Impact craters

Impact signatures

Since 1946, convincing, if not totally unequivocal, evidence for the impact origin of many terrestrial craters has been unearthed. Evidence comes from at and near the crater site itself, and from material ejected from the crater and broadcast far and wide. The evidence at the crater itself takes the form of three different, though related, signatures of impacts: shatter cones, shock-metamorphosed forms of silica (coesite and stishovite), and shocked quartz crystals. All of these features probably result from the immense pressures created in the rocks around an impact site. None of them provides incontrovertible evidence of meteoritic impact because mechanisms other than bolide impaction might have caused the necessary shock and intense pressures. However, Dietz (1968) argued that any other mechanism must involve an intraterrestrial explosion triggered by an unknown geological process. The only alternative mechanism offered has been volcanism, but whether even the biggest volcanic explosions are sufficiently energetic to produce the shock-metamorphic features is open to question. In any case, the protagonists of the bombardment hypothesis are aware of the problems of interpreting impact signatures. They seldom rely on just one piece of evidence as a sure sign that a cosmic body produced a particular crater; they are only prepared to accept an impact origin for craters in which a range of evidence is suggestive of bombardment.

Figure 3.3 Formation of a simple hypervelocity impact crater. The series of cross-section diagrams shows the progressive development of a small, bowl-shaped simple impact structure in a horizontally layered target. (a) Contact and compression stage: initial penetration of projectile and 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 ground surface reflects the near-surface part of original shock wave downwards; interaction of rarefaction wave with ground surface to accelerate near-surface material upwards and outwards. (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. Source: After French (1998).

The vaporized material ejected from a hypervelocity crater condenses in the atmosphere to various small, rounded, glassy objects - tektites (from the Greek tektos, meaning molten) - that fall to the ground, sometimes forming aerodynamic shapes as they partially melt on their downward journey. Although a volcanic origin was one mooted, most scientists now believe tektites are melt products of hypervelocity impact. The discovery of shocked quartz and coesite within some tektites bolstered the case for an impact origin. Tektites occur as strewn-fields, of which the chief are the Australasian, Ivory Coast, Czechoslovakian, and North American. Strewn-fields include tektites (usually about 1 cm in size, but can be 20 cm), which are found on land, and microtektites which are microscopic tektites (less than 1 mm) found in deep-sea sediments. Impact spherules are spherical particles the size of sand grains formed by the condensation of silicate mineral vaporized by a hypervelocity impact, and may be deposited hundreds to thousands of kilometres from the crater (Simonson and Glass 2004). They commonly occur abundantly in thin, discrete layers that form rapidly and may have global extent. If unaltered, impact spherules consist entirely of glass (microtektites) or a combination of glass and crystals grown in flight (microkrystites). Always found in a stratigraphical context, spherule layers are probably superior to terrestrial craters and related structures for assessing the environmental and biotic effects of large impacts. Indeed, they can provide evidence of past impacts, even in cases where the craters no longer exist.

Crater form and distribution

In terms of morphology, terrestrial impact structures are either simple or complex (Figure 3.4). Simple structures, such as Brent Crater in Ontario, Canada, are bowl-shaped (Figure 3.4(a)). The rim area is uplifted and, in the most recent cases, is surmounted by an overturned flap of near-surface target rocks with inverted stratigraphy. Fallout ejecta commonly lie on the overturned flap. Autochthonous target rock that is fractured and brecciated marks the base of a simple crater. A lens of shocked and unshocked allochthonous target rock partially fills the true crater. Craters with diameters larger than about 2 km in sedimentary rocks and 4 km in crystalline rocks do not have a simple bowl shape. Rather, they are complex structures that, in comparison with simple structures, are rather shallow (Figure 3.4(b)). The most recent examples, such as Clearwater Lakes in Quebec, Canada, typically have three distinct form facets. First, a structurally uplifted central area, displaying shock-metamorphic effects in the autochthonous target rocks, that may be exposed as a central peak or rings; second, an annular depression, partially filled by autochthonous breccia, or an annular sheet of so-called impact melt rocks, or a mixture of the two; and third, a faulted rim area.

Impact craters occur on all continents. By 2 November 2004, 172 had been identified as impact craters from the presence of meteorite fragments, shock metamorphic features, or a combination of the two (Figure 3.5). This is a small total compared with the number identified on planets retaining portions of their earliest crust. However, impact structures are likely to be scarce on the Earth owing to the relative youthfulness and the dynamic nature of the terrestrial geosphere. Both factors serve to obscure and remove the impact record by erosion and sedimentation (Grieve 1987). Craters would have originally marked sites of all impacts. Owing to erosion, older sites are now obscure, all that remains being signs of shock metamorphism in the rocks. Thus, impacts will always leave a very long-lasting, though not indelible, signature in rocks, but the landforms (craters) they produce will gradually fade, like the face of the Cheshire cat. The current list of known impact structures is certainly incomplete, for researchers discover about five new impact sites every year.

(a) Simple impact structure

Overturned flap

(a) Simple impact structure

Overturned flap

jj Sedimentary fill

Allochthonous fallback breccia l| Fallout ejecta

Impact melt and melt fragments

Strongly shocked target rocks Fractured and brecciated target rocks | / | Fault

(b) Complex impact structure

Faulted Breccia and impact rim melt annulus

Central peak and rings

Breccia and impact Faulted melt annulus rim

Faulted Breccia and impact rim melt annulus

Central peak and rings

Breccia and impact Faulted melt annulus rim

Figure 3.4 Simple and complex impact structures.

The spatial distribution of terranean impact structures reveals a concentration on the Precambrian shield areas of North America and Europe (Figure 3.5). This concentration reflects the facts that the Precambrian shields in North America and Europe have been geologically stable for a long time, and that the search for, and study of, impact craters have been conducted chiefly in those areas. It is not a reflection of the impaction process, which occurs at random over the globe (but see p. 48). The ages of known impact craters and structures range from Precambrian to Recent, and their diameters range from a few tens of metres to over 200 km. There are more younger structures than older, more than half the structures with diameters greater than 10 km being younger than 200 million years. This results not so much from a recent increase in the meteorite collision rate as from erosion, which can rapidly render the crater form, but to a lesser extent the underlying chemical and structural signature, unrecognizable. All geological traces of craters more than 20 km in diameter and located in glaciated areas, unless protected from erosion by a blanket of sediment laid down after impact, can be lost within 100 million years (Grieve 1984).

Research since the mid-1980s has revealed a few underwater impact craters. The first of these, called Montagnais, was identified on the North Atlantic continental shelf some 200 km south-east of Nova Scotia, Canada. Multi-channel reflection seismic surveys revealed a circular structure (Jansa and Pe-Piper 1987). Seismic profiles showed a crater at least 45 km in diameter with a central irregular uplift 1.8 km high and 11.5 km wide, partially filled by a seismically isotropic mass interpreted as fallback breccia, which exhibits shock deformation features. The projected depth at the centre of the crater is about 2.8 km; shallowing towards the edges. Tertiary marine deposits that bury the crater overlie the fallback

Impact Crater Seismic
Figure 3.5 The distribution of known impact craters and structures.

breccia. Lack of enrichment of the melt rocks in siderophile elements (which indicate the impact of an iron meteorite) compared with basement rocks, and a slight enrichment in iridium, suggest that the bolide was either a stony meteorite or a comet nucleus with a diameter some 2-3 km. Other submarine impact craters include the Chesapeake Bay Crater, Virginia, USA (Koeberl et al. 1995, 1996), the Mj0lnir structure in the Barents Sea, off the northern Norwegian coast (Gudlaugsson 1993; Dypvik et al. 1996; Tsikalas et al. 1998), the Silverpit Crater in the North Sea (Stewart and Allen 2002), and Bedout, an end-Permian impact structure lying off northwestern Australia (Becker et al. 2004).

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