metamorphicconditions may be able to attain several kilobars pressure and perhaps locallyup to 1000 °C temperatures, impact conditions involve shock pressures up toseveral tens of gigapascals (GPa) and several thousands of degrees of shock or


post-shock temperatures. This induces the specific deformation effects such asplanar fractures, planar deformation features, diaplectic glass, high-pressurepolymorphs that would be entirely out of place in an upper crustal setting, suchas coesite and stishovite (after quartz), diamond (after graphite), and reidite(after zircon); and finally mineral and bulk rock melting (e.g., Stöffler, 1972,1974; Stöffler and Langenhorst, 1994; Grieve et al., 1996; Huffman andReimold, 1996; Reimold, 2007a). In many cases such features may berecognizable in thin section, but where doubt about the presence of planarshock deformation features remains, electron microscopy may have to beemployed (e.g., Goltrant et al. 1991; Gratz et al. 1996; see Figs. 3d and e).Where accessible, impact breccias may provide the necessary study object forshock deformation, but highly stressed target rocks from the central part of animpact structure may also suffice. Melt fragments in suevite and impact meltrocks are the most sought-after impactite lithologies, as besides possibly carryingshock metamorphosed clasts, they may also be suitable for age dating of animpact event. Rocks in the crater rim region are usually only weakly shocked(<5 GPa) and may not provide unambiguous evidence of shock deformation(no systematic investigations of possibly diagnostic shock damage producedunder such low pressures have been carried out yet), however, the crater floorand rim zone may be intruded by so-called dike breccias, including veins anddikes of suevite and impact melt rock. The need to identify additional criteriafor the recognition of impact structures, especially indicators of low-level (<10 GPa) shock metamorphism is obvious, to counteract the effects of erosion,as well as the dispersal of highly shocked impact debris within a majorityproportion of low-shock deformed material. Further dispersal is encounteredwhen eroded material is shed into sediment reservoirs.

The second method to obtain unambiguous evidence of impact is basedon several techniques to detect traces of the meteoritic projectile. Detailedreviews of the background to these techniques and the state of art ofidentification of meteoritic projectiles have been published, for example, byKoeberl (1998, 2007). In the absence of actual meteorite fragments, it isnecessary to chemically search for traces of meteoritic material that is mixedin with the target rocks in breccias and melt rocks. Meteoritic componentshave been identified for about 45 impact structures, out of the more than 170impact structures that have so far been identified on Earth (see list of successfulcases in Koeberl, 2007). The presence of a meteoritic component can be verifiedby measuring abundances and interelement ratios of siderophile elements,especially the platinum group elements (PGE), which are much more abundantin meteorites than in terrestrial upper crustal rocks. Often the content of theelement iridium is measured as a proxy for all PGEs, because it can be measuredwith the best detection limit of all PGEs by neutron activation analysis; but


taken out of context (without supplementary chemical and petrographicinformation), small Ir anomalies alone have little diagnostic value (as has beenshown in, for example, the case of the P/Tr boundary; Koeberl et al. 2004).

More reliable results can be achieved by measuring whole suites ofelements, for example, the PGEs, which also avoids some of the ambiguitiesthat result if only moderately siderophile elements (e.g., Cr, Co, Ni) are used.It is difficult to distinguish among different chondrite types based on siderophileelement (or even PGE) abundances, which has led to conflicting conclusionsregarding the nature of the impactor at a number of structures. In such cases,the Os and Cr isotopic systems can be used to establish the presence of ameteoritic component in a number of impact melt rocks and breccias. Both ofthese methods are based on the observation that the isotopic compositions ofthe elements Os and Cr, respectively, are different in most meteorites comparedto terrestrial rocks; the Cr isotopic method allows, in addition, the identificationof the type of meteoritic material involved (see, e.g., Koeberl et al. 2007).

With tools like these, the question arises, why can’t we identify moreimpact structures on Earth? First of all, our planet – in contrast to the othersolid bodies of the solar system that have retained a long impact record ontheir surfaces – is dynamic. Plate tectonics have erased most of the impactscars created over time. It takes roughly 150 Ma to completely resurface theoceanic crust on this planet, constituting some 70% of the entire surface area.Impact structures, especially the large ones, are relatively shallow morphologicalfeatures, and are quite quickly eroded. Small, simple structures are also notvery deep and are quickly filled in or eroded. There can be no doubt, however,for example from studies of the Moon, that impact was the fundamental surfacemodifying process in the early Archean (e.g., Grieve et al. 2006; Koeberl,2006a,b). In fact the Moon likely originates from a major planetary collision –probably the last major impact event in the history of the Solar System (papersin Canup and Righter, 2000). Koeberl (2006a,b) discusses what the possibleeffects of impact bombardment in the early Archean could have been for Earth.Quite a debate has also raged about whether a so-called “Late HeavyBombardment” was experienced by the planetary bodies at around 3.9-3.8 Ga(in favor (e.g.): Ryder et al. 2000; against: Stöffler et al. 2006). The final decisionis still outstanding on this issue, but as Koeberl (2006a,b) reported, evidencefor a LHB on Earth is still lacking. The search for Archean evidence of impacthas remained an important task for impact researchers.

Another important research topic is the role that impact catastrophes,especially at a very much higher impact flux than at present – would have hadregarding the evolution of life on this planet. How did impact cratering affectthe presence of life-sustaining (e.g., Westall et al. 2006; Schopf, 2006) wateron Earth? The possibility that a barrage of cometary projectiles onto the early


Earth could have contributed to the hydrosphere development has beendiscussed. The role of early impact bombardment on the development of anearly atmosphere must be investigated further. And the fact that “litho-panspermia” – the transfer of primitive life between planets – is a potentiallyviable process has been strongly supported by impact experimentationand study of meteorites originating from Mars by a group around D. Stöffler(Stöffler et al. 2007; Horneck et al. 2008), as well as by earlier work by Andersonet al. (2002) and Wells et al. (2003).

As discussed before, the current evidence for impact in the Archean islimited to a handful of spherule layers. Such ejecta have not been detected yetfrom the 2.02 Ga Vredefort impact – however, they are now known from theSudbury impact event (Addison et al. 2005; Kring et al. 2006). It is hoped thatmore stratigraphers/sedimentologists/exploration geologists will takecognizance of the importance of such ejecta, in order to improve the Proterozoicand Phanerozoic impact ejecta record. This, in turn, may lead to furthersuccesses in pinpointing the source craters for such ejecta layers, as it waspossible in the K/T boundary-Chicxulub case in the 1990s.

Basic research on shock deformation – or the response of differentmaterials to shock pressures, temperatures and ultra-high strain rates – is stillrequired. Much of the experimental work to calibrate shock deformation inspecific minerals has been done with single-crystal target specimens, and alsothe shock recovery experimentation with rock samples is still quite limited. Byfar the majority of calibration experiments with rocks were done with crystallinerock targets, and porous, water-bearing rocks (such as sedimentary rocks) havebeen comparatively neglected. And yet, it is sedimentary rocks that areubiquitously found in target areas on continents – their shock response must beevaluated further. Shock metamorphic studies are required to confirm the originof a suspected impact structure. In recent years, it has become more andmore fashionable to propose an impact structure or to relate seeminglycatastrophic demise in the terrestrial bioevolutionary record with impact.Unfortunately, in many of these instances, the promoters of such stories havefound editors willing enough to publish their sensationalist yet unfoundedwritings. The criteria for the recognition of an impact structure or impact affectedmaterial have been laid out clearly. Confirmation of the presence of meteoriticmaterial – even if only in the form of traces determined by analytical chemicalmethods – and/or of shock metamorphic deformation is required. These criteriamust be promoted amongst the geoscientific community and especially amongststudents of geoscience. And the impact cratering community has to remainvigilant to avoid having such reports entering into the mainstream literature.This dilemma has been discussed with a number of examples by Reimold(2007b) and Pinter and Ishman (2008).


On the positive side, since about 20 years hydrocode modelling (e.g., agood introduction to this topic is contained in Weiss and Wünnemann, 2007;also Pierazzo and Collins, 2003) has become so advanced that it is now possibleto investigate individual physical parameters at play during impact and isolatetheir influences, in 2D and 3D modelling experiments. In Figures 13a and btwo examples of numerical modelling runs in 2D and 3D presentation areshown. Both models were calculated for the case of vertical impact of a 1 kmdiameter asteroid into a granitic target at 12 km/s. The sequence of timesteps shown ranges from the moment just before impact to 95 s after, at whichtime the final crater is complete. This sequence of events would be anapproximation for the formation of a ca. 10 km wide, complex impact structure– such as the Bosumtwi structure in Ghana (Koeberl and Reimold, 2005.Important features of the series of time steps (e.g., Fig. 13a) include theformation of the transient cavity, development of the ejecta curtain and flapof overturned target strata in the rim zone of the collapsed complex crater,central uplift formation (upward bending of stratigraphic lines in the middle ofthe section), and finally the collapse of the central uplift indicated by theoutward directed (i.e., outward flowing) uppermost strata around the centraluplift. Note the strong deformation of strata around the outer edge of thecollapsed crater, which corresponds to significant faults developed in thisoutermost crater zone. Modern numerical modeling tools allow to investigatethe fate of a single limited-size block of material throughout the ultra-fastimpact event (e.g., Gohn et al. 2008), or even to study on the micro-scale theinteraction of a single pore space with a shock wave. The simple example of a3D model shown in Fig.13b alludes to the amazing visualization possibilitiesthat these most modern hydrocode developments allow. Numerical modelling,in conjunction with shock experimentation and ground based research willcontinue to improve our knowledge about interaction of shock waves andmaterials. Bridging the Gap between modellers and field workers has beenthe task of the last years, and important contributions can be found in MAPS(2004b).

Tektite and Impact Glass Issues

Tektites are a form of distal impact ejecta. They are chemicallyhomogeneous, often spherically symmetric natural glasses, with most being afew centimeters in size. Mainly due to chemical studies, it is now commonlyaccepted that tektites are the product of melting and quenching of terrestrialrocks during hypervelocity impact on the Earth (see, e.g., the reviews byKoeberl, 1994, and Montanari and Koeberl, 2000). The chemistry of tektites isin many respects identical to the composition of upper crustal material. Tektitesare currently known to occur in four strewn fields of Cenozoic age on the


Fig.13a


Fig.13. Results of numerical simulations of the vertical impact of a 1 km diameter asteroid intoa granitic target, calculated with the iSALE hydrocode (Amsden et al. 1980; Wünnemannet al. 2006). (a) Cross section through the crater center showing tracer lines to visualizedeformation in the target. Four time steps are shown covering the interval from 0 s (justbefore impact) to 95 s after impact, by which time the formation of the final crater iscomplete. Step two shows the development of a somewhat hemispherical transient cratercavity, by which time also the ejecta curtain begins to develop at the edge of the cavity.Step three corresponds to the phase of rebound of the strongly compressed central craterfloor and concomitant development of the central uplift (note the upward bent tracerlines in the central part of the crater. Overturning of target material at the edge of thestructure is prominent. Step 4 at completion of the final crater shows the collapsed centraluplift, with tracer lines clearly illustrating the outward and downward flow of theuppermost section of the central uplift. (b) Various time steps, calculated for the sameimpact experiment, but showing density contours on the front face of a half space of theimpact structure. The “ripple marks” occurring in some of the illustrations are visualizationartifacts of the numerical modeling. Red colors indicate high density, and blue is lowdensity. Both series of results were kindly made available for this review by KaiWünnemann and Dirk Elbeshausen, Museum for Natural History, Berlin.

a) t=0 s b) t=0.5 s

c) t=7.5 s d) t=25 s

e) t=40 s f) t=95 s

13b Fig.13b

surface of the Earth. Strewn fields can be defined as geographically extendedareas over which tektite material is found. The four strewn fields are: the NorthAmerican, Central European (moldavite), Ivory Coast, and Australasian strewnfields. Tektites found within each strewn field have the same age and similarpetrological, physical, and chemical properties. As mentioned above, relativelyreliable links between craters and tektite strewn fields have been established


between the Bosumtwi (Ghana), the Ries (Germany), and the Chesapeake Bay(USA) craters and the Ivory Coast, Central European, and North Americantektite strewn fields, respectively. The source crater of the Australasian strewnfield has not yet been identified. Tektites have been the subject of much study,but their discussion is beyond the scope of the present review.

In addition to the “classical” tektites on land, microtektites (<1 mm indiameter) from three of the four strewn fields have been found in deep-seacores. Microtektites have been very important for defining the extent of thestrewn fields, as well as for constraining the stratigraphic age of tektites, andto provide evidence regarding the location of possible source craters.Microtektites have been found together with melt fragments, high-pressurephases, and shocked minerals (e.g., Glass and Wu, 1993) and, therefore, confirmthe association of tektites with an impact event. The variation of the microtektiteconcentrations in deep-sea sediments with location increases towards theassumed or known impact location, both spatially and temporally (see, e.g.,Glass and Koeberl, 2006).

The definition of a tektite has been discussed to some extent, but thefollowing characteristics should probably be included (see Koeberl 1994;Montanari and Koeberl 2000): (1) they are glassy (amorphous); (2) they arefairly homogeneous rock (not mineral) melts; (3) they contain abundantlechatelierite; (4) they occur in geographically extended strewn fields (not justat one or two closely related locations); (5) they are distal ejecta and do notoccur directly in or around a source crater, or within typical impact lithologies(e.g., suevitic breccias, impact melt breccias); (6) they generally have low watercontents and a very small extraterrestrial component; and (7) they seem tohave formed from the uppermost layer of the target surface (see below). Thus,the term “tektite” should be used only for true glasses, not for recrystallizedobjects, and, if the criteria listed above are not fulfilled, use the (in such a casemuch better) general term “impact glass”.

An interesting group of tektites are the Muong Nong-type tektites(Fig. 12a), which, compared to “normal” (or splash-form) tektites are larger,more heterogeneous in composition, of irregular shape, have a layered structure,and show a much more restricted geographical distribution (Fig.13b; cf.Montanari and Koeberl, 2000). They also contain relict mineral grains thatindicate the nature of the parent material and contain shock-produced phasesthat indicate the conditions of formation, and that sedimentary source rockswere involved.

Despite knowing the source craters of three of the four tektite strewnfields, we still do not know exactly when and how during the impact processtektites form. Detailed reviews of the parameters that have to be consideredwere provided by, for example, Koeberl (1994) and Montanari and Koeberl


(2000). These characteristics include, for example, the following observations:(a) vapor fractionation played no major role in tektite formation; (b) tektitesare very poor in water with contents ranging from about 0.002 to 0.02 wt%;(c) bubbles in tektites contain residues of the terrestrial atmosphere at lowpressures; (d) meteoritic components in tektites are very low or below thedetection limit; (e) the 10Be content of Australasian tektites cannot haveoriginated from direct irradiation with cosmic rays in space or on Earth, butcan only have been introduced from sediments that have absorbed 10Bethat was produced in the terrestrial atmosphere. Tektites might be produced inthe earliest stages of impact, which are poorly understood. It is clear, however,that tektites formed from the uppermost layers of terrestrial target material(otherwise they would not contain any 10Be; see Serefiddin et al. 2007 for arecent discussion). However, the question how the tektite production anddistribution occurred in detail remains the subject of further research.Furthermore, it is possible that new locations of tektites are found in thefuture. There is a discrepancy between modeling calculations that indicatethat very early melts formed by jetting should have a high meteoritic component,whereas tektites – which are inferred to have formed very early in the impactprocess – do have a very low meteoritic component (cf. Koeberl, 1994, 2007).This disagreement needs to be studied and resolved.

Planetological Issues

Studies of impact structures on planetary surfaces must continue in orderto determine whether the various crater production curves, which are extensivelyutilized for the age calibration on the terrestrial planets (e.g., Hartmann et al.2000; Neukum et al. 2001; Ivanov et al. 2003), but also on asteroidal surfaces,are compatible with each other (e.g., Stöffler et al. 2006). To date only arelatively small number of lunar rocks have been dated by absolute radiometrictechniques and serve as absolute calibration of crater production curves. Arethe lunar curves directly applicable to Mars or Mercury? The problem of thestill debated “Late Heavy Bombardment” issue has already been discussed.How do the terrestrial crater statistics (e.g., Grieve and Shoemaker, 1994;Hughes, 2000) compare with the extraterrestrial curves?

Large tracts of the Martian surface, and certainly also on Venus, arecovered with extrusive rocks. However, on Earth, only two impact structuresformed in basalt are known – 1.8 km Lonar Lake in India and the 17 km sizeLogoisk structure in Russia (and in the latter case no basalt is exposed). Mostknown terrestrial impact structures were formed in granitoids and sedimentaryrocks – and much still needs to be learned about those target materials thatform extraterrestrial planetary surfaces, including those regolith coveredasteroids – and also cometary matter.


The Impact Hazard

The 1994 impact of fragments of comet Shoemaker-Levy 9 into theatmosphere of Jupiter demonstrated to humankind that impact is not a threat ofthe past. Hardly a year goes by without a call of danger from a “rogue asteroid”to earth – although so far, all of them seem to have remained without cause.However, the terrestrial cratering record predicts a Tunguska event – impact ofa roughly 50-m-sized projectile - for every 1500 years or so (Harris, 2008), andif this event had occurred in a highly populated region, this relatively minorevent comparable to ca. 1000 Hiroshima atom bombs could have caused a lossof life potentially going into the millions. A large event of Chicxulub magnitudemight occur every 100 million years, but this statistical approach does notpredict when it might happen next. Depending on the actual velocity of theChicxulub impactor, that projectile might have measured between 5 and 15 kmin diameter, although scaling from the amount of extraterrestrial material aroundthe world indicates a value of about 10 km. However, current predictions ofimpact consequences suggest that even a 1 km sized bolide might be sufficientto seriously cause harm to mankind through global environmental catastrophe.Projectiles enough are out there – comets in the Kuiper-Edgeworth belt and inthe Oort cloud, and near-earth objects (those asteroids that approach theSun closer than 1.3 astronomical units) are not particularly rare in the asteroidbelt (according to Chandler, 2008, some 750 large asteroids and further5500 asteroids in total have now been mapped in the asteroid belt, largely aresult of the NASA sponsored international Spaceguard surveying project).Harris (2008) relates that “the risk of any person’s death by impact prior toSpaceguard was estimated at about the same as dying in an airplane crash” –something like 1 in 30,000 (also Chapman and Morrison 1994; Chapman, 2004),but since the successful work of Spaceguard, this estimate has changed to 1 in600,000 as the danger from likely still undetected asteroids. In fact, Harrispresents a chart that suggests that before Spaceguard the short-term risk ofimpact was in the order of 250 fatalities per year for 3 km sized projectiles,since Spaceguard this risk has been reduced to about 20 fatalities per year.

And still, much has remained to be learnt about the catastrophic impacteffect. Space search programs for possible still unknown asteroids continue,and the danger from suddenly appearing comets remains anyway. The lattercase provides a challenge for impact workers, as the behavior of large, low-density projectiles, and the effects of their impacts onto various planetarysurfaces are far from understood. The terrestrial impact record has to beimproved to further constrain the past impact flux onto Earth, and the energythreshold for truly global devastation by impact still needs to be identified.Does it take an impact event of the magnitude of Chicxulub? Or was that impact


particularly lethal because of the sulfate- and carbonate-rich target area atYucatán? Can our highly evolved and through its intricate civilization highlyvulnerable race survive a much smaller impact event, perhaps only creating a40 km impact structure? And that will only take a 2-3 km sized impactor….

Impact Benefits

Finally, impact cratering is not only to be considered a catastrophic,negative process. A number of groups have investigated that in fact impactstructures could provide very useful habitats for primitive life forms – andthus, may have been critical for the development of life also on Earth (e.g.,Lindsay and Brasier, 2006; Cockell and Lee, 2002; Cockell et al. 2007). Otherbenefits of impact cratering include the development of, at times, major oredeposits – such as the gigantic Sudbury ores, many sites of significant – oreven major (e.g., the gas fields around the Alaskan Avak impact structure orthe Campeche oil field in the vicinity of the Chicxulub structure in Mexico)hydrocarbon showings, and the effect of the Vredefort impact onto theWitwatersrand gold and uranium resources. A number of comprehensive reviewsof this topic have been published: Grieve and Masaitis (1994); Grieve (2005);Reimold et al. (2005). No developed and certainly no developing nation canafford to ignore the possibility that its impact structures could carry importantmetal or energy resources.

Drilling of impact structures, especially through a series of recent ICDP(International Continental Scientific Drilling Program) ventures, has stronglyfertilized impact knowledge (see the review by Koeberl and Milkereit, 2007).Deep drilling at Chicxulub (MAPS, 2004a); Bosumtwi (Ghana) (MAPS, 2007),and Chesapeake Bay (Geological Society of America Special Paper, in review)has resulted in much new information regarding the famous K/T boundaryimpact, impact in the marine realm, and regarding post-impact sedimentaryand biological processes.

Moreover, well-exposed impact structures may serve as unique sitemuseums, for instruction of laypeople and students in geoscience, planetaryscience, astronomy, life and environmental science. A number of excellentmuseums have been built already (e.g., at Meteor Crater, and especially in theRies Crater), or are in final stage of development (at Vredefort). The RiesCrater in southern Germany has been identified as a unique geotope and hasbeen declared a German Geopark. And a portion of the Vredefort impactstructure in South Africa was declared a World Heritage Site by UNESCO in2005, and is currently developed into a major geotouristic and educationalterrane. The newly discovered impact structure Waqf as Suwwan in Jordanwas found in May 2008 to have been locally affected by apparently illegitimatelimestone exploration activity. Strong efforts are being made there to preserve


this well exposed impact structure and geological site, and it is hoped that thismay lead to responsible development and sustainable educational andgeotouristic utilization.

Outlook

This essay has shown that a lot has been learned about the impact processand its influence on Earth and Life evolution. Impact cratering has been afundamental process in the planetological sense, with regard to the earliestevolution of our own planet, and throughout its history – also decisivelyinfluencing the evolution of Life. However, the terrestrial cratering record isfar from impressive, and must – und undoubtedly will – be improved in comingyears. While the geological community has been rather slow in embracingknowledge about this process – it has only been in the last few decades thatimpact cratering has found acknowledgement in geoscience textbooks, it isnow widely accepted that the effects of impact cratering permeate everydiscipline of the earth and planetary science.

It is hoped that this article will contribute perhaps somewhat to widerrecognition of impact as a fundamental, and for Earth and mankindfundamentally important, planetological process. The recent discovery of Waqfas Suwwan in Jordan, with its well preserved and exposed central uplift, or thefinding of the remnants of a previously unknown impact structure near SantaFe (USA), have demonstrated that any new discovery can significantly enhanceour knowledge about the impact process and the response of different materialsto it.

Fifty years of impact research have gone by, parallel to 50 years of spaceexploration. And yet, with regard to the many problem areas and open questionsthat have been raised above we are still at the beginning of gathering basicinformation. Impact cratering studies have transformed into a multidisciplinaryintegrated science discipline – and this approach will be successful to providethe missing answers.

ACKNOWLEDGMENTS

We are most grateful to the Editor of this special volume for the invitationto represent this important discipline - impact cratering research - in thisbenchmark Golden Anniversary issue. Claudia Crasselt, Thomas Kenkmann,Cristiano Lana, Jared Morrow, and Jayanta Pati kindly provided several images.Nils Hoff expertly redrafted Figure 12b, and Antje Dittmann and ClaudiaCrasselt assisted with other graphics. Figures 13a and b were contributed byKai Wünnemann and Dirk Elbeshausen. We are grateful to Dr. B.P. Radhakrishnaand Dr. Fareeduddin for comments on the manuscript.


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