meteorite impact structures: the good and the...

13© Blackwell Publishing Ltd, Geology Today, Vol. 24, No. 1, January–February 2008

FEATURE

Meteorite impact structures are found on all planetary bodies in the Solar System with a solid surface.On many planets, impact craters are the dominant landform. Earth’s active geology, however, tends torapidly erase impact structures from the geological record, although we know currently of 174confirmed impact sites. Impact events are destructive and have been linked to at least one of the ‘bigfive’ mass extinctions over the past 540 Ma. But they also provide certain economic benefits, includingthe formation of metalliferous ore deposits and hydrocarbon reservoirs. Impact structures can also formnew biological niches, which can provide favourable conditions for the survival and evolution of life.Despite this, it was only in the past 40 years that the importance of impact cratering as a geologicalprocess was recognized and only during the past 15–20 years that the study of meteorite impactstructures has moved into the geological mainstream. There is, therefore, still considerable potential fornew and exciting advancements.

until 1906 that the first terrestrial impact crater wasrecognized: Meteor or Barringer Crater in Arizona(Fig. 2).

Despite the identification of Meteor Crater as animpact site by Daniel Barringer, there remained littleawareness in the geological community of theimportance of impact cratering. By the 1930s severalsmall craters on Earth were suspected as being ofimpact origin. As with Meteor Crater, the mainevidence for the impact origin was the presence offragments of meteorites within and around thesesites. Robert Dietz established the first reliablegeological criterion for the recognition of impactstructures, in the absence of meteorites, in 1947,with the recognition of shatter cones—distinctiveconical fractures with radiating and branchingstriations. This prompted a surge in the number of

FeatureMeteorite impact structures: the good andthe bad

Gordon R. OsinskiDepartments of Earth

Science/Physics and

Astronomy, University of

Western Ontario, 1151

Richmond St., London N6A

5B7, Canada

[email protected]

Impact cratering is a ubiquitous geological processthat affects all planetary objects with a solid surface.Meteorite impact structures are one of the mostcommon geological landforms on the all theterrestrial planets (Figs 1a,b), except Earth, and manyof the rocky and icy moons of Saturn and Jupiter(Fig. 1c). The study of meteorite impact craters hashad a long and varied history; however, it was notuntil the 1960s and 1970s that the importance ofimpact cratering as a geological process began to berecognized. Galileo Galileo made one of the firstrecorded observations of impact craters in 1609. Heobserved circular features on the Moon with rimmeddepressions that we now know are impact craters,although he did not speculate as to their origin. In1893 the American geologist Grove Gilbert proposedan impact origin for these lunar craters but it was not

Fig. 1. a. The 28 km diameterEuler Crater on the Moon is agood example of a compleximpact crater. Note the centralpeak, flat floor and faultedcrater rim. Image: NASA. b.Image taken by Mars Express ofthe 140 km diameter Holdenimpact structure, Mars. Image:ESA. c. Galileo image of the26 km diameter Pwyll Crater onJupiter’s moon, Europa. Image:NASA.

© Blackwell Publishing Ltd, Geology Today, Vol. 24, No. 1, January–February 200814

FEATURE

impact craters being recognized in the 1950s and1960s, particularly in Canada, where Carlyle Beals,the Dominion Astronomer, initiated a program tosystematically search for Canadian impact sites. Theincreased recognition of impact sites, together withthe impetus provided by the Apollo landings on theMoon, led to a more complete understanding of theformation of impact craters in the 1960s thanks tothe pioneering work of a small group of scientists,including Ralph Baldwin, Carlyle Beals, Robert Dietz,Bevan French, and Eugene Shoemaker.

An improved understanding of the impactcratering process continued during the 1970s and1980s, with the recognition of several shockmetamorphic criteria and dozens more impact sites.(Shock metamorphism is defined as themetamorphism of rocks and minerals caused by shockwave compression and decompression due tometeorite impact, or due to the detonation of high-energy chemical or nuclear explosives.) Finally, in the1990s, two events or discoveries resulted in impactcratering entering the geological mainstream,namely: the spectacular impact of comet Shoemaker–Levy 9 into Jupiter in July 1994; and, the discovery ofthe ~200 km diameter Chicxulub impact structure,Mexico, and its probable link to the ~ 65 MaCretaceous–Tertiary mass extinction event.

Formation of impact craters

Impact cratering is unlike any other geologicalprocess. There has been much confusion andcontroversy surrounding impacts in the past, whichis due, in part, to their rarity compared with otherendogenous geological events (Fig. 3). There are alsomajor differences between impact events and othergeological processes, such as volcanism and platetectonics, including the extreme physical conditions(Fig. 3), the concentrated nature of the energy releaseat a single point on the Earth’s surface, the virtuallyinstantaneous nature of the impact process (e.g.,

Fig. 2. Oblique aerialphotograph of the 1.2 kmdiameter, 50 ka Meteor Crater,Arizona. Image courtesy of TedBunch.

Fig. 3. A comparison of the energy released during impact eventswith endogenous geological processes and man-made explosions. Thevertical axis represents the frequency of impact events expressed asthe estimated interval in years for a particular size of event. Compiledwith data from Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures, byB.M. French. For a definition of ‘D’ and ‘DA’, see Fig. 4.

seconds to minutes), and the high strain ratesinvolved (~104 s–1 to 106 s–1). Despite these unusualcharacteristics, the formation of an impact crater canbe divided into three main stages (Fig. 4): 1, contactand compression; 2, excavation; and, 3, modification.

The contact and compression stage begins at theinstant when the projectile, which could be anasteroid or comet, contacts the surface of the target(Fig. 4). The projectile, travelling at velocities ofanywhere from ~10 to 75 km/s, penetrates no morethan 1–2 times its diameter before transferring itskinetic energy into the target in the form of shockwaves. These shock waves radiate out both into thetarget rocks and back into the projectile itself.Numerical models have shown that when thisreflected shock wave reaches the upper surface of theprojectile, it is reflected back into the projectile as ararefaction, or tensional wave. This causes theprojectile to rapidly decompress from high shockpressures, resulting in the virtually complete meltingand/or vaporization of the projectile itself. Thepassage of the initial shock wave and subsequentrarefaction wave through the target rocks also resultsin the melting and vaporization of a large volume oftarget material close to the point of impact. Theduration of the contact and compression stage lastsno more than a few seconds for all but the largestbasin-forming impacts.

The crater itself forms during the subsequentexcavation stage (Fig. 4). Complex interactionsbetween the outward-directed shock waves and thedownward-directed rarefaction waves generate a so-


FEATURE

called ‘transient crater’. The transient cratercomprises an upper ‘excavated zone’ and a lower‘displaced zone’: material in the excavated zone isejected beyond the transient cavity rim (formingimpact ejecta), while material in the displaced zoneremains within the transient cavity (forming crater-fill impactites) (Fig. 4). Excavation ceases when theshock and rarefaction waves can no longer excavateor displace the target rocks. This stage is longer thanthe initial contact and compression stage, but for a200 km diameter crater, calculations suggest that theexcavation stage requires ~90 s to complete.

For relatively small impact events (<2–4 km onEarth) the transient cavity undergoes only minormodification resulting in the formation of a bowl-shaped ‘simple crater’ (Fig. 4). For larger impactevents, however, the transient cavity isgravitationally unstable and a ‘complex impactcrater’ is formed (Fig. 4). Two competing processesare at work during the modification stage: uplift of thetransient crater floor, which results in a central uplift,and collapse of the initially steep walls of the transientcrater. The modification stage typically takes only afew minutes to complete, although readjustment ofthe crater, associated with minor faulting and massmovement, continues indefinitely. The modification ofan impact crater also occurs through impact-

associated hydrothermal activity, which can lead tosubstantial alteration and mineralization of impact-produced and altered target rocks.

How to recognize a meteorite impactstructure

Meteorite impact structures are typically recognizedby their roughly circular form, even when eroded,and the presence of a variety of distinctive types ofrocks, such as breccias, melt rocks andpseudotachylyte (e.g., Fig. 5). However, theseattributes also hold true for other geological features,such as volcanic craters, so that on their own, thesecriteria do not provide definitive evidence for animpact event. Unequivocal evidence for meteoriteimpact takes the form of shock metamorphicindicators—features formed via shock metamorphism(Fig. 6)—that can be either macroscopic (e.g. shattercones) or microscopic (e.g. planar deformationfeatures (PDFs) and diaplectic glass) (Fig. 7), and thepresence of high-pressure polymorphs of rock-forming minerals (e.g. coesite and stishovite,polymorphs of SiO

2).

The recognition of shock metamorphic indicatorsexplicitly requires the investigation and preservationof suitable rocks within a suspected structure.

Fig. 4. Series of schematiccross-sections depicting theformation of a meteorite impactstructure. At small diameters(i.e. diameter <2–4 km), asimple impact crater forms. Fordiameters >2–4 km, the initialtransient crater is unstable and acomplex impact crater forms,which following erosion, istermed an impact structure.


FEATURE

However, this is often not possible for eroded orburied structures and for the majority of structurespresently in the marine environment, even thoughthere may be strong structural evidence for an impactorigin. A prime example is the controversysurrounding the origin of the Silverpit structure inthe North Sea. Simon Stewart and Phil Allenoriginally proposed that this structure was an impactcrater in 2002 based on high-resolution 3-D seismicdata (Fig. 8), and despite some opposition, mostimpact workers would accept this interpretation.However, without drilling to retrieve samples, thisstructure is not regarded as a confirmed impactstructure and so is not listed in any database ofterrestrial impact structures. The same holds formany other suspected offshore impact sites, which isparticularly unfortunate, as many of these structureshave been surveyed by the petroleum industry so thatexcellent geophysical datasets are often available.

Fig. 5. a. Aerial view of the~80 m high cliffs of impact meltrock at the Mistastin impactstructure, Canada. Note thewell-formed columnar joints.Photograph courtesy of DerekWilton. b. Pseudotachylyte fromthe Sudbury impact structure,Canada. Pseudotachylyte istypically formed via in situfrictional melting of wall rocks,and can be glassy orrecrystallized. It can also befound in endogenous faultzones. 18 cm long GPS for scale.

Fig. 6. Pressure–temperature(P–T) plot showing comparativeconditions for shockmetamorphism and ‘normal’crustal metamorphism. Figureredrawn with permission of theLunar Planetary Institute fromTraces of Catastrophe: AHandbook of Shock-Metamorphic Effects inTerrestrial Meteorite ImpactStructures, by B.M. French.

Fig. 7. Shock metamorphic effects in rocks from the Haughtonimpact structure, Canada. a. Shatter cones in carbonates. 10 cm longpenknife for scale. b. Quartz grain displaying planar deformationfeatures (PDFs). Plane polarized light photomicrograph. Field of viewis 2 mm. c, d. Plain and cross polarized light photomicrographs,respectively, reveal that the majority of the quartz grains in thissandstone have been transformed into diaplectic glass—impact glass,which displays the shape of the parent crystal(s) and that lacks anymorphological evidence of flow. Field of view is 4 mm.

The terrestrial impact cratering record

Unlike other planetary bodies such as the Moon,Mercury and even Mars, which have retainedportions of their earliest crust, on Earth, erosion,volcanic resurfacing and tectonic activity arecontinually erasing impact craters from the rockrecord. Despite this, 174 confirmed impact structureshave been documented to date with several more sitesbeing recognized each year (Fig. 9), although therecord is notably incomplete. In particular, there arefew confirmed impact sites in South America, CentralAfrica and large parts of Asia; however, it is not clearwhether this is due to the ‘unsuitable’ regionalgeology of these regions (e.g. lack of ancient, stablecratons), or if the scarcity of impact sites is due to alack of detailed field and remote sensing studies and/or other factors, such as vegetation coverage or


FEATURE

enhanced erosion. It is also notable from Fig. 9 thatthe majority of the recognized impact sites also occuron land. This is due to two main factors: the relativelyunexplored nature of the ocean floor, and the lack ofcriteria to validate suspected impact sites withoutactual samples, as mentioned above.

Impact events and mass extinctions

In 1980, Luis and Walter Alvarez and colleagues,published a paper in Science outlining evidence for anextraterrestrial origin for the most recent of the ‘bigfive’ mass extinctions: the Cretaceous–Tertiary (nowthe Cretaceous–Palaeogene) mass extinction event at~65 Ma. Early work focused upon the discovery ofgreatly enhanced iridium concentrations insediments at the Cretaceous–Palaeogene boundary.This iridium was clearly extraterrestrial and evidencesuggested that is was not from a nearby supernova,but from an impact event that formed a large, thenunidentified, impact crater. Since the discovery of theiridium anomaly, considerable further evidence hasbeen documented in boundary sediments, supportinga large end-Cretaceous impact, including shockedquartz displaying PDFs, spherules of impact glass, andother extraterrestrial chemical and isotopicsignatures. However, it was not until the early 1990sthat the source crater—the Chicxulub impactstructure—was found lying beneath ~1 km ofsediment below and offshore the present-day YucatanPeninsula, Mexico.

The investigation of old petroleum exploration drillcores, combined with the analysis of recent scientificdrill core and geophysical surveys, have resulted in agood understanding of the crater itself. Much of theongoing research focuses on understanding theextinction mechanisms at the Cretaceous–Palaeogeneboundary. The local and regional effects of the impactevent include the air blast and heat from the impact

explosion, tsunamis, and earthquakes. Global effectsincluded forest fires ignited by impact ejecta re-entering the Earth’s atmosphere, the injection of hugeamounts of dust into the upper atmosphere, whichmay have inhibited photosynthesis for as much astwo months, and the production of vast quantities ofN

2O from the shock heating of the atmosphere.

However, one of the most important findings hasbeen that, in terms of global effects, the severity of theChicxulub impact was due, in part, to thecomposition of the target rocks: ~3 km of carbonatesand evaporites overlying crystalline basement.

While it was initially thought that thevaporization and decomposition of carbonates—producing CaO and releasing CO

2, resulting in global

warming—was important, it appears that the mostdestructive effect(s) came from the release of sulphurspecies from the evaporite target rocks. We knowfrom studies of sulphur-rich volcanic eruptions, suchas Mount Pinatubo in 1992, that sulphur aerosolscan significantly reduce the amount of sunlight thatreaches the Earth’s surface, resulting in short-termglobal cooling. Estimates for Chicxulub suggest asmuch as a 15 °C decrease in average globaltemperatures, which, when coupled with the othereffects of the impact event, would have resulted insevere global environmental consequences.

Beneficial effects of impact events

Ever since the proposal of a link between meteoriteimpacts and mass extinctions, the deleterious effectsof impact events have received much attention.However, research conducted over the past few yearsindicates that although meteorite impacts are indeeddestructive, catastrophic events, there are severalpotential beneficial effects, particularly in terms ofproviding new habitats for microbial communities.This may have important implications forunderstanding the origin and evolution of life onEarth and other planets such as Mars.

One of the most important beneficial effects is the

Fig. 8. Perspective view of thetop chalk surface at the Silverpitstructure, North Sea, UK, asuspected meteorite impactstructure. The central crater is2.4 km wide and is surroundedby a series of concentric faults,which extend to a radialdistance of ~10 km from thecrater centre. False coloursindicate depth (yellow =shallow; purple = deep). Imagecourtesy of Phil Allen and SimonStewart.

Fig. 9. Distribution of the 174recognized terrestrial impactstructures superimposed on adigital elevation map (providedby ESRI). Coordinates are fromthe Earth Impact Database,which provides an up-to-dateinventory of the world’s impactstructures (http://www.unb.ca/passc/ImpactDatabase/index.html).


FEATURE

generation of a hydrothermal system within animpact crater immediately following its formation(Fig. 10). Recent work suggests that impact-associated hydrothermal systems will form followingimpacts into any H

2O-bearing solid planetary body,

with exceptions for small <2–4 km-size impacts andthose in extremely arid regions. Numerical models ofthese hydrothermal systems suggest that they maylast several Ma for large 100 km-size impactstructures. This may have important astrobiologicalimplications because many researchers believe thathydrothermal systems in general might haveprovided habitats or ‘cradles’ for the origin andevolution of early life on Earth, and possibly otherplanets such as Mars. Other potential habitatsexclusive to impact craters include impact-processedcrystalline rocks, which have increased porosity andtranslucence compared to unshocked materials,

improving microbial colonization (Fig. 11), andimpact crater lakes, which form protectedsedimentary basins that can provide protectiveenvironments and increased preservation potential offossils and organic material.

Economic potential of impact craters

One of the less well-known aspects of meteoriteimpact craters is the potential association of economicmineral and hydrocarbon deposits and thus, theirsuitability as exploration targets. This is exemplifiedby the large, ~200–250 km diameter Sudbury(Canada) and Vredefort (South Africa) impactstructures, which host some of the world’s largestand most profitable mining camps. The ~2 GaVredefort impact event led to the preservation of pre-impact (progenetic) gold and uranium deposits in the

Fig. 10. Schematic cross-section of a complex impactstructure showing the locationand nature of post-impacthydrothermal deposits. Fieldphotographs are from theHaughton impact structure,Canada.

Fig. 11. a. Shocked gneissfrom the Haughton impactstructure, which has beencolonized by endolithicmicroorganisms (i.e. this is ahabitat within (‘endo’) the fabricof the rock (‘lith’)—see thegreen layer at ~2 mm depthhighlighted by the rectangle).The impact event led to anincrease in porosity andtranslucence resulting in greaterhabitat availability, whichpermitted endolithiccolonization in a rock type thatwould be otherwise resistant. b.Endolithic colony ofChroococcidiopsis sp. withinshocked gneiss. The inset showsthe UV epifluorescence image ofthe same fragment of rock.Images courtesy of CharlesCockell.


FEATURE

Witwatersrand Basin and their subsequentmobilization and concentration during impact-induced hydrothermal alteration, producing theworld’s richest gold province. A different scenarioexists at the 1.8 Ga Sudbury structure, where theworld’s largest nickel–copper ore deposits occur at thebase of the impact melt sheet and in radial dykes.These ore deposits are syngenetic and formedthrough the separation of immiscible sulfide liquidsfrom the silicate impact melt. Subsequent post-impact(epigenetic) hydrothermal alteration also led to theformation of zinc–copper–lead economic ore depositsat Sudbury. Economic ore deposits also occur at anumber of other smaller terrestrial impact structuresand the lack of detailed studies of many impact sitesleaves room for further discoveries.

In addition to economic metalliferous ore deposits,several meteorite impact structures have beenexploited for hydrocarbons. The fracturing andfaulting of rocks in central uplifts and faulted craterrims (e.g. see Fig. 10), results in enhanced porosityand permeability, providing valuable reservoirs for oiland gas, even in rocks such as granites that aretypically not suitable hydrocarbon reservoirs (e.g.Ames structure, USA). Writing in the Oil & Gas Journalin 1998, Richard Donofrio noted that approximatelyhalf of the impact structures present in the world’smajor petroleum provinces contain commercial oiland gas fields. Post-impact sedimentary crater-filldeposits can also generate suitable reservoir rocksand source rocks.

Future directions for research

The formation of meteorite impact structures is unlikeany other geological process; however, this shouldnot hinder their study. Far from it; coming to termswith understanding a geological process that takesplace in only a few minutes, with energies that can begreater than the total annual energy release from theEarth, provides a stimulating environment forresearch and teaching. The relatively young andemergent nature of impact geology also leaves plentyof room for future research. For example, only ahandful of the world’s impact craters have beenstudied systematically. Basic processes such as themechanics of complex crater formation are still notfully understood. Outstanding problems include theeffect of target properties (e.g., volatiles, porosity,layering) on the impact cratering process and the

environmental effects of impact events. As theexploration of our Solar System continues, furtheringour understanding of impact cratering will becomeeven more important. Images from the Mars GlobalSurveyor spacecraft recently provided provocativeevidence for liquid water on the surface of Mars, atthe present-day, within an impact crater—how didthe water get there? Looking ahead, the preferred sitefor a permanent base on the Moon, to be establishedby 2020, is an impact crater near one of the poles.Impact cratering as a planetary geological process ishere to stay.

Suggestions for further reading

Cockell, C.S. & Lee, P. 2002. The biology of impactcraters: a review. Biological Reviews, v.77, pp.279–310.

Collins, G.C., Melosh, H.J. & Marcus, R.A. 2005. EarthImpact Effects Program: A web-based computerprogram for calculating the regionalenvironmental consequences of a meteoroidimpact on Earth. Meteoritics & Planetary Science,v.40, pp.817–840. Program available online at:http://www.lpl.arizona.edu/impacteffects/

Earth Impact Database. 2006. http://www.unb.ca/passc/ImpactDatabase/.

French, B.M. 1998. Traces of Catastrophe: A Handbookof Shock-Metamorphic Effects in Terrestrial MeteoriteImpact Structures. Lunar and Planetary Institute,Houston. Available online at: http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html

Kring, D.A. 2000. Impact events and their effect onthe origin, evolution, and distribution of life. GSAToday, v.10, pp.1–7.

Melosh, H.J. 1989. Impact Cratering: A Geologic Process.Oxford University Press, New York.

Osinski, G.R., Parnell, J., Lee, P., Spray, J.G., & Baron,M.J. 2005. A case study of impact-inducedhydrothermal activity: The Haughton impactstructure, Devon Island, Canadian High Arctic.Meteoritics & Planetary Science, v.40, pp.1859–1878

Reimold, W.U., Koeberl, C., Gibson, R.L. & Dressler,B.O. 2005. Economic mineral deposits in impactstructures and their geological settings. In:Koeberl, C. & Henkel, H. (eds) Impact Tectonics,Impact Studies Series v.6, pp.479–552. Springer-Verlag, Berlin.

meteorite impact structures: the good and the...

Documents