The recognition of impact structures

Until recently, the importance of meteoritic impacts on the development of the Earth has received scant attention from most geologists. This is perhaps understandable, as geologists usually spend their professional lives trying to understand and explain geological phenomena wholly in terms of terrestrial processes. One of the reasons for neglecting the importance of meteoritic impacts is an uncritical application of the concept of uniformitarianism. We contend that catastrophes are an integral part of uniformitarianism.

It is fortunate for mankind that even quite a small impact event, which gave rise to a crater with a diameter of no more than about 1 km, has probably not occurred in continental areas during the period of recorded history. Statistically there should have been as many as four such events in this period. Possibly, of course, these modest-size meteorites fell into the oceans.

However, even this lack of activity may be more apparent than real. For example, Lewis (1996) and Vershuur (1996) cite historic examples of small impacts, while Tollmann and Tollmann (1993, 1994) present arguments for moderately large oceanic impacts that occurred around 7300 BC. Moreover, it is now being suggested that layers of burned vegetation, which had initially been attributed to grass fires, are now considered to be the result of more extensive and distant forest fires, which may have been initiated by impacts. Such events, which probably occurred in the period between 2200 and 1200 BC may have resulted in the destruction of Bronze Age communities in the Middle East, China and possibly the E Mediterranean area.

A potentially catastrophic event occurred in the morning of 30 June 1908 in the Tanguska region of N Siberia. Fortunately, this area was almost completely uninhabited, so there was no recorded loss of human life. However, two brothers, on a hunting expedition, experienced considerable shock when their tent was 'blown away'.

Scientific expeditions were not allowed into the area for 20 years, so the epicentre was not found until 1928. Despite repeated searches no evidence of cratering or of extra-terrestrial material has been found.

The cause of the 'wind' was an air-blast that flattened the trees from a central zone, throughout an area of 2000 km2. However, the 'blow-down' pattern was not circular, but exhibited a 'butterfly shape'. From the bilateral symmetry of this butterfly pattern, Russian scientists set up an experiment using small, cylindrical, explosive charges, set at a small distance above a board which contained a 'forest' of small pegs which were initially set perpendicular to the peg-board. Each peg was attached to the baseboard by a thin ductile wire, which permitted the individual 'trees' to be blown down in a direction wholly determined by the explosive blast. They very quickly established that the line of symmetry of the butterfly pattern gave the track of the bolide and, by varying the angle of inclination of the cylindrical explosive and comparing the pattern of 'blow-down' in the models with the known pattern in the field, they were able to establish that the angle of track the bolide made with the horizontal was 10°. They also found that immediately beneath the charge, the model trees remained upright. This was exactly what had been seen in the field where, immediately beneath the focus of the blast, the trees in the Tanguska forest had been stripped of their branches but the trunks remained standing like 'telegraph poles'.

With these experimental results available, other scientists showed that the explosion was almost certainly caused by the vapourisation of a fragment of an icy comet which occurred at a height of 6-8 km above ground. The energy of the explosion has been estimated to be 1 megaton (1 Mt) of TNT (Toon et al., 1997), over 60 times larger than the bomb that destroyed much of Hiroshima. The most probable cause of this event can be attributed to the impacting in the atmosphere of one unit of a comet stream, made up of fragments of a larger cometary body which orbit around the Sun. Between 40 and 50 such comet streams are known. However, the most probable 'culprit' is the B-Taurus stream which makes an angle of 5° with the plane of

Earth's orbit. The two orbits intersect in November and June, with the maximum likelihood of collision occurring on 30 June. Such collisions are thought to be probable every 100-300 years.

Had this event occurred above London, the devastated area would have covered almost the whole of Greater London within the M25 ring road. Indeed, if this air-burst had occurred above any of the cities on Earth, the devastation and loss of life would have focused the attention of mankind in general, and the scientific community in particular, on the study of what is arguably the most important of all the catastrophic dangers that face mankind.

Subconsciously, or even consciously, such relatively recent events are categorised as rare or remote occurrences and tend to be dismissed from our thoughts. This luxury of self-delusion, has now been swept away by Earthwatch. Toon et al. (1997) note that several objects, with energies comparable with the device which devastated Hiroshima, enter and are destroyed in the Earth's atmosphere every year. They further comment that 'the populace is not generally aware of these events'. However, they are routinely recorded by surveillance satellites operated by the U.S Department of Defense (Tagliaferri et al., 1994). An incident occurred on 1 February 1994, when a bolide exploded over Micronesia, with an energy estimated as 20-100 kT, after penetrating the atmosphere to an altitude of 21 km (McCord et al., 1995). As we shall see, impacts which produce a crater 1.0 km or greater in diameter, are only apparently rare events when measured in terms of the recorded history of mankind. They are far from rare events when measured by more usual geological time-scales.

The two most important, practical reasons for the lack of acceptance of the importance of major impact structures were, firstly, the lack of a sufficiently large 'world view', so that extensive circular features were not readily recognised. This difficulty has now, of course, disappeared with the arrival of satellite imagery. Secondly, relatively small features, which have now been shown to be of impact origin, were, in the main, for many years attributed to volcanism, or crypto-volcanism, where 'crypto' means secret, hidden or concealed (so that 'crypto-volcanism' really translates into 'unknown-mechanism').

Lists of impact events on Earth have been compiled by, among others, Grieve and Dence (1979) and Hodge, (1994). Hodge lists over 150 craters in 139 different sites, which he regarded as certain, up to the year 1992. He closely follows the compilation of Grieves and Dence, but adds eight new sites. However, he notes that there are many other structures around the world which have been proposed by various scientists to be probable impact structures; and some of these, with further study, are likely to be upgraded to certain. In order that such an upgrade from probable to certain can be achieved, evidence must be adduced to show that the impacted rocks of the structures have experienced shock-metamorphism, with the generation of exotic minerals and structures.

It is interesting to note that, currently, a crater may be classified as either certain or dubious. We suggest that this simplistic classification could now be expanded to (1) certain, (2) probable, (3) possible and (4) unlikely or dubious. There are certain features, some of which are illustrated in this chapter, which exhibit a variety of characteristics that render them as almost certain (i.e. probable). However, because such events have been only recently observed by satellite imagery in a remote part of the globe, or they are buried features which have been inferred from remote sensing and have not been surveyed on the ground or drilled to establish whether or not they contain diagnostic evidence of impact (which we discuss below), they would be termed dubious, a rating which is far too low.

The thoughts expressed in the previous paragraph are, of course, contentious. However, from what has been reported earlier, it is clearly not in question that the Earth has experienced bombardment by comets or meteorites. Nevertheless, when one compares the number and size of known impact craters on Earth with those of our satellite or of Venus it must be admitted that the Earth structures appear trivial, both as to number and size of crater. The reason for this, however, is obvious. The Earth is tectonically active with a surface that is rapidly eroded, or replaced. Other bodies in the Solar System record a longer history of bombardment.

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