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Formation of Impact Craters 17
17
The processes by which large impact craters form, and
the sudden releases of huge quantities of energy involved,
cannot be duplicated in the laboratory, and, fortunately, no
such structure has formed during recorded human history.
All our knowledge about large impact structures is therefore
indirect, and it has come from combining several areas of
once-separate research: theoretical and experimental studies
of shock waves (for reviews and literature, see Melosh,
1989), experimental production of small craters (e.g., Gault
et al., 1968; Gault, 1973; Holsapple and Schmidt, 1982, 1987;
papers in Roddy et al., 1977), and geological studies of larger
terrestrial impact structures (Shoemaker, 1963; Dence, 1968;
Dence et al., 1977; Grieve and Cintala, 1981; Grieve et al.,
1981; Schultz and Merrill, 1981; Stöffler et al., 1988). The
cratering process is complex, many details are still uncertain,
and neither calculations nor predictions can be made with
firm confidence. But these studies provide the essential basis
for understanding how impact craters form and for deciphering
the geological features they display.
3.1. SHOCK WAVES AND
CRATER FORMATION
The general term “impact crater” is used here to designate
a hypervelocity impact crater, the structure formed by
a cosmic projectile that is large enough and coherent enough
to penetrate Earth’s atmosphere with little or no deceleration
and to strike the ground at virtually its original cosmic
velocity (>11 km/s). Such projectiles tend to be relatively
large, perhaps >50 m in diameter for a stony object and >20 m
for a more coherent iron one.
Smaller projectiles, typically a few meters or less in size,
behave differently in passing through the atmosphere. They
lose most or all of their original velocity and kinetic energy
in the atmosphere through disintegration and ablation, and
they strike the ground at speeds of no more than a few hundred
meters per second. In such a low-velocity impact, the
projectile penetrates only a short distance into the target (depending
on its velocity and the nature of the target material),
and the projectile’s momentum excavates a pit that is
slightly larger than the projectile itself. The projectile survives,
more or less intact, and much of it is found in the
bottom of the pit. Such pits, sometimes called penetration
craters or penetration funnels, are typically less than a few
tens of meters in diameter.
Examples of these features include Brenham (Kansas),
the many small pits made by the Sikhote-Alin (Russia) meteorite
shower in 1947, and the pit dug by the largest piece
of the Kirin (China) meteorite fall in 1976. The process of
excavation is strictly a mechanical one, and high-pressure
shock waves are not produced.
In sharp contrast, a hypervelocity impact crater starts to
form at the instant that an extraterrestrial object strikes the
ground surface at its original cosmic velocity. These impact
velocities are much greater than the speed of sound in the
target rocks, and the crater is produced by intense shock
waves that are generated at the point of impact and radiate
outward through the target rocks. Shock waves are intense,
transient, high-pressure stress waves that are not produced
by ordinary geological processes (for details, see Melosh, 1989,
Chapter 3 and references therein). Peak shock pressures produced
at typical cosmic encounter velocities may reach several
hundred GPa. These pressure are far above the stress
levels (~1 GPa) at which terrestrial rocks undergo normal
elastic and plastic deformation, and the shock waves produce
unique and permanent deformation effects in the rocks
through which they pass.
The shock waves radiate from the impact point at high
velocities that may exceed 10 km/s, much greater than the
speed of sound in the target rocks. As they expand, they interact
with the original ground surface to set a large volume
of the target rock into motion, thus excavating the impact
crater. The formation of an impact crater by shock waves,
Formation of Impact Craters
18 Traces of Catastrophe
and the immediate modification of the newly formed crater
by gravity and rock mechanics, is a complex and continuous
process. However, it is convenient to divide this process,
somewhat arbitrarily, into three distinct stages, each dominated
by different forces and mechanisms: contact and compression,
excavation, and modification (Gault et al., 1968;
see also Melosh, 1989, Chapters 4, 5, and 8).
3.1.1. Contact/Compression Stage
This stage begins at the instant that the leading edge of
the moving projectile makes contact with the ground surface.
If the target is solid rock, the projectile is stopped in a
fraction of a second and penetrates no more than 1–2× its
own diameter (Fig. 3.1) before its immense kinetic energy is
transferred to the target rocks by shock waves generated at
the interface between projectile and target (Kieffer and
Simonds, 1980; O’Keefe and Ahrens, 1982, 1993; Melosh, 1989,
Chapter 4).
The general features of this conversion of kinetic energy
into shock waves have been determined from experiments
and theoretical studies (e.g., O’Keefe and Ahrens, 1975, 1977,
1993; Ahrens and O’Keefe, 1977; papers in Roddy et al., 1977;
Melosh, 1989, Chapter 4), although many details are still not
well understood. One clear result is that, as one set of shock
waves is transmitted outward from the interface into the target
rocks, a complementary shock wave is reflected back into
the projectile (Fig. 3.1) (Melosh, 1989, Chapter 4; O’Keefe
and Ahrens, 1993).
The shock waves transmitted into the target rocks lose
energy rapidly as they travel away from the impact point.
Two factors are involved in this energy loss: (1) the expanding
shock front covers an increasingly larger hemispherical
area with increasing radial distance, thus reducing the overall
energy density; (2) additional energy is lost to the target
rocks through heating, deformation, and acceleration. The
peak pressures of the shock waves therefore also drop rapidly
with distance from the impact point. Theoretical models
(Melosh, 1989, pp. 60–66) and geological studies of
shock-metamorphosed rocks in individual structures (Dence,
1968; Robertson, 1975; Grieve and Robertson, 1976; Dence et
al., 1977; Robertson and Grieve, 1977; Dressler et al., 1998)
indicate that the peak shock-wave pressure (Ps) drops exponentially
with the distance R from the impact point according
to an equation of the form Ps a R–n. Various field and
laboratory studies indicate a dependence of R–2 to R–4.5; the
exact value of the exponent depends on projectile size and
impact velocity (Ahrens and O’Keefe, 1977).
On the basis of these studies, it is possible to regard the
impact point as surrounded by a series of concentric, roughly
hemispherical shock zones, each zone distinguished by a
certain range of peak shock pressure (Fig. 3.2) and characterized
by a unique suite of shock-metamorphic effects produced
in the rocks. At the impact point, peak shock-wave
pressures may exceed 100 GPa (= 1000 kbar or 1 Mbar) for
typical cosmic encounter velocities, producing total melting,
if not vaporization, of the projectile and a large volume of
surrounding target rock. Further outward, pressures of 10–
50 GPa may exist over distances of many kilometers from
the impact point, producing distinctive shock-deformation
effects in large volumes of unmelted target rock.
At even greater distances from the impact point, the peak
shock-wave pressures eventually drop to about 1–2 GPa
(Kieffer and Simonds, 1980). At this point, near the eventual
crater rim, the shock waves become regular elastic waves or
seismic waves, and their velocity drops to that of the velocity
of sound in the target rocks (typically 5–8 km/s). These seismic
waves can be transmitted throughout the entire Earth,
like similar waves generated by earthquakes and volcanic
eruptions. Because of their low pressures, they do not produce
any permanent deformation of the rocks through which
they pass. However, seismic waves may produce fracturing,
brecciation, faulting, and (near the surface) landslides, and
the results may be difficult to distinguish from those of normal
geological processes.
The duration of the contact/compression stage is determined
by the behavior of the shock wave that was reflected
back into the projectile from the projectile/target interface
(Fig. 3.1) (Melosh, 1989, pp. 57–59). When this shock wave
reaches the back end of the projectile, it is reflected forward
into the projectile as a rarefaction or tensional wave (also
Fig. 3.1. Contact/compression stage: shock-wave generation
and projectile deformation. Theoretical cross-section showing
calculated conditions immediately after the impact of a large,
originally spherical, projectile (stippled) onto a uniform target. The
projectile has penetrated about half its diameter into the target,
and intense shock waves (pressures in GPa) are radiating outward
into the target from the interface. The projectile itself has become
intensely compressed, and similar shock waves from the interface
are spreading toward the rear of the projectile. When this shock
wave reaches the rear of the projectile, it will be reflected forward
as a tensional wave or rarefaction, unloading the projectile and
allowing it to transform, virtually instantaneously, into melt and
vapor. The original model, developed for large lunar impact events
(O’Keefe and Ahrens, 1975), represents conditions about 1 s after
the impact of a 46-km-diameter anorthosite projectile at 15 km/s
onto a gabbroic anorthosite target, but similar conditions will be
produced by smaller impacts and other material compositions.
(Modified from Melosh, 1989, Fig. 4.1a, p. 47.)
Formation of Impact Craters 19
called a release wave). As the release wave passes through
the projectile from back to front, it unloads the projectile
from the high shock pressures it had experienced. Because
the shock pressures, and the associated temperatures, have
been so high, this release results in the virtually complete
melting and vaporization of the projectile. At the instant at
which the release wave reaches the front end of the projectile,
the whole projectile is unloaded, and the release wave
continues forward into the target and begins to decompress
it as well. This point, at which the release wave reaches the
front of the projectile and begins to enter the adjacent compressed
target, is taken as the end of the complete contact/
compression stage.
The contact/compression stage lasts no more than a few
seconds, even for impacts of very large objects. The time
required for the shock wave to travel from the projectile/
target interface to the rear edge of the projectile is approximately
equal to the time it takes the projectile to travel the
distance of one diameter at its original velocity. Even for
large projectiles, this time is short: 2 s for a 50-km-diameter
projectile traveling at 25 km/s, and less than 0.01 s for a
100-m-diameter object traveling at the same speed. The
additional time required for the release wave to travel from
the rear to the front edge will be no more than a few times
this value, depending on the properties of projectile and target
rock (Melosh, 1989, pp. 48 and 58). For most impact
events, the entire contact/compression stage is over in less
than a second.
After the release wave has reached the front end of the
projectile and unloaded it completely, the projectile itself plays
no further role in the formation of the impact crater, and the
actual excavation of the crater is carried out by the shock
waves expanding through the target rocks. The vaporized
portion of the projectile may expand out of the crater as part
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Fig. 3.2. Contact/compression stage: initial shock-wave pressures and excavation flow lines around impact point. Schematic crosssection
showing peak shock pressure isobars (pressures in GPa) developed in the target around the impact point near the end of the
contact/compression stage. The originally spherical projectile, after penetrating about two diameters into the target, has been almost
completely destroyed and converted to melt and vapor. Shock waves radiating from the projectile-target interface decline rapidly outward
in peak pressure (isobars in GPa on left side of cavity), creating concentric, approximately hemispherical zones of distinctive shock effects
(right side of cavity). From the original interface outward, these zones involve: (1) melting (>50 GPa) and formation of a large melt unit;
(2) shock-deformation effects (5–50 GPa); (3) fracturing and brecciation (1–5 GPa). The subsequent excavation stage involves two
processes: (1) upward ejection (spalling) of large near-surface fragments and smaller ejecta (ejecta curtain) (upward-pointing arrows
above ground surface); (2) subsurface flow of target material to form the transient crater (arrow paths crossing isobars at left side).
(Modified from Melosh, 1989, Fig. 5.4, p. 64.)
20 Traces of Catastrophe
of a vapor plume (Melosh, 1989, pp. 68–71), and the remainder,
virtually all melted, may be violently mixed into the
melted and brecciated target rocks.
3.1.2. Excavation Stage: The Transient Crater
The brief contact/compression stage grades immediately
into a longer excavation stage, during which the actual impact
crater is opened up by complex interactions between
the expanding shock waves and the original ground surface
(Fig. 3.3) (Melosh, 1989, Chapter 5; Grieve, 1991). As the
contact/compression stage ends, the projectile is surrounded
by a roughly hemispherical envelope of shock waves that
expand rapidly through the target rock. Because the projectile
has penetrated a finite distance into the target, the center
of this hemisphere actually lies within the original target
rock at a point below the original ground surface.
Within this hemispherical envelope, the shock waves that
travel upward and intersect the original ground surface are
reflected downward as rarefactions (release waves). In a nearsurface
region where the stresses in the tensional release
wave exceed the mechanical strength of the target rocks,
the release wave is accompanied by fracturing and shattering
of the target rock (Fig. 3.2). This reflection process also
converts some of the initial shock-wave energy to kinetic
energy, and the rock involved is accelerated outward, much
of it as individual fragments traveling at high velocities
(Fig. 3.4).
These complex processes drive the target rock outward
from the impact point, producing a symmetric excavation
flow around the center of the developing structure. Exact
flow directions vary with location within the target rocks
(Fig. 3.4). In the upper levels, target material moves dominantly
upward and outward. At lower levels, target material
moves dominantly downward and outward. These movements
quickly produce a bowl-shaped depression (the transient
cavity or transient crater) in the target rocks (Maxwell,
1977; Grieve at al., 1977; Grieve and Cintala, 1981; Melosh,
1989, pp. 74–78).
The transient crater is divided into approximately equal
upper and lower zones (Figs. 3.4 and 3.5). Within the upper
ejection zone, velocities imparted to the target rocks may be
as high as several kilometers per second, high enough to excavate
the fragmented material and to eject it beyond the
rim of the final crater (Grieve et al., 1977; Dence et al., 1977;
Fig. 3.3. Development of a simple impact structure. Series of cross-section diagrams showing progressive development of a small,
bowl-shaped simple impact structure in a horizontally layered target: (a) contact/compression stage: initial penetration of projectile,
outward radiation of shock waves; (b) start of excavation stage: continued expansion of shock wave into target; development of tensional
wave (rarefaction or release wave) behind shock wave as the near-surface part of original shock wave is reflected downward from ground
surface; interaction of rarefaction wave with ground surface to accelerate near-surface material upward and outward; (c) middle of
excavation stage: continued expansion of shock wave and rarefaction wave; development of melt lining in expanding transient cavity;
well-developed outward ejecta flow (ejecta curtain) from the opening crater; (d) end of excavation stage: transient cavity reaches maximum
extent to form melt-lined transient crater; near-surface ejecta curtain reaches maximum extent, and uplifted crater rim develops; (e) start
of modification stage: oversteepened walls of transient crater collapse back into cavity, accompanied by near-crater ejecta, to form
deposit of mixed breccia (breccia lens) within crater; (f ) final simple crater: a bowl-shaped depression, partially filled with complex
breccias and bodies of impact melt. Times involved are a few seconds to form the transient crater (a)–(d), and minutes to hours for the
final crater (e)–(f ). Subsequent changes reflect the normal geological processes of erosion and infilling.
Kieffer and Simonds, 1980; Melosh, 1989, pp. 74–76). Even
at significant distances from the impact point, shock pressures
and the resulting ejection velocities remain high enough
(>100 m/s) to eject material. For this reason, the diameter of
the final crater is many times larger (typically 20–30×) than
the diameter of the projectile itself.
At deeper levels, tensional stresses in the release waves
are lower. As a result, fracturing is less pronounced, excavation
flow velocities are lower, and the excavation flow lines
themselves are not oriented to eject material beyond the crater
rim (Fig. 3.4). This region forms a displaced zone in
which material is driven downward and outward more or
less coherently.
Both zones in the transient crater continue to expand,
accompanied by the uplift of near-surface rocks to form the
transient crater rim, as long as the expanding shock waves
and release waves are strong enough to eject or displace material
from the developing cavity. However, these waves continually
lose energy by deforming and ejecting the target rocks
through which they pass. Eventually, a point is reached at
which the shock and release waves can no longer excavate or
displace target rock. At that point the growth of the transient
crater ceases. There is an instant of theoretical balance
in which the energies of the shock wave no longer act, and
the waiting forces of gravity and rock mechanics have not
yet reasserted themselves. At this instant, the transient crater
reaches its maximum extent, the excavation stage ends,
and the subsequent modification stage begins immediately.
The excavation stage, although longer than the contact/
compression stage, is still brief by geological standards. If
the near-surface excavation flow has a minimum average velocity
of 1 km/s, then a 200-km-diameter transient crater
can be excavated in less than 2 min. More detailed calculations
(Melosh, 1989, p. 123) indicate that excavation of a
l-km-diameter crater (e.g., Barringer Meteor Crater [Arizona])
will occur in about 6 s, while a 200-km-diameter
crater requires only about 90 s.
The concept of the transient crater has been developed
from a combination of theoretical studies (Melosh, 1989,
Chapter 5) and geological investigations (Dence, 1968; Grieve
and Cintala, 1981; Grieve et al., 1981). The ideal transient
crater is a bowl-shaped depression with a structurally uplifted
rim (Figs. 3.4 and 3.5). Its shape is approximately
hemispherical but is actually a paraboloid of revolution
Formation of Impact Craters 21
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22 Traces of Catastrophe
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Fig. 3.4. Excavation stage: formation of transient crater. Theoretical cross section showing development of the transient crater
immediately after the contact/compression stage. Original peak shock pressures (units in GPa) around the impact point are shown for
simplicity as hemispherical isobars (for details, see Fig. 3.2). Complex interactions of the shock wave, the ground surface, and the
subsequent rarefaction wave produce an outward excavation flow (dashed arrows) that opens up the transient crater. In the upper part of
this region (excavated zone; ruled area), target material is fractured, excavated, and ejected beyond the transient crater rim. In the lower
region (displaced zone), target material is driven downward and outward, more or less coherently, and does not reach the surface. This
model yields two important geological results: (1) ejected material is derived only from the upper part (approximately the top one-third
to one-half ) of the transient cavity; (2) because the excavation flow lines in the excavated zone cut across the initially hemispherical shock
isobars, ejected material will reflect a wide range of original shock pressures and deformation effects, ranging from simple fracturing to
complete melting and vaporization. (Modified from Grieve, 1987, Fig. 5; Hörz et al., 1991, Fig. 4.3a, p. 67.)
Fig. 3.5. Transient crater: locations of shock-metamorphosed materials. Cross section through a theoretical transient crater, showing
discrete zones from which various shock-metamorphosed materials are derived. The “vaporized” zone closest to the original impact point
(stippled) contains a mixture of vaporized target rock and projectile, which expands upward and outward into the atmosphere as a vapor
plume. The adjacent “melt” zone (solid black) consists of melt that moves downward and then outward along the floor of the final
transient cavity (for details, see Fig. 6.2). Material in the upper “ejected” zones on either side of the melt zone, which contains a range of
shock-metamorphic effects, is ejected outward to and beyond the transient crater rim. The lower “displaced” zone moves downward and
outward to form the zone of parautochthonous rocks below the floor of the final transient crater. Hat = the final transient crater depth;
Hexc = the depth of excavation, which is significantly less than the total depth. (From Melosh, 1989, Fig. 5.13, p. 78.)
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Formation of Impact Craters 23
(Dence, 1973). Its maximum depth is approximately onethird
its diameter, and this proportion seems to remain
approximately constant for craters of widely different size
(Maxwell, 1977; Croft, 1985).
The theoretical instant of ideal overall balance in a transient
crater at the end of the excavation stage may not be
actually attained during formation of a real crater. For example,
in these models, the maximum diameter is normally
attained after the maximum depth is reached. Subsequent
modification of one part of an actual transient crater might
therefore begin while other parts are still being excavated.
Even so, the transient crater is a key concept in models of
crater formation. All impact structures, regardless of their
final size or the complexity of their subsequent development,
are assumed to pass through the transient-crater stage, making
this stage of critical importance in comparing impact
structures of different sizes or on different planets. Defining
the transient crater is also an essential step in determining
critical characteristics of an impact structure: its original
(pre-erosion) diameter and depth, the energy of impact, the
size and velocity of the projectile, the distribution of shock
pressures and shock effects within the crater, the amount of
material melted and ejected during formation of the crater,
the amount of structural uplift during formation of the central
peak of complex impact structures, and the depth from
which excavated materials were derived.
3.1.3. Modification Stage
The excavation stage ends when the transient crater has
grown to its maximum size, and the subsequent modification
stage begins immediately. The expanding shock waves
have now decayed to low-pressure elastic stress waves beyond
the crater rim, and they play no further part in the
crater development. Instead, the transient crater is immediately
modified by more conventional factors like gravity and
rock mechanics.
The immediate part of the modification stage, during
which the major impact-related changes occur, lasts only
slightly longer than the excavation stage: less than a minute
for a small structure, a few minutes for a large one (Melosh,
1989, Chapter 8, pp. 141–142). (One simple definition is
that the modification stage ends “when things stop falling.”)
However, the modification stage has no clearly marked end,
and the modification processes of uplift and collapse merge
gradually into the normal processes of geological mass movement,
isostatic uplift, erosion, and sedimentation.
3.2. SIMPLE AND COMPLEX
IMPACT STRUCTURES
The extent to which the transient crater is altered during
the modification stage depends on its size and (to a lesser
extent) on the structure and properties of the target rock.
Small transient craters are altered chiefly by the collapse of
their upper walls, and the shape of the final crater is little
changed from that of the original transient crater. In larger
structures, modification may involve major structural
changes: uplift of the central part of the floor and major
peripheral collapse around the rim. Depending on the extent
to which the transient crater is modified, three distinct
types of impact structures can be formed: simple craters,
complex craters, and multiring basins.
3.2.1. Simple Craters
The smallest impact structures occur as bowl-shaped depressions
(simple craters) less than a few kilometers across,
which help to preserve the shape and dimensions of the original
transient cavity (Figs. 1.1 and 3.6). In evolving to a simple
crater, the transient crater is modified only by minor collapse
of the steep upper walls into the crater cavity and by
redeposition of a minor amount of ejected material in the
crater. As a result, the crater diameter may increase by as
much as 20%, but the original transient crater depth remains
largely unaffected (Fig. 3.7) (Melosh, 1989, p. 129).
During modification, the simple crater is immediately
filled, to perhaps half its original depth, by a mixture of redeposited
(fallback) ejecta and debris slumped in from the
walls and rim (Fig. 3.7). This crater-filling unit, variously
called the breccia lens or crater-fill breccia, is a mixture of
rock fragments, both shocked and unshocked, together with
fragments or lenses of shock-melted rock (impact melt).
Fig. 3.6. A simple lunar impact crater. This small, well-preserved
crater (Moltke: D = 7 km) shows features typical of simple impact
craters: a circular outline, a bowl-like shape, an uplifted rim, and
hummocky deposits of ejecta around the rim. In the relatively low
gravity of the Moon, this structure formed as a simple crater; a
terrestrial structure of the same diameter, formed under Earth’s
higher gravity, would have formed as a complex crater with a central
uplift. (Apollo 10 image AS10-29-4324.)
24 Traces of Catastrophe
Fig. 3.7. Simple impact structure: locations of impactite types. Schematic cross section of a typical simple impact structure, showing
the simple bowl shape and the locations of various types of impactites in and around the structure. The parautochthonous rocks below
the true crater floor are fractured and brecciated but generally show no distinctive shock effects, except in a small zone (fine vertical
ruling) in the center of the structure. The crater is filled, to approximately half its original height, with a variety of allogenic breccias and
impact melts, which forms the crater-fill units or the breccia lens. A thinner layer of ejected material (fallout ejecta) overlies the uplifted
crater rim and surrounds the crater. This unit is easily eroded and is present only in the youngest and best-preserved structures. D = final
crater diameter, which is 10–20% greater than the diameter of the original, premodification transient crater; dt = true depth of the final
crater, which is approximately the depth of the original transient crater; da = apparent depth of the crater, or the depth from the final rim
to the top of the crater-fill units. The diagram represents the state of the final crater before any subsequent geological effects, e.g., erosion,
infilling. The model is based on drilling studies at Barringer Meteor Crater (Arizona) (Roddy et al., 1975; Roddy, 1978), Brent Crater
(Canada) (Dence, 1968; Grieve and Cintala, 1981), and similar structures (e.g., Masaitis et al., 1980; Gurov and Gurova, 1991). (From
Grieve, 1987, Fig. 1.)
Depending on the subsequent geological history, the breccia
lens may be eroded or may be covered and preserved by a
cap of later sedimentary fill.
3.2.2. Complex Craters
The bowl-shaped form of simple craters appears only in
relatively small structures less than a few kilometers across.
Larger impact structures (complex craters) display a different
and more complicated form, characterized by a centrally
uplifted region, a generally flat floor, and extensive inward
collapse around the rim (Figs. 1.3, 3.8, and 3.9) (Dence, 1968;
Grieve et al., 1977, 1981; Grieve, 1991). For terrestrial structures,
the transition between simple and complex craters
occurs at a diameter of about 4 km in massive crystalline
rocks, but at only about 2 km in sediments. (However, these
values apply only to Earth. The transition diameter varies
inversely with gravitational acceleration, and it is different
on different planets.) The larger impact events that form
complex craters apparently release enough energy to overcome
the fundamental strength of the target rocks over a
large volume beneath the large transient crater. As a result,
late-stage modification involves complex interactions between
shock-wave effects, gravity, and the strength and structure
of the target rocks, and the modification is characterized
by outward, inward, and upward movements of large volumes
of the subcrater rocks.
The details of these interactions are uncertain, but the
general result is that the original bowl-shaped transient crater
is immediately modified as deep-seated rocks beneath
the center of the transient crater rise to form a central uplift
(Dence, 1968; Grieve et al., 1981). At the same time, rocks
around the periphery of the transient crater collapse downward
and inward along concentric faults to form one or more
depressed rings (ring grabens) and a series of terraces along
the outer margins of the final structure (Fig. 3.10). [A simple
model of the formation of a complex crater and its central
uplift is presented by the familiar slow-motion movies of a
drop of liquid hitting a liquid surface (e.g., Melosh, 1989,
p. 148; Taylor, 1992, p. 168). There is the same initial cavity
formation, the same outward and downward ejection of target
material, the same upward rebound of the central cavity
floor, and the same collapse of the periphery back into the
cavity. However, in impact events, these processes take place
in solid rock and may operate over distances of tens to hundreds
of kilometers.]
The idea that such rapid deformation and subsequent
uplift can occur in large volumes of crustal rocks has been
difficult for many geologists to appreciate. Key evidence has
come from studies of impact structures formed in sedimentary
rocks, in which the actual uplift of key stratigraphic
markers has been established beyond question through drilling
and geophysical studies (e.g., Milton et al., 1972, 1996a,b;
Formation of Impact Craters 25
Grieve et al., 1981; Grieve and Pilkington, 1996). Geological
studies have also established that the amount of actual stratigraphic
uplift (SU) in impact structures is about one-tenth
the final diameter (D) of the structure. A detailed statistical
relation derived from studies of well-constrained complex
impact structures (Grieve et al., 1981, p. 44) is SU = 0.06 D1.1
(both SU and D are in kilometers). A subsequent analysis,
using more craters (Grieve and Pilkington, 1996, p. 404),
gave SU = 0.086 D1.03. The two equations are virtually identical,
and a value of SU = 0.1 D is a reasonable approximation
to either. For large (D = 100–200 km) impact structures,
these relations imply that the crustal rocks beneath the structure
are uplifted vertically by 10–20 km during the impact
event. An uplift of this magnitude has been estimated for
the Vredefort (South Africa) structure on geological grounds
(Reimold and Gibson, 1996; Therriault et al., 1997; Turtle and
Pierazzo, 1998).
Both theoretical and field studies indicate that central
uplifts form in only a few minutes, almost instantaneously
by geological standards, even in the largest structures (Melosh,
1989, pp. 129 and 141–142). Theoretical studies also suggest
that the central uplifts of structures 200–300 km in
Fig. 3.8. A complex lunar crater. This relatively young crater
(Theophilus: D = 100 km) displays well-preserved features that
are typical of complex impact structures: a central uplift, a scalloped
circular outline, ruggedly terraced walls with possible landslide
deposits inside the rim, and hummocky ejecta deposits just outside
the rim. This view also indicates the continuing nature of
lunar cratering; an older impact crater (upper right) has been partly
destroyed by Theophilus, while a younger small crater has formed
within Theophilus itself (near rim, lower right). The flat dark
area in the background (upper left) is made up of lava flows covering
part of Mare Nectaris. The spiral-like rod at left center is an
instrument boom on the Apollo 16 spacecraft, from which this
orbital picture was taken. (Apollo 16 image AS16-M-0692.)
Fig. 3.9. A complex impact basin on Venus. A large, wellpreserved
multiring impact basin on the surface of Venus
(Meitner: D = 150 km) is revealed beneath the planet’s opaque
atmosphere by the imaging radar system of the Magellan spacecraft.
Meitner, the third-largest impact structure identified on
Venus, shows a flat smooth (dark-colored) interior, two circular
rings, and a rough, irregular blanket of lobate ejecta (light-colored).
The crater was formed on a surface of smooth plains, possibly
underlain by lava flows and cut by abundant parallel fractures
(white lines). (Magellan image F-MIDRP .55S319;201.)
diameter, such as Vredefort (South Africa), formed in less
than 15 minutes (Melosh, 1989, pp. 141–142; Turtle and
Pierazzo, 1998).
Despite the extensive evidence that central uplifts do form
in large impact structures, the details of the process are still
the subject of continuing uncertainty and active debate
(Dence, 1968; Grieve et al., 1981; Melosh, 1989, Chapter 8;
Hörz et al., 1991; Spudis, 1993). Even so fundamental a quantity
as the ratio between the diameter of the initial transient
crater and the diameter of the final complex impact structure
has not been well established; values estimated by various
workers, using both theoretical and geological studies,
range from about 0.5 to 0.7 (see, e.g., Therriault et al., 1997,
Table 2).
At larger crater diameters, the resulting structures, and
especially the centrally uplifted area, become even more complicated.
As the crater size increases the character of the central
uplift changes, and the single central peak is progressively
replaced by a more complex series of concentric rings and
basins. At least three types of complex impact structures can
be distinguished with increasing crater diameter: centralpeak
structures, central-peak-basin structures, and peak26
Traces of Catastrophe
Fig. 3.10. Development of a complex impact structure. Series of cross sections showing progressive development of a large, complex
impact structure in a horizontally layered target: (a) formation of a large transient crater by the excavation process is virtually identical to
transient crater formation in smaller structures (compare with Fig. 3.3a–d); (b) initial development of central uplift during the subsequent
modification stage; (c) start of peripheral collapse, accompanied by continuing development of the central uplift and the thinning and
draping of the original melt layer (black) over the uplifted rocks; (d) final structure, which is of the central-uplift type, consists of a central
uplift of deeper rocks, surrounded by a relatively flat plain and by a terraced rim produced by inward movement along stepped normal
faults. The central uplift is surrounded by an annular deposit of allogenic breccias and impact melt (black), which may be absent from the
central peak itself. An ejecta layer (stippled) covers the target rocks around the structure. The diameter of the final structure, measured at
the outer rim beyond the outermost fault, may be 1.5–2× the diameter of the original transient crater. This central-peak morphology is
observed in terrestrial structures ranging from about 2–25 km in diameter; larger structures tend to develop one or more concentric rings
within the crater (for details, see text).
Formation of Impact Craters 27
ring basin structures (Grieve at al., 1981; Melosh, 1989,
Chapter 8; Spudis, 1993). As the terms suggest, these structures
are characterized by the initial development of a basin
in the central peak and eventually by the complete conversion
of the central peak area to a ring structure (Figs. 1.3,
3.9, and 3.11).
These distinctions, and the transition diameters at which
they occur, have been most clearly established on airless bodies
like the Moon, where even large ancient structures have
been well preserved (Figs. 3.6, 3.8, and 3.11) (e.g., Taylor,
1982, 1992; Melosh, 1989, pp. 131–135; Spudis, 1993). Classification
of large terrestrial structures (e.g., papers in Schultz
and Merrill, 1981; Spudis, 1993, pp. 24–41) is more difficult
and uncertain, because the impact structures, especially their
critical upper parts, tend to be removed by erosion or buried
by later sediments. Furthermore, the critical diameters at
which one form changes to another depend inversely on the
planetary gravity, making it difficult to apply data from structures
on other planets to terrestrial features. For example,
the transition between simple and complex craters occurs at
about 20 km diameter on the Moon but at only 2–4 km on
Fig. 3.11. A lunar impact basin. This large impact structure
(Schrödinger: D = 320 km) is located on the lunar farside near
the Moon’s South Pole. Although ancient and highly degraded,
it still preserves features distinctive of larger complex impact
structures: a central uplift and terraced walls. However, in this
large structure, the central uplift appears as an interior peak ring
about 150 km in diameter (arrows), in sharp contrast to the simpler
central peak formed in smaller complex structures. (Lunar Orbiter
image LO-IV-8M.)
Earth. The subsequent transition between a central-peakbasin
structure to a peak-ring structure occurs at about 150–
200 km on the Moon, but at only about 20–25 km on Earth.
Despite the various difficulties, it has been possible to
establish rough boundaries for different types of terrestrial
complex structures (Grieve et al., 1981, p. 42, Fig. 2). These
limits, and some typical examples, are: central-peak structures
(D = 4–22 km) [Steinheim (Germany), Sierra Madera
(Texas)]; central-peak-basin structures (D = 22–30 km)
[Mistastin (Canada)]; peak-ring-basin structures (D = 30–
62 km) [West Clearwater (Canada); Fig. 1.3]. These values
are only approximations, and they will almost certainly change
as more structures are studied in detail and as the formation
of complex craters is better understood.
3.2.3. Multiring Basins
The largest planetary impact structures so far identified
have diameters of a few hundred kilometers to more than
1000 km (e.g., papers in Schultz and Merrill, 1981; Melosh,
1989, Chapter 9; Spudis, 1993). In contrast to smaller impact
structures, they appear as huge geological bulls-eyes,
composed of multiple concentric uplifted rings and intervening
down-faulted valleys (ring grabens) (Fig. 3.12). These
features, designated multiring basins, are defined as structures
that have two or more interior rings in addition to the
outer rim of the structure.
Multiring impact basins have been produced by the impact
of projectiles tens to hundreds of kilometers in diameter,
and they date mainly from an early period in the solar
system (>3.9 Ga), when such large objects were more abundant
and collisions were more frequent. The best multiring
basins are best observed on planets with well-preserved ancient
surfaces, such as the Moon, Mercury, parts of Mars,
and some of the moons of Jupiter. Mare Orientale, on the
Moon, with a diameter of at least 900 km, is one of the most
prominent and best-known multiring basins (Fig. 3.12),
but even larger features exist, such as the Valhalla Basin
(D ~4000 km) on Jupiter’s icy moon Callisto. In addition,
there are numerous large basins in the solar system that
do not display a pronounced multiring structure, possibly
because they have been deeply eroded since they formed.
These include the Caloris Basin (Mercury; D = 1300 km),
the Argyre Basin (Mars; D > 900 km) (Fig. 1.9), and the
recently identified South Pole-Aitken Basin on the Moon
(D ~2500 km).
On the Moon, the transition to multiring basins occurs
at diameters of about 400–600 km. Because the transition
diameters for different crater forms vary inversely with planetary
gravity, this observation implies that multiring basins
should begin to form on Earth at crater diameters greater
than about 100 km. Because the few terrestrial impact structures
in this size range have been deeply eroded or buried
(e.g., Fig. 1.4), it has not yet been possible to demonstrate
clearly that any multiring basins exist on the Earth. The
few possible candidates (and their current estimated diameters)
are Manicouagan (Canada, 100 km), Popigai (Russia,
100 km), Vredefort (South Africa, >200 km), Sudbury
28 Traces of Catastrophe
Fig. 3.12. A lunar multiring impact basin. One of the largest,
freshest, youngest, and best-known multiring impact basins in the
solar system, Mare Orientale (D = 930 km) lies on the boundary
between the Earth-facing lunar nearside (right) and the lunar
farside. The structure, formed at about 3.8 Ga, is bounded by an
outer ring about 930 km in diameter (Cordillera Mountains), and
inner rings with diameters of 620, 480, and 320 km can be
distinguished. Mare Orientale is surrounded by radial features
(especially at lower right) that may have been produced by the
low-angle ejection of large blocks of excavated material. The
postimpact history of the structure is also complex, and much of
the area inside the rings has been modified by later volcanic activity.
The flat dark areas at upper right are the younger lava flows that
cover Oceanus Procellarum. (Lunar Orbiter image LO-IV-187M.)
(Canada, >200 km), and Chicxulub (Mexico, >180 km). It
has not proved possible to establish beyond question the
multiring character of these structures for various reasons,
including deep erosion, postcrater deformation, or insufficient
geological study. The strongest current candidate for a
terrestrial multiring structure is Chicxulub, which, although
buried, appears well preserved (Sharpton et al., 1993, 1996b;
Morgan et al., 1997).
Multiring basins represent the most energetic and catastrophic
impact events in the solar system, and the postimpact
movements — upward, downward, and inward — of
the target rock that modify the transient crater are far more
complex and widespread than in smaller structures. It is therefore
not surprising that the formation of multiring basins is
even more uncertain and hotly debated than is the origin of
smaller complex impact structures (e.g., papers in Schultz
and Merrill, 1981; Melosh, 1989, Chapter 9; Spudis, 1993).
For example, it is not clear whether the transition between
smaller impact structures and multiring basins is a
natural development with increasing crater diameter (Herrick
et al., 1997), or whether multiring basins only form when
special conditions are present within the target, e.g., a crustmantle
structure with a weak layer (asthenosphere) at depth
within the planet (see Melosh, 1989, pp. 176–180). Nor is it
understood why some planetary features in the 1000–2000-
km-diameter range have a pronounced multiring form
(Fig. 3.12) and others do not (Fig. 1.9). Finally, it is not yet
established whether multiring impact structures — ancient
or modern — do exist on Earth and which large structures
they may be.
3.3. SUBSEQUENT DEVELOPMENT OF
IMPACT STRUCTURES
When the crater formation process ends, the resulting
circular structure, whether simple or complex, consists of
deformed subcrater rocks covered by an ejecta blanket outside
the crater and with crater-fill deposits (usually a mixture
of breccias and bodies of impact melt) within it (Figs. 3.7
and 3.13). This assemblage of distinctive near-surface rocks
is immediately subject to more normal geological processes:
erosion, burial, and tectonic deformation. If the crater
forms on land and remains exposed after formation,
erosion will quickly remove the surface ejecta blanket and
destroy any surviving meteorite fragments. At the same time,
however, a lake may form in the crater depression, covering
the crater-fill material with a preserving cap of sediments,
e.g., as at Brent (Canada) (Dence, 1968; Grieve, 1978) and
the Ries Crater (Germany) (von Engelhardt, 1990).
If the original impact site is covered by water, the formation
and subsequent history of the resulting crater may be
more complex. At the moment of impact, the overlying layer
of water will be excavated with the underlying bedrock, and
the development of the crater and the deposition of the impact-
produced rock units will be modified by the immediate
and violent resurge of this displaced water back into the crater
cavity (Therriault and Lindström, 1995; Lindström et al.,
1996). If the crater remains below the water level, it will
immediately begin to fill with sediments, and its subsequent
history will depend on whether it remains below water level
(continuous sediment filling) or is uplifted at some future
time (beginning of erosion). A number of such submarine
impact structures have now been recognized; some have subsequently
been raised above sea level [e.g., Lockne (Sweden)
(Therriault and Lindström, 1995; Lindström et al., 1996)]
and others still remain buried [e.g., Montagnais (Canada)
(Jansa and Pe-Piper, 1987); the Chesapeake Bay Crater
(USA) (Poag, 1996, 1997); and the recently discovered
Mjølnir structure (Norway) in the Barents Sea (Dypvik et
al., 1996)].
Formation of Impact Craters 29
Fig. 3.13. Complex impact structure: locations of impactite types. Schematic radial cross section across a complex impact structure of
the central-uplift type, from the central uplift (right) to the outer, downfaulted rim (left). (Vertical scale is exaggerated.) The subcrater
parautochthonous rocks, exposed in the central uplift, are highly fractured and brecciated and may contain distinctive shock features such
as shatter cones. These rocks may also contain widespread pseudotachylite breccias and dike-like intrusive bodies of allogenic breccias
and impact melts. Larger and thicker subhorizontal units of allogenic breccias and melts occur as an annular unit of crater-fill material
that covers the parautochthonous rocks between the central uplift and the rim. The bulk of these crater-fill deposits consist of melt-free
lithic breccias, with lesser amounts of melt-bearing suevite breccias. The melt component in the crater-fill deposits becomes more
abundant toward the center and upward, and a discrete layer of impact melt (solid black) may occur at or toward the top of the crater fill.
(Modified from Stöffler et al., 1988, Fig. 12, p. 290.)
Because impact is a near-surface process, the deformation
associated with impact structures dies away rapidly
with depth. Typical impact structures are relatively shallow,
and impact-produced rocks form comparatively thin units.
The distinctive rock types and shock effects in a structure
tens of kilometers in diameter may extend only a few kilometers
below the original ground surface. Impact structures
are therefore especially vulnerable to erosion. Initial erosion
will preferentially remove the near-surface ejecta deposits
and the distinctively shocked and melted materials they contain,
thus rapidly destroying the most convincing evidence
for impact. Deeper erosion over longer periods of time will
eventually produce major destructive changes in the crater.
The breccias and melt units that fill the crater, and the distinctive
shocked materials they contain, together with any
protecting cap of sediments, will be reduced to small remnants
or completely removed. The original circular outline
will disappear. Eventually, all trace of the crater will be removed
except for the weakly shocked subcrater rocks. If
erosion continues long enough, the whole impact structure
will be erased.
Impact structures that are not destroyed by erosion may
be entirely filled and buried by younger sediments, so that
their detection depends on geophysical methods and drilling
rather than on surface field geology. About one-third of
the presently known impact structures are subsurface (Grieve,
1991, 1997; Grieve and Masaitis, 1994; Grieve et al., 1995);
they were first discovered during geophysical explorations,
and their impact origin has been verified by the discovery of
shocked rocks in drill core samples. This group includes several
continental structures that are actual or potential petroleum
producers [Ames (Oklahoma); Avak (Alaska); Marquez
(Texas); Red Wing Creek (North Dakota)] (Donofrio, 1997),
as well as a few submarine impact structures [e.g., Montagnais
(Canada) (Jansa and Pe-Piper, 1987)]. Several large and relatively
young buried impact structures have also been identified
by geophysical techniques: the 90-km-diameter
Chesapeake Bay Crater (USA) (Poag, 1996, 1997); the larger
(>180-km diameter) Chicxulub structure (Mexico), which
is associated with the K/T event (Hildebrand et al., 1991;
Sharpton et al., 1992; papers in Ryder et al., 1996); and the
large (>70 km?) Morokweng structure (South Africa) (Corner
et al., 1997; Koeberl et al., 1997a). Many more impact
structures remain to be found, and the evidence for their
existence may already be sitting unrecognized in existing
drill cores and geophysical records around the world.
Impact structures may also be caught up in subsequent
tectonic deformation, with varying results. Horizontal compression
may deform the original circular shape, making study
and interpretation more difficult [as at Sudbury (Canada)].
30 Traces of Catastrophe
Tectonism can also break up regions of original shocked rocks
and disperse them as large discrete areas across the geological
landscape [e.g., the Beaverhead (Idaho) structure
(Hargraves et al., 1990; Fiske et al., 1994)]. Sufficient tectonism
and metamorphism could destroy even large impact
structures or make them totally unrecognizable.
Geologists must therefore be prepared to recognize impact
structures in all states of preservation, from young, fresh,
well-exposed circular structures filled with distinctive shocked
breccias to older features in which distinctive shock effects
are scattered, barely recognizable, or deeply buried. It is essential
to be able to recognize the variety of distinctive shock
effects associated with impact structures and to understand
where different types of shock effects may be located in the
original crater.
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