GOLDEN JUBILEE MEMOIR OF THE GEOLOGICAL SOCIETY OF INDIA
No.66, 2008, pp.69-110
Catastrophes, Extinctions and Evolution:
50 Years of Impact Cratering Studies
W. UWE REIMOLD1 and CHRISTIAN KOEBERL2
1
Museum for Natural History (Mineralogy), Humboldt University Berlin,
Invalidenstrasse 43, 10115 Berlin, Germany
2
Department of Lithospheric Research, University of Vienna,
Althanstrasse 14, A-1090Vienna, Austria
Email: [email protected]; [email protected]
Abstract
Following 50 years of space exploration and the concomitant recognition
that all surfaces of solid planetary bodies in the solar system as well as of comet
nuclei have been decisively structured by impact bombardment over the 4.56 Ga
evolution of this planetary system, impact cratering has grown into a fully
developed, multidisciplinary research discipline within the earth and planetary
sciences. No sub-discipline of the geosciences has remained untouched by this
“New Catastrophism”, with main areas of research including, inter alia, formation
of early crust and atmosphere on Earth, transfer of water onto Earth by comets
and transfer of life between planets (“lithopanspermia”), impact flux throughout
Earth evolution, and associated mass extinctions, recognition of distal impact
ejecta throughout the stratigraphic record, and the potential that impact structure
settings might have for the development of new – and then also ancient – life. In
this contribution the main principles of impact cratering are sketched, the main
findings of the past decades are reviewed, and we discuss the major current
research thrusts. From both scientific interests and geohazard demands, this
discipline requires further major research efforts – and the need to educate every
geoscience graduate about the salient facts concerning this high-energy geological
process that has the potential to be destructive on a local to global scale.
INTRODUCTION
The year 2008 approximately coincides with the 50th anniversary of space
exploration, which has gone hand in hand with the second major 20th century
revolution of the earth sciences (the first one having been the eventual victory
of continental drift theory and then plate tectonics) transformed into planetary
science (e.g., Reimold, 2007a). It has been demonstrated that all solid surfaces
70
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of planetary bodies in the solar system – from the terrestrial planets to the
minor planets, also known as asteroids, and even comet nuclei have been
bombarded with cosmic projectiles (e.g., Fig. 1). Eugene M. Shoemaker, the
famous pioneer of planetary geology and impact cratering studies, already in
1977 declared that impact was the fundamental process forming planetary
surfaces.
In July 1994, the fragments of comet Shoemaker-Levy 9 crashed into the
atmosphere of planet Jupiter, producing impact plumes the size of our own
planet, 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 January
2008). This area near Mercury’s terminator shows a range of different impact crater
geometries, from small simple bowl-shape craters to complex crater structures with central
uplift features, and a very large crater with a peak ring structure (upper right). This very
large crater measures about 135 km in diameter. Note the smooth interior of many large
craters, which is interpreted as being due to flooding with lava. Clearly these smooth
terranes are much younger than the regions between large impact structures, as they carry
much 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 happen
any time, any place in the solar system – also on Earth. And this article is
written just about 100 years after the only well documented impact catastrophe
of the 20th century, namely the Tunguska explosion of 30 June 1908 that
devastated some 2000 km2 of Siberian forests, and caused environmental effects
as 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 the
explosion of a nuclear UFO, it is now widely accepted that this explosion was
caused by the explosive disruption of a small (maybe 30 to 50 m wide) asteroid
within the atmosphere, with an explosive energy of about 5 to 10 megatons
TNT-equivalent.
Even more recently, a small cosmic projectile impacted in Peru. The
Carancas 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 that
country, and besides several people sustaining a significant scare, no casualties
are known. However, even though this impact event was of insignificant size
on 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, measures
some 2500 km in diameter), there would have been hundreds of victims if this
event had occurred in a densely populated area of our planet. Clearly the
understanding of impact processes is an important issue for mankind – especially
if one considers what happened to the dinosaurs and their contemporaries 65
million years ago. The impact of a 5-15 km asteroid at Chicxulub on the Yucatán
peninsula generated then a 180 km diameter impact structure with global
catastrophic 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 cratering
as a planetary process, as a catastrophic process that may severely affect
biodiversity on our planet, as well as how the investigation of this process and
its 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 by
Stöffler et al. (2006), who reviewed the lunar and terrestrial impact record. In
terms 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 is
discussed, the term “impact structure” is more appropriate. Generally, four
different impact structure types are distinguished according to geometry, and
related to increasing size of a crater structure: simple, bowl-shaped structures,
complex impact structures with modified crater rims (terraces) and a central
72
W. UWE REIMOLD AND CHRISTIAN KOEBERL
uplift feature, and multi-ring impact structures. Amongst the complex impact
structures, the central uplift formations can be peak-shaped or, when the peak
has 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 part
of the surface of planet Mercury, taken in fly-by in January 2008 by NASA’s
Messenger space probe. General geometric/morphological consideration of
and nomenclature for impact structures have been discussed by Turtle et al.
(2005).
In the terrestrial impact record, simple bowl-shaped craters are formed in
sedimentary and crystalline rocks when the crater diameter is smaller than
about 2 or 4 km, respectively. Complex peak or peak-ring structures are known
from 2 to 4 km upward to more than 100 km diameters. Fresh simple craters
have an apparent depth (i.e., the distance from top of crater rim to present-day
crater floor) of about one-third of the apparent crater diameter (clearly both
these apparent values depend on the degree of erosion). Apparent crater depth
for complex structures is more like one-fifth to one-sixth of a diameter. And
crater geometry depends strongly on planetary gravity and atmospheric
conditions. The lunar gravitational acceleration, which is approximately onesixth of the terrestrial value, leads to the formation of comparatively deeper
impact craters on the Moon, and a larger changeover diameter from simple to
complex craters compared to the terrestrial situation. The reason for this is that
gravity counteracts excavation of material and the development of topography.
Finally, only three very large impact structures of suspected “multi-ring
basin” type (Spudis, 1993) are known on Earth: Chicxulub in Mexico of 65
Ma age and 180 km diameter, Sudbury in Canada – 1.85 Gyr old and ca. 200250 km original diameter, and Vredefort in South Africa (Fig. 2), at 2.02 Gyr
age and 250-300 km original diameter widely held the largest and oldest known
impact structure on Earth. Stöffler et al. (2006) reviewed some structural
parameters of lunar and terrestrial impact structures. Melosh (1989) is still the
only dedicated treatise of the impact cratering process, with regard to the various
stages of development of impact structures, and ejecta distribution. Other
reviews providing an entry into impact cratering studies have been published
by, e.g., French (1998), Grieve et al. (1996; 2007), Reimold (2007a), Koeberl
(2002).
The enormous kinetic energy supplanted by hypervelocity extraterrestrial
projectiles (at ca.11 to 72 km velocity, with average asteroidal and cometary
velocities of 15 and 30 km per second, respectively) onto target rock triggers a
catastrophic deformation and disruption process. The target region is partially
excavated, and partially the target rocks are vaporized, melted, or subjected to
plastic and elastic deformation. For example, French (1989), Stöffler and
Langenhorst (1994), and Grieve et al. (1996) have provided some introductory
50 YEARS OF IMPACT CRATERING STUDIES
73
Fig.2. Vredefort Dome – the deeply eroded central uplift of the Vredefort impact structure. The
central part (core) of the circular dome is ca. 45 km in diameter, and the prominent ridges
of the metasedimentary/-volcanic collar extend the diameter of the dome to ca. 90 km.
Source: GoogleEarth
works relating to some aspects of the formation of an impact crater and
associated impact metamorphism of the target materials. Also known as “shock
metamorphism”, this process involves those mineral and rock deformations
that demand extreme pressures and temperatures, as well as ultra-high strain
rates, 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 three
main stages (compression, excavation, and modification) are provided by e.g.,
Melosh (1989), Melosh and Ivanov (1999), and Stöffler et al. (2006). Upon
contact of an extraterrestrial projectile with the target surface, shock waves
are generated and travel both into the target and into the impactor. Behind a
shock wave, a release wave leads to material flowing out of the forming crater
with enormous velocity. Obviously the material properties of target and impactor
are relevant with regard to the development of the flow field, but also projectile
velocity and angle of impact are important parameters.
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
The compression phase involves the time between the initial contact of
the projectile with the target until the moment when the entire impactor has
been affected by the shock compression wave. At impact velocities of tens of
kilometers per second, the compression phase involves extreme stresses, high
temperatures, and very high strain rates in the affected materials. Most material
of 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 be
mixed with similarly affected target material. When the shock front travelling
through the projectile reaches the free surface at its end, it is reflected forward
as a so-called “rarefaction wave”. At small and young craters that have formed
by iron meteorites (e.g., Meteor Crater, Arizona; Henbury, Australia), fragments
amounting to a few percent of the original mass of the impactor are commonly
found (e.g., Schnabel et al. 1999; Melosh and Collins, 2005; Bevan, 1996).
After decades of investigation of terrestrial impact structures, only very little
and extremely rare fragments of meteoritic matter have been recovered from
impact melt rock or ejecta. One exception is a meteoritic fragment a few 10s of
centimeters in size that was recovered from the Morokweng impact melt rock
by Meier et al. (2006).
The excavation stage begins once the kinetic energy of the projectile has
been completely transferred to the target. The shock front travelling into the
target – radially away from the contact point – is gradually weakened. Energy
is converted into heat and used up in irreversible change of material states. In
the words of Stöffler et al. (2006), “passage of the shock front imparts a velocity
to 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 the
rarefaction front moves through the shocked material, the overall movement
of target material will be at first directed downward and outward but then
gradually turn upward toward the surface – as a consequence of the
decompression process. One distinguishes two volumes within the transient
crater – the ejected material and the displaced material (see also French, 1989).
The ejected material seems to come primarily from the upper part of the transient
crater, whereas the remaining – about half – volume is pushed (displaced)
downward/outward.
The modification stage begins at that time when the maximum depth of
the transient cavity has been reached. The crater floor then is raised – in small
craters just a bit but in larger ones significantly. Material strength is of great
importance, as is gravity on the impacted body. The enormous amount of impact
deposited energy in the compressed target likely plays an important role as
well. It has been suggested that the rising material can be compared with a
Bingham fluid, and that its motion be regarded as a process known as “viscous
50 YEARS OF IMPACT CRATERING STUDIES
75
fluidization” (Melosh, 1989; Melosh and Ivanov, 1999). Stratigraphic uplift
has been scaled, based on natural observations (e.g., Melosh, 1989). In the
case of the very large Vredefort impact structure, the amount of stratigraphic
uplift recorded in the central uplift known as the Vredefort Dome is considered
some 20 km, against a 250-300 km diameter of the original impact structure.
Shock Metamorphism
Shock pressure and related post-shock temperature decrease radially
away from the point of explosion of the projectile. Where entire impact
structures are accessible for detailed mineralogical analysis, the concept of
progressive shock metamorphism (i.e., progressive increase of shock pressure
and temperature towards the epicenter of the event) could be developed. With
increasing grade of deformation, specific rock mineral deformations and
transformations have been recognized. These, in turn, have been calibrated in
numerous shock recovery experiments with constrained shock pressure and
pre-shock temperature of the target rock. To date, by far the majority of shock
recovery experiments has been conducted with targets held at room temperature,
but the limited number of experiments with pre-heated targets has shown
unequivocally that equivalent shock degrees are reached in pre-heated
specimens at significantly lower shock pressures compared to room temperature
experiments. The effect of pre-cooling of targets on shock degree has not been
evaluated yet, but is important for those bodies in the solar system that have
cold 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 that
for crystalline rocks. Upper crustal, relatively cold rock reacts differently from
a 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) recently
investigated the conditions of impact melting in sedimentary rock. An example
for the progressive change of deformation with increasing shock pressure is
provided by the mineral quartz. Shocked at room temperature, quartz crystals
are elastically deformed only below pressures of ca. 8 GPa. So-called shock
extension 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 pressure
as two or more sets. Up to pressures of ca. 30 GPa PDFs are the dominant
shock deformation effect, which involves partial transformation of crystalline
to amorphous silica, as well as the production of a whole host of different
lattice defects (e.g., Gratz et al. 1992; Stöffler and Langenhorst, 1994). In
addition, transformation of quartz to coesite can occur from ca 15 GPa on, and
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
b
a
c
d
e
Fig.3. Planar deformation features: (a-c) Three examples of planar deformation features in quartz
and feldspar in a sample of alkali granite from the Schurwedraai complex in the Vredefort
Dome (courtesy Claudia Crasselt, Museum for Natural History): (a) Two sets of PDF in
a plagioclase crystal. Crossed polarizers. Width of field of view 0.52 mm. (b) Decorated
PDF in a partially “toasted” quartz grain. Plane polarized light, width of field of view
1.03 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 deformation
features obtained at the TEM (transmission electron microscope) scale – courtesy Thomas
Kenkmann (Museum for Natural History Berlin): (d) Transmission electron micrograph
of an experimentally shock-deformed quartzite-dunite compound specimen: visible are
amorphous PDF lamellae in olivine (length of the short edge is 1.8 µm). Original work
by Kenkmann et al. (2000). (e) Bright field transmission electron micrograph of a quartz
grain from the Kayenta Formation of the Upheaval Dome impact structure (for more
details see Buchner and Kenkmann, 2008). The lamellae along rhomboidal quartz planes
are straight and parallel and have variable thickness. The original glassy lamellae are
healed 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.
50 YEARS OF IMPACT CRATERING STUDIES
77
above ca. 30 GPa, also transformation to stishovite. From this shock degree
on, amorphization of quartz has progressed so strongly that entire crystals are
converted to so-called diaplectic glass (formed through solid-state
transformation not involving a liquid phase). Once the shock temperature of
minerals begins to exceed the liquidus temperatures of individual minerals,
shock melting takes place, from about 45 GPa onward; above 55-60 GPa, bulk
rock melting is initiated. For a large number of important rock-forming minerals,
Pati and Reimold (2006) provided a summary diagram detailing the important
transitions of progressive shock pressure/temperature increase. The optical and
electron microscopic (e.g., Goltrant et al.1991; Gratz et al. 1996) confirmation
of shock metamorphic effects in rock represents the best recognition criterion
for an impact deformed rock or the presence of an impact structure. Only one
macroscopic 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 impact
structures.
Impact-derived rocks, so-called impactites, are generated in different
settings of and around an impact structure (Stöffler and Grieve 2007; Stöffler
and Reimold, 2006). One distinguishes proximal and distal impactites, according
to a very detailed schematic proposed recently by Stöffler and Grieve (2007).
This classification of impactites involves crater facies, especially cataclasites
a
b
Fig.4. Shatter cones: (a) Large shatter cone from the newly discovered Santa Fe site in New
Mexico (Fackelman et al. 2008). Note the prominent horse-tailing on the surface of the
large cone. Scale bar in centimeter intervals. (b) Shatter cones in limestone from the
Waqf as Suwwan impact structure in Jordan (Salameh et al. 2006, 2008).
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
(so-called monomict or polymict lithic breccias), suevites (clastic matrix
breccias with a melt fragment component, and impact melt rock composed of
melt matrix and a more or less abundant clast component. Proximal ejecta
include suevitic and lithic impact breccia ejecta, and distal ejecta include
tektites and spherule beds such as the one known from the Cretaceous-Tertiary
boundary (Smit, 1999). In addition, one observes impact breccia injections
into the crater floor and those may involve lithic, suevitic, and impact melt
veins, as well as a range of rocks (Fig. 5) that in the past have been summarily
termed “pseudotachylite” but that recently have been classified (e.g., Reimold
Fig.5. Bedding-parallel generation plane with several cm thick pseudotachylitic breccia. An
injection perpendicular to bedding extends off the generation vein. Hammer for scale
ca. 30 cm long. Thwane Hill, Hospital Hill Quartzite, western collar of the Vredefort
Dome (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/shock
melting, friction melting, decompression melting, possibly pre- or post-impact
tectonic breccias in the target area/impact structure). Distal impact ejecta in
the terrestrial stratigraphic record have been reviewed by Simonson and Glass
(2004). Detailed accounts of impact breccias, also dealing with lunar and
meteorite breccias, include Stöffler et al. (1980, 1991), French (1989), and
Dressler and Reimold (2001, 2004).
Impact Cratering - A New, Integrated Planetological/Geoscientific Discipline
Historically, the first impact structures have been recognized by the
50 YEARS OF IMPACT CRATERING STUDIES
79
presence of remnants of the projectile (remnants of iron meteorites, as at
Meteor Crater or Henbury, among others). It was not before the mid-20th century
that detailed geological studies of so-called cryptoexplosion structures were
carried out, and only in the early 1960s the first recognition criteria for impact
structures were established (see e.g., reviews in Mark, 1987; Koeberl 2001;
Reimold, 2007a, b). To date, impact cratering studies have become a multidisciplinary research field, involving astronomy and planetary exploration,
geology, geophysics, remote sensing, mineralogy, geochemistry, biological
sciences, and chronology, inter alia. The four legs that this new discipline is
build on include geological site studies, laboratory-based mineralogical and
chemical analysis, shock experimentation for the calibration of shock pressure
and temperature levels and associated microdeformation, and the relatively
new 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 mineral
deformation, but it is also possible to isolate individual parameters, such as
size of projectile, velocity of projectile, impact angle, mineral composition
and porosity of the target, water content, etc., to get down to the basics
determining the physical and chemical response of a material to shock wave
compression and decompression. In this way, impact cratering has come a long
way from its infancy just 50 years ago.
The Terrestrial Cratering Record – With Special Reference to Asia
The Earth Impact Database (2008) of the University of New Brunswick
provides detailed information on ca. 175 known impact structures. While this
includes at least 3 unconfirmed structures (the two Arkenu structures in Libya
and the so-called Suavjärvi structure in Russia, for which no unambiguous
impact evidence has been provided - see e.g., Reimold, 2007), and does not yet
include three recently confirmed impact structures for which publications are
in 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 imagery
and literature about the world’s impact structures. Figure 6a illustrates the
distribution of the impact structures listed in the Earth Impact Database (2008).
It is immediately obvious that the spatial distribution is skewed, and that the
number of impact structures known in parts of Asia, in Africa, and South
America is inferior in terms of landmass in comparison with the other parts of
the world. Reasons for this are multifold: on the one hand, impact structures in
North America and Russia have been investigated extensively in the context of
space exploration, once the predominance of crater structures on the Moon
had become known. The Australian impact record is partly the result of two
people’s lifework – Carolyn and Gene Shoemaker – and some of their friends.
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
a
b
Fig.6. (a) World map of impact structures – modified after the compilation accessible on the
Earth 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 impact
structures. Note the locations of Waqf as Suwwan (WaS) and Dhala (D) in Jordan and
India, respectively. Both these structures have only recently been discovered – the former
through reassessment of older published reports of a cryptoexplosion structure, followed
by field work (Salameh et al. 2006, 2008), the latter through remote sensing and subsequent
ground-based studies (Pati et al. 2008).
North African impact structures are known from the extensive oil exploration
phase of the 1960s and 1970s, and some dedicated impact work has been done
in South Africa in more recent decades (Reimold, 2006). In South America,
only a handful interested geologists have been studying impact, and the vast
rain-forest terrane has hampered recognition of circular features from space.
50 YEARS OF IMPACT CRATERING STUDIES
81
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 1
provides a summary of the Asian structures and some detail about them. Only
Russia and some other countries derived from the former USSR show some
sites of known impact structures. Quite strangely only one impact structure is
known from the vast territory of Mongolia and none from China. Regarding
the latter, only one feature, a so-called Duolon structure (Wu, 1987), was ever
referred in this huge country – and despite some detailed analytical work could
not be confirmed. And in the whole of southern Asia and the Middle East, only
three impact structures have been confirmed, the existence of which has been
determined through the presence of impact melt rock and extensive evidence
of shock metamorphism, to date. Whereas major parts of southern and southeast
Asia 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
Dia.
(km)
Location
Latitude
Longitude
BeyenchimeSalaatin
Bigach
Russia
Kazakhstan
N 71°0'
N 48°34'
E 121°40'
E 82°1'
8
8
Chiyli
Chukcha
Kazakhstan
Russia
N 49°10'
N 75° 42'
E 57°51'
E 97° 48'
Dhala
El’gygytgyn
India
Russia
N 25° 18'
N 67° 30'
Gusev
Jänisjärvi
Russia
Russia
Kaluga
Kamensk
Age
(Ma)
Exposed Drilled
Target
40 ± 20
5±3
Y
Y
N
Y
S
M
5.5
6
46±7
<70
Y
Y
Y
Y
S
M
E 78° 8'
E 172° 5'
11
18
1600 - 2500
3.5 ± 0.5
Y
Y
(Y)
N
C
C
N 48° 26'
N 61° 58'
E 40° 32'
E 30° 55'
3
14
49.0 ± 0.2
700 ± 5
N
Y
Y
N
S
C-M
Russia
Russia
N 54° 30'
N 48° 21'
E 36° 12'
E 40° 30'
15
25
380 ± 5
49.0 ± 0.2
N
N
Y
Y
M
S
Kara
Kara-Kul
Russia
Tajikistan
N 69° 6'
N 39° 1'
E 64° 9'
E 73° 27'
65
52
70.3 ± 2.2
<5
N
Y
Y
N
M
C
Karla
Kursk
Russia
Russia
N 54° 55'
N 51° 42'
E 48° 2'
E 36° 0'
10
6
5±1
250 ± 80
Y
N
Y
Y
S
M
Longancha
Lonar
Russia
India
N 65° 31'
N 19° 58'
E 95° 56'
E 76° 31'
204
1.83
0 ± 20
0.05 ± 0.01
N
Y
N
Y
M
C
Macha
Mishina Gora
Russia
Russia
N 60° 6'
N 58° 43'
E 117° 35'
E 28° 3'
0.3
2.5
<0.007
300 ± 50
Y
Y
N
Y
S
M
Popigai
Puchezh-Katunki
Russia
Russia
N 71° 39'
N 56° 58'
E 111° 11'
E 43° 43'
100
80
35.7 ± 0.2
167 ± 3
Y
N
Y
Y
M
M
Ragozinka
Shunak
Russia
Kazakhstan
N 58° 44'
N 47° 12'
E 61° 48'
E 72° 42'
9
2.8
46 ± 3
45 ± 10
N
Y
Y
Y
M
C
Sikhote Alin
Sobolev
Russia
Russia
N 46° 7'
N 46° 18'
E 134° 40'
E 137° 52'
0.02
0.05
0.000059
<0.001
Y
Y
N
Y
C
M
Tabun-Khara-Obo
Wabar
Mongolia
Saudi Arabia
N 44° 7'
N 21° 30'
E 109° 39'
E 50° 28'
1.3
0.11
150 ± 20
0.00014
Y
Y
N
N
C
S
Waqf as Suwwan
Zhamanshin
Jordan
Kazakhstan
N 31° 3'
N 48° 24'
E 36° 48'
E 60° 58'
5.5
14
<20
0.9 ± 0.1
Y
Y
N
Y
S
M
82
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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).
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use – both not conducive to remote sensing investigations, much of the Middle
East lacks dense vegetation. There is only the 1.8 km wide Lonar impact
crater (Fig.7) in India, at 19°59’N, 76°31’E, which has been known and
confirmed for many years (e.g., Fredriksson et al. 1979; Fudali et al. 1980;
Hagerty and Newsom, 2003; Kumar, 2005; Osae et al. 2005; Chakrabarti and
Basu, 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 km
wide ancient impact structure, the existence of which has been confirmed by
the presence of impact melt rock and extensive evidence of shock metamorphism
in 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 consisting
of a coarse-grained, pink granitic gneiss. This circular structure is centered
at 25º17’59.7"; 78º8'3.1" on the Central Indian Bundelkhand craton, Shivpuri
district, 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 the
11 km Dhala impact structure, India. The expansive monomict impact breccia exposures
are obvious in the form of a ring around the CEA. For more detail, refer to Pati et al.
(2008).
84
a
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 composed
of 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 the
eroded yet still very well exposed central uplift. Note the strong deformation of the strata. Dark rocks are chert, the lighter strata comprise limestone and
minor sandstone. The width of the central uplift is about 0.9 km.
W. UWE REIMOLD AND CHRISTIAN KOEBERL
b
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area comprising post-impact sediments. Without any drilling evidence it has
not been possible yet to establish whether this central elevation covers relics
of the central uplift structure.
Waqf as Suwwan (Fig. 9a and b), located at 31°3’N / 36°48’E in the desert
of 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 of
occurrences of shatter cones, proof for the definite existence of an impact
structure. Waqf as Suwwan is about 5.5 km in diameter and despite some erosion
since its formation at estimated 10-15 Myr ago, still shows a well exposed
central 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 kind
in sedimentary rock. Detailed multidisciplinary analysis (remote sensing,
geology, geophysics) is underway. Jordanian scientists and some authorities
are considering the possibility of making this site a protected conservation
area 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 (Fackelman
et al. 2008). The cones occur as a penetrative feature in intrusive igneous and
supracrustal metamorphic rocks; they are unusually large (up to 2 m long and
0.5 m wide at the base; Fig. 4a). In thin section, quartz grains within the cone
surfaces show optical mosaicism, as well as decorated planar fractures (PFs)
and rare planar deformation features (PDFs). The PFs and PDFs are dominated
by basal (0001) crystallographic orientation, which indicates a peak shock
pressure of up to 10 GPa that is consistent with shatter cone formation (French,
1998). Regional structural and exhumation models, together with anomalous
breccia units that overlie and crosscut the shatter cone-bearing rocks, should
provide size and age constraints for the impact event. The observed shatter
cone outcrop area suggests that the minimum final crater diameter of the Santa
Fe impact structure was about 6–13 km and the age is so far only vaguely
constrained 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), located
at N25°20’, E76°37’ in India, has been repeatedly related to impact (e.g., Master
and Pandit, 1999), most recently by Sisodia et al. (2006a,b). These authors
claimed to have recorded impact-diagnostic shock deformation, including
alleged planar deformation features (PDF) in quartz, diaplectic glass, and
impact spherules. This claim was, however, rejected by Reimold et al. (2006),
who discussed point by point that none of the evidence presented constituted
bona fide impact diagnostic findings. And in the vicinity of Lonar impact
crater, 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)
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Fig.10. Ramghar “crater” structure (modified, courtesy GoogleEarth). The “crater” structure is
about 4 km wide.
and Fudali et al. (1980) suggested that Little Lonar is a second impact crater
formed by a fragment of the larger bolide that was responsible for Lonar crater.
Master (1999) claimed the presence of breccias and alleges shatter cones at
Little Lonar. This evidence, however, remains to be confirmed – shatter cones
are unknown to form from such very small craters, and the ejecta found in the
area are simply part of the close-by Lonar crater (e.g., Osae et al. 2005). Detailed
observations by Maloof et al. (2008) also make it very unlikely that Little
Lonar is an impact feature.
Karanth (2006) and Karanth et al. (2006) reported on an unusual, 1.2 km
wide feature in Kachhh, western India, and related some unusual observations
to an impact origin. As in the case of Little Lonar and Ramghar, the evidence at
this site needs to be examined carefully to evaluate this claim.
When talking about the terrestrial impact record, the question of age
constraints on the known impact structures and on distal impact ejecta, and
what it tells us about the flux of large extraterrestrial projectiles onto this planet
and their catastrophic effects on Earth and evolution of life on it, must be
considered. Careful checking of the age data available for the known terrestrial
impact structures (Jourdan et al. 2007a; Renne et al. 2007) reveals that only a
good handful (in fact, only 11) impact events have been dated with uncertainties
better than 1%. In many cases, no absolute age data are available at all, or error
50 YEARS OF IMPACT CRATERING STUDIES
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limits as high as 10-20% have been reported. Major problems related to the
precise dating of impact melt rocks are inherent to the nature of the complex
impact breccias. Impact melt rocks are mixtures of impact generated melt
plus clastic debris from the target rock. The latter may not be fully reset with
respect to the isotopic system used in the dating experiment. A preferred method
is U-Pb single zircon dating, whereby success depends largely on the time of
crystallization of the impact melt. In some cases, a newly grown zircon phase
could 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 target
rock have become available (e.g., Dhala – unpublished work by our group).
The use of argon dating techniques may also be obstructed by insufficient
degassing of target rock clasts – especially in those cases where the age
difference between the impact event and the formation of the target rocks is
very large (Jourdan et al. 2007b).
Recently new analytical improvements in zircon dating allowed age
determinations 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 noritic
phase of the Sudbury impact melt gives 1849.53 ± 0.21 Ma, whereas a phase
from several hundred meters higher in the noritic layer is resolvably younger
at 1849.11±0.19 Ma (95% confidence errors). This technique should help to
improve the dating record of impact craters if appropriate samples are available.
However, it is obvious that such a limited impact chronological data base
causes severe problems when investigating the projectile flux onto our planet
through time. What is worse, by far the majority of impact structures known to
date is younger than 350 Ma – which means that >90% of Earth evolution are
not represented in this database. Only two impact structures have been positively
identified as being of early Proterozoic origin – Sudbury and Vredefort. In
addition, there are three (possibly 4) known distal ejecta layers, so-called
spherule layers (Fig. 11), of ca. 3.4 Ga age in the Barberton Mountain Land in
South 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). These
ancient ejecta layers represent the entire record of Archean impact on this planet
(Koeberl, 2006a,b; Grieve et al. 2006). There are also three spherule layers
known in the stratigraphy of the Transvaal Supergroup in South Africa, and
also three in the Hamersley Basin succession of Western Australia (e.g.,
Simonson et al. 2004, 2008). The spherules in these strata have similar petrographic appearance, which has been studied and described in great detail by
Bruce Simonson and his co-workers. Simonson et al. (2008) propose that two
of these respective layers in South Africa and Western Australia seem to be
correlatives, whereas the respective third cases may not be related to each
other.
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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 Koeberl
et al. (1993) and Koeberl and Reimold (1995).
Two other spherule layer occurrences have been reported in the literature:
one in South Greeenland and that has been, on very poor chronological
constraints, tentatively related to either the Sudbury or the Vredefort event
(Chadwick et al. 2005), but the impact origin of that layer has not yet been
confirmed. Secondly, actual Sudbury ejecta have been detected in Minnesota
and Michigan (Addison et al., 2005; Pufahl et al., 2007). It is certain that there
are remnants of further impact events that so far have gone undetected, but
traces of which could be identified in the extensive records of Phanerozoic
rocks that have been drilled and studied in outcrop widely. Therefore, it is
mandatory that every geology student be informed about the existence and
characteristics of these spherule horizons.
In the Phanerozoic, a number of other stratigraphic horizons are associated
with mass extinctions, and these have been repeatedly linked with impact.
However, to date there is no conclusive evidence that mass extinctions in the
Late 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 extensive
debate has centered on a possible link between the mass extinction at the
Permian/Triassic boundary (252 Ma ago) and impact (e.g., Chapman, 2005),
and a number of possible impact sites have been promoted for this alleged
event. It was suggested that an alleged 120 km sized impact structure in Western
Australia, known as Woodleigh, could be the smoking-gun for the P/Tr
catastrophe, but this impact structure turned out to be too small at 60 km diameter
to have caused a global catastrophe capable of more or less instantaneously
destroying about 90% of all species at that time (Reimold et al. 2003). Also the
50 YEARS OF IMPACT CRATERING STUDIES
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age of Woodleigh, for which at first a P/Tr boundary age was favored, was
quickly changed to a Late Devonian age (conveniently also at a time associated
with 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 Bedout
impact structure northwest of Australia (Becker et al. 2004). However, the
very existence of a large circular structure there and a fortuitous age for an
associated melt rock (impact or volcanic in origin) of allegedly 250 Ma have
both been rejected since as evidence for impact (Renne et al. 2004; Müller et
al. 2005). Also, reports of shock metamorphosed quartz from Antarctic and
Australian 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 evidence
for impact activity at the Permian/Triassic boundary and found no evidence
for 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 termed
the K/P – Cretaceous/Paleogene) boundary (Smit, 1999; Simonson and Glass,
2004). In contrast to the Archean ejecta layers, in which only very rare shocked
material (Rasmussen and Koeberl, 2004) has been detected so far and the origin
of which by impact is solely constrained by trace chemical and isotopic
signatures, there are shock deformed and impact generated particles (shocked
quartz, glass, microtektites, high-Ni spinel), besides PGE enrichments and Cr
and Os isotopic impact signatures (see below), in this world-wide stratigraphic
marker. In addition, the impact site for the K/T catastrophe has been discovered
and extensively studied. Two special volumes (MAPS, 2004a) have been
dedicated to a major drilling project in this large impact structure, and these
contributions provide a useful entry into the Chicxulub and K/T impact
literature. Interestingly, while some groups took a long time to become
comfortable with the idea of a K/T impact catastrophe, some of these workers
have since switched camps and are now favoring two, related closely in time,
impact events – with only one of them linked to the Chicxulub impact (Keller
et al. 2003, 2007; Stüben et al. 2005). This idea has, however, found stern
opposition 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 described
in more detail below. However, for completion of this section it must be noted
that three of these tektite strewn fields have been unequivocally related to
source impact craters: the Ivory Coast tektites to the 1.07 Ma Bosumtwi crater
in Ghana, the moldavites of central Europe to the 14.5 Ma Ries crater in southern
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a
b
Fig.12. Australasian tektites: (a) Examples of different tektite forms and shapes (Australasian
tektites). Top left: drop-shaped indochinite. Bottom left: button-shaped thailandite with
flow textures. Right: Fragment of a Muong Nong-type (layered) indochinite. (b) Map of
the Australasian tektite/microtektite strewn field, after Glass and Koeberl (2006). Tektite
occurrences on land are indicated schematically by Xs. Microtektite-bearing sites in the
ocean are indicated with open circles and black dots. The black dots indicate sites with
the highest concentrations of microtektites and associated unmelted ejecta (the two
concentric lines indicate sites that have microtektite abundances between 20 and
200/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 known
occurrences of tektites on land and microtektites in marine sites are enclosed with a
dashed 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 that
best explains the geographic variation in abundance of unmelted ejecta and microtektites.
However, any location within the dark grey oval, which extends from southern China
through northern Vietnam into the Gulf of Tonkin, explains the geographic variation in
abundance of impact ejecta and microtektites equally well. The still undiscovered
Australasian tektite/microtektite source crater is expected to lie within this region.
50 YEARS OF IMPACT CRATERING STUDIES
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Germany, and the North American tektite strewn field to the 35.4 Ma
Chesapeake Bay impact structure in Virginia (USA).
Important Impact Cratering Research Issues and Thrusts
An extremely significant issue is the question of “how to recognize impact
structures”! From remote sensing and morphological observations, and
geophysical data important initial observations can be made. For example,
circular drainage patterns or surface forms, and circular positive or negative
potential 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 show
distinct magnetic anomalies; sink-holes and roundish pans derived from other
geologic or human activity may indicate negative gravity anomalies or shapes
similar to those of impact craters). In large, complex impact structures the
central uplift of raised, relatively denser, originally deeper seated rocks may
indicate a slightly positive circular anomaly, surrounded by the negative annular
anomaly related to low-density impact breccia fill of the ring basin around the
central uplift. In the case of very old, deeply eroded impact structures, this may
well be the only remaining feature. Potential field anomalies for impact
structures have been reviewed by, for example, Pilkington and Grieve (1992)
and Grieve and Pilkington (1996). Magnetic anomalies of impact structures, in
particular, may have quite variable signatures, due to the combination of induced
magnetic character of the various target rocks and the impact-induced remanent
magnetization superposed on target rocks and impact breccias (see, e.g., the
review by Henkel and Reimold, 2002). Seismic studies may show loss of seismic
coherence due to the impact-induced rock deformation. In general, remote
sensing, morphology, and geophysics provide useful first-order indications of
the possible presence of an impact structure. Indeed, in the last decades, a
number of impact structures have initially been suggested by such data.
Especially some observations from ore exploration have proven useful in
this regard. However, none of these observations constitutes unambiguous
evidence for an impact origin.
Definite confirmation of an impact origin can only be obtained from
mineralogical or chemical observations; consequently, ground truthing or
investigating a candidate structure by drilling and subsequent rock analytical
studies are required. The impact specific high shock pressures and temperatures
induce – in upper crustal rocks – the distinct deformation features already
introduced as “shock metamorphism”. Whereas normal crustal metamorphic
conditions may be able to attain several kilobars pressure and perhaps locally
up to 1000 °C temperatures, impact conditions involve shock pressures up to
several tens of gigapascals (GPa) and several thousands of degrees of shock or
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post-shock temperatures. This induces the specific deformation effects such as
planar fractures, planar deformation features, diaplectic glass, high-pressure
polymorphs that would be entirely out of place in an upper crustal setting, such
as 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 and
Reimold, 1996; Reimold, 2007a). In many cases such features may be
recognizable in thin section, but where doubt about the presence of planar
shock deformation features remains, electron microscopy may have to be
employed (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 for
shock deformation, but highly stressed target rocks from the central part of an
impact structure may also suffice. Melt fragments in suevite and impact melt
rocks are the most sought-after impactite lithologies, as besides possibly carrying
shock metamorphosed clasts, they may also be suitable for age dating of an
impact 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 produced
under such low pressures have been carried out yet), however, the crater floor
and rim zone may be intruded by so-called dike breccias, including veins and
dikes of suevite and impact melt rock. The need to identify additional criteria
for 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 majority
proportion of low-shock deformed material. Further dispersal is encountered
when eroded material is shed into sediment reservoirs.
The second method to obtain unambiguous evidence of impact is based
on several techniques to detect traces of the meteoritic projectile. Detailed
reviews of the background to these techniques and the state of art of
identification of meteoritic projectiles have been published, for example, by
Koeberl (1998, 2007). In the absence of actual meteorite fragments, it is
necessary to chemically search for traces of meteoritic material that is mixed
in with the target rocks in breccias and melt rocks. Meteoritic components
have been identified for about 45 impact structures, out of the more than 170
impact structures that have so far been identified on Earth (see list of successful
cases in Koeberl, 2007). The presence of a meteoritic component can be verified
by measuring abundances and interelement ratios of siderophile elements,
especially the platinum group elements (PGE), which are much more abundant
in meteorites than in terrestrial upper crustal rocks. Often the content of the
element iridium is measured as a proxy for all PGEs, because it can be measured
with the best detection limit of all PGEs by neutron activation analysis; but
50 YEARS OF IMPACT CRATERING STUDIES
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taken out of context (without supplementary chemical and petrographic
information), small Ir anomalies alone have little diagnostic value (as has been
shown in, for example, the case of the P/Tr boundary; Koeberl et al. 2004).
More reliable results can be achieved by measuring whole suites of
elements, for example, the PGEs, which also avoids some of the ambiguities
that result if only moderately siderophile elements (e.g., Cr, Co, Ni) are used.
It is difficult to distinguish among different chondrite types based on siderophile
element (or even PGE) abundances, which has led to conflicting conclusions
regarding 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 a
meteoritic component in a number of impact melt rocks and breccias. Both of
these methods are based on the observation that the isotopic compositions of
the elements Os and Cr, respectively, are different in most meteorites compared
to terrestrial rocks; the Cr isotopic method allows, in addition, the identification
of 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 more
impact structures on Earth? First of all, our planet – in contrast to the other
solid bodies of the solar system that have retained a long impact record on
their surfaces – is dynamic. Plate tectonics have erased most of the impact
scars created over time. It takes roughly 150 Ma to completely resurface the
oceanic crust on this planet, constituting some 70% of the entire surface area.
Impact structures, especially the large ones, are relatively shallow morphological
features, and are quite quickly eroded. Small, simple structures are also not
very 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 surface
modifying 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 (papers
in Canup and Righter, 2000). Koeberl (2006a,b) discusses what the possible
effects of impact bombardment in the early Archean could have been for Earth.
Quite a debate has also raged about whether a so-called “Late Heavy
Bombardment” 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 decision
is still outstanding on this issue, but as Koeberl (2006a,b) reported, evidence
for a LHB on Earth is still lacking. The search for Archean evidence of impact
has 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 had
regarding the evolution of life on this planet. How did impact cratering affect
the presence of life-sustaining (e.g., Westall et al. 2006; Schopf, 2006) water
on Earth? The possibility that a barrage of cometary projectiles onto the early
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Earth could have contributed to the hydrosphere development has been
discussed. The role of early impact bombardment on the development of an
early atmosphere must be investigated further. And the fact that “lithopanspermia” – the transfer of primitive life between planets – is a potentially
viable process has been strongly supported by impact experimentation
and 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 Anderson
et al. (2002) and Wells et al. (2003).
As discussed before, the current evidence for impact in the Archean is
limited to a handful of spherule layers. Such ejecta have not been detected yet
from the 2.02 Ga Vredefort impact – however, they are now known from the
Sudbury impact event (Addison et al. 2005; Kring et al. 2006). It is hoped that
more stratigraphers/sedimentologists/exploration geologists will take
cognizance of the importance of such ejecta, in order to improve the Proterozoic
and Phanerozoic impact ejecta record. This, in turn, may lead to further
successes in pinpointing the source craters for such ejecta layers, as it was
possible in the K/T boundary-Chicxulub case in the 1990s.
Basic research on shock deformation – or the response of different
materials to shock pressures, temperatures and ultra-high strain rates – is still
required. Much of the experimental work to calibrate shock deformation in
specific minerals has been done with single-crystal target specimens, and also
the shock recovery experimentation with rock samples is still quite limited. By
far the majority of calibration experiments with rocks were done with crystalline
rock targets, and porous, water-bearing rocks (such as sedimentary rocks) have
been comparatively neglected. And yet, it is sedimentary rocks that are
ubiquitously found in target areas on continents – their shock response must be
evaluated further. Shock metamorphic studies are required to confirm the origin
of a suspected impact structure. In recent years, it has become more and
more fashionable to propose an impact structure or to relate seemingly
catastrophic demise in the terrestrial bioevolutionary record with impact.
Unfortunately, in many of these instances, the promoters of such stories have
found editors willing enough to publish their sensationalist yet unfounded
writings. The criteria for the recognition of an impact structure or impact affected
material have been laid out clearly. Confirmation of the presence of meteoritic
material – even if only in the form of traces determined by analytical chemical
methods – and/or of shock metamorphic deformation is required. These criteria
must be promoted amongst the geoscientific community and especially amongst
students of geoscience. And the impact cratering community has to remain
vigilant 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).
50 YEARS OF IMPACT CRATERING STUDIES
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On the positive side, since about 20 years hydrocode modelling (e.g., a
good introduction to this topic is contained in Weiss and Wünnemann, 2007;
also Pierazzo and Collins, 2003) has become so advanced that it is now possible
to investigate individual physical parameters at play during impact and isolate
their influences, in 2D and 3D modelling experiments. In Figures 13a and b
two examples of numerical modelling runs in 2D and 3D presentation are
shown. Both models were calculated for the case of vertical impact of a 1 km
diameter asteroid into a granitic target at 12 km/s. The sequence of time
steps shown ranges from the moment just before impact to 95 s after, at which
time the final crater is complete. This sequence of events would be an
approximation 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 the
formation of the transient cavity, development of the ejecta curtain and flap
of overturned target strata in the rim zone of the collapsed complex crater,
central uplift formation (upward bending of stratigraphic lines in the middle of
the section), and finally the collapse of the central uplift indicated by the
outward directed (i.e., outward flowing) uppermost strata around the central
uplift. Note the strong deformation of strata around the outer edge of the
collapsed crater, which corresponds to significant faults developed in this
outermost crater zone. Modern numerical modeling tools allow to investigate
the fate of a single limited-size block of material throughout the ultra-fast
impact event (e.g., Gohn et al. 2008), or even to study on the micro-scale the
interaction of a single pore space with a shock wave. The simple example of a
3D model shown in Fig.13b alludes to the amazing visualization possibilities
that these most modern hydrocode developments allow. Numerical modelling,
in conjunction with shock experimentation and ground based research will
continue to improve our knowledge about interaction of shock waves and
materials. Bridging the Gap between modellers and field workers has been
the 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 chemically
homogeneous, often spherically symmetric natural glasses, with most being a
few centimeters in size. Mainly due to chemical studies, it is now commonly
accepted that tektites are the product of melting and quenching of terrestrial
rocks during hypervelocity impact on the Earth (see, e.g., the reviews by
Koeberl, 1994, and Montanari and Koeberl, 2000). The chemistry of tektites is
in many respects identical to the composition of upper crustal material. Tektites
are currently known to occur in four strewn fields of Cenozoic age on the
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
Fig.13a
50 YEARS OF IMPACT CRATERING STUDIES
13b
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
97
Fig.13b
Fig.13. Results of numerical simulations of the vertical impact of a 1 km diameter asteroid into
a granitic target, calculated with the iSALE hydrocode (Amsden et al. 1980; Wünnemann
et al. 2006). (a) Cross section through the crater center showing tracer lines to visualize
deformation in the target. Four time steps are shown covering the interval from 0 s (just
before impact) to 95 s after impact, by which time the formation of the final crater is
complete. Step two shows the development of a somewhat hemispherical transient crater
cavity, 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 crater
floor and concomitant development of the central uplift (note the upward bent tracer
lines in the central part of the crater. Overturning of target material at the edge of the
structure is prominent. Step 4 at completion of the final crater shows the collapsed central
uplift, with tracer lines clearly illustrating the outward and downward flow of the
uppermost section of the central uplift. (b) Various time steps, calculated for the same
impact experiment, but showing density contours on the front face of a half space of the
impact structure. The “ripple marks” occurring in some of the illustrations are visualization
artifacts of the numerical modeling. Red colors indicate high density, and blue is low
density. Both series of results were kindly made available for this review by Kai
Wünnemann and Dirk Elbeshausen, Museum for Natural History, Berlin.
surface of the Earth. Strewn fields can be defined as geographically extended
areas over which tektite material is found. The four strewn fields are: the North
American, Central European (moldavite), Ivory Coast, and Australasian strewn
fields. Tektites found within each strewn field have the same age and similar
petrological, physical, and chemical properties. As mentioned above, relatively
reliable links between craters and tektite strewn fields have been established
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
between the Bosumtwi (Ghana), the Ries (Germany), and the Chesapeake Bay
(USA) craters and the Ivory Coast, Central European, and North American
tektite strewn fields, respectively. The source crater of the Australasian strewn
field 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 in
diameter) from three of the four strewn fields have been found in deep-sea
cores. Microtektites have been very important for defining the extent of the
strewn fields, as well as for constraining the stratigraphic age of tektites, and
to provide evidence regarding the location of possible source craters.
Microtektites have been found together with melt fragments, high-pressure
phases, and shocked minerals (e.g., Glass and Wu, 1993) and, therefore, confirm
the association of tektites with an impact event. The variation of the microtektite
concentrations in deep-sea sediments with location increases towards the
assumed 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 the
following characteristics should probably be included (see Koeberl 1994;
Montanari and Koeberl 2000): (1) they are glassy (amorphous); (2) they are
fairly homogeneous rock (not mineral) melts; (3) they contain abundant
lechatelierite; (4) they occur in geographically extended strewn fields (not just
at one or two closely related locations); (5) they are distal ejecta and do not
occur directly in or around a source crater, or within typical impact lithologies
(e.g., suevitic breccias, impact melt breccias); (6) they generally have low water
contents and a very small extraterrestrial component; and (7) they seem to
have formed from the uppermost layer of the target surface (see below). Thus,
the term “tektite” should be used only for true glasses, not for recrystallized
objects, and, if the criteria listed above are not fulfilled, use the (in such a case
much 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 that
indicate the nature of the parent material and contain shock-produced phases
that indicate the conditions of formation, and that sedimentary source rocks
were involved.
Despite knowing the source craters of three of the four tektite strewn
fields, we still do not know exactly when and how during the impact process
tektites form. Detailed reviews of the parameters that have to be considered
were provided by, for example, Koeberl (1994) and Montanari and Koeberl
50 YEARS OF IMPACT CRATERING STUDIES
99
(2000). These characteristics include, for example, the following observations:
(a) vapor fractionation played no major role in tektite formation; (b) tektites
are 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 low
pressures; (d) meteoritic components in tektites are very low or below the
detection limit; (e) the 10Be content of Australasian tektites cannot have
originated from direct irradiation with cosmic rays in space or on Earth, but
can only have been introduced from sediments that have absorbed 10Be
that was produced in the terrestrial atmosphere. Tektites might be produced in
the 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 a
recent discussion). However, the question how the tektite production and
distribution occurred in detail remains the subject of further research.
Furthermore, it is possible that new locations of tektites are found in the
future. There is a discrepancy between modeling calculations that indicate
that very early melts formed by jetting should have a high meteoritic component,
whereas tektites – which are inferred to have formed very early in the impact
process – 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 order
to determine whether the various crater production curves, which are extensively
utilized 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 a
relatively small number of lunar rocks have been dated by absolute radiometric
techniques and serve as absolute calibration of crater production curves. Are
the lunar curves directly applicable to Mars or Mercury? The problem of the
still 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, are
covered with extrusive rocks. However, on Earth, only two impact structures
formed in basalt are known – 1.8 km Lonar Lake in India and the 17 km size
Logoisk structure in Russia (and in the latter case no basalt is exposed). Most
known terrestrial impact structures were formed in granitoids and sedimentary
rocks – and much still needs to be learned about those target materials that
form extraterrestrial planetary surfaces, including those regolith covered
asteroids – and also cometary matter.
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The Impact Hazard
The 1994 impact of fragments of comet Shoemaker-Levy 9 into the
atmosphere of Jupiter demonstrated to humankind that impact is not a threat of
the 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 of
a roughly 50-m-sized projectile - for every 1500 years or so (Harris, 2008), and
if this event had occurred in a highly populated region, this relatively minor
event comparable to ca. 1000 Hiroshima atom bombs could have caused a loss
of life potentially going into the millions. A large event of Chicxulub magnitude
might occur every 100 million years, but this statistical approach does not
predict when it might happen next. Depending on the actual velocity of the
Chicxulub impactor, that projectile might have measured between 5 and 15 km
in diameter, although scaling from the amount of extraterrestrial material around
the world indicates a value of about 10 km. However, current predictions of
impact consequences suggest that even a 1 km sized bolide might be sufficient
to seriously cause harm to mankind through global environmental catastrophe.
Projectiles enough are out there – comets in the Kuiper-Edgeworth belt and in
the Oort cloud, and near-earth objects (those asteroids that approach the
Sun closer than 1.3 astronomical units) are not particularly rare in the asteroid
belt (according to Chandler, 2008, some 750 large asteroids and further
5500 asteroids in total have now been mapped in the asteroid belt, largely a
result of the NASA sponsored international Spaceguard surveying project).
Harris (2008) relates that “the risk of any person’s death by impact prior to
Spaceguard 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 in
600,000 as the danger from likely still undetected asteroids. In fact, Harris
presents a chart that suggests that before Spaceguard the short-term risk of
impact 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 impact
effect. Space search programs for possible still unknown asteroids continue,
and the danger from suddenly appearing comets remains anyway. The latter
case provides a challenge for impact workers, as the behavior of large, lowdensity projectiles, and the effects of their impacts onto various planetary
surfaces are far from understood. The terrestrial impact record has to be
improved to further constrain the past impact flux onto Earth, and the energy
threshold 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
50 YEARS OF IMPACT CRATERING STUDIES
101
particularly lethal because of the sulfate- and carbonate-rich target area at
Yucatán? Can our highly evolved and through its intricate civilization highly
vulnerable race survive a much smaller impact event, perhaps only creating a
40 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 impact
structures could provide very useful habitats for primitive life forms – and
thus, 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). Other
benefits of impact cratering include the development of, at times, major ore
deposits – such as the gigantic Sudbury ores, many sites of significant – or
even major (e.g., the gas fields around the Alaskan Avak impact structure or
the Campeche oil field in the vicinity of the Chicxulub structure in Mexico)
hydrocarbon showings, and the effect of the Vredefort impact onto the
Witwatersrand gold and uranium resources. A number of comprehensive reviews
of this topic have been published: Grieve and Masaitis (1994); Grieve (2005);
Reimold et al. (2005). No developed and certainly no developing nation can
afford to ignore the possibility that its impact structures could carry important
metal or energy resources.
Drilling of impact structures, especially through a series of recent ICDP
(International Continental Scientific Drilling Program) ventures, has strongly
fertilized 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 boundary
impact, impact in the marine realm, and regarding post-impact sedimentary
and biological processes.
Moreover, well-exposed impact structures may serve as unique site
museums, for instruction of laypeople and students in geoscience, planetary
science, astronomy, life and environmental science. A number of excellent
museums have been built already (e.g., at Meteor Crater, and especially in the
Ries Crater), or are in final stage of development (at Vredefort). The Ries
Crater in southern Germany has been identified as a unique geotope and has
been declared a German Geopark. And a portion of the Vredefort impact
structure in South Africa was declared a World Heritage Site by UNESCO in
2005, and is currently developed into a major geotouristic and educational
terrane. The newly discovered impact structure Waqf as Suwwan in Jordan
was found in May 2008 to have been locally affected by apparently illegitimate
limestone exploration activity. Strong efforts are being made there to preserve
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W. UWE REIMOLD AND CHRISTIAN KOEBERL
this well exposed impact structure and geological site, and it is hoped that this
may lead to responsible development and sustainable educational and
geotouristic utilization.
Outlook
This essay has shown that a lot has been learned about the impact process
and its influence on Earth and Life evolution. Impact cratering has been a
fundamental process in the planetological sense, with regard to the earliest
evolution of our own planet, and throughout its history – also decisively
influencing the evolution of Life. However, the terrestrial cratering record is
far from impressive, and must – und undoubtedly will – be improved in coming
years. While the geological community has been rather slow in embracing
knowledge about this process – it has only been in the last few decades that
impact cratering has found acknowledgement in geoscience textbooks, it is
now widely accepted that the effects of impact cratering permeate every
discipline of the earth and planetary science.
It is hoped that this article will contribute perhaps somewhat to wider
recognition of impact as a fundamental, and for Earth and mankind
fundamentally important, planetological process. The recent discovery of Waqf
as Suwwan in Jordan, with its well preserved and exposed central uplift, or the
finding of the remnants of a previously unknown impact structure near Santa
Fe (USA), have demonstrated that any new discovery can significantly enhance
our knowledge about the impact process and the response of different materials
to it.
Fifty years of impact research have gone by, parallel to 50 years of space
exploration. And yet, with regard to the many problem areas and open questions
that have been raised above we are still at the beginning of gathering basic
information. Impact cratering studies have transformed into a multidisciplinary
integrated science discipline – and this approach will be successful to provide
the missing answers.
ACKNOWLEDGMENTS
We are most grateful to the Editor of this special volume for the invitation
to represent this important discipline - impact cratering research - in this
benchmark 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 Claudia
Crasselt assisted with other graphics. Figures 13a and b were contributed by
Kai Wünnemann and Dirk Elbeshausen. We are grateful to Dr. B.P. Radhakrishna
and Dr. Fareeduddin for comments on the manuscript.
50 YEARS OF IMPACT CRATERING STUDIES
103
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