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Moon-like circular assemblages on Earth

For several decades of the 20th century, the main source of visual data by remote sensing came from aerial photographic images. As many such photographs were desperately lacking in contrast, it took a trained and practised eye to see and interpret the subtle details, faintly in evidence, in such images. Because of the subtlety of the data, a person not trained in such interpretation does not have to be a natural sceptic to doubt the validity of the interpretation. However, when presented with an image-intensified version of the same view, or enhanced satellite images, the practised interpreter is usually vindicated, though the intensified image may show details that even the practised observer did not detect.

Evidence which indicates a concentration of impact activity on Earth which could be considered significant, seems to have evaded discovery. But such evidence that crater density almost comparable with that on the Moon, does exist and was 'brought to light' (literally) in a study by Saul (1978). His investigation showed that there are numbers of moderate-sized circular features in Arizona, USA (Figure 5.26).

These features were not inferred from field observations or photogeological techniques, but are the result of studying the geomorphology of this portion of Arizona, as represented by an accurate three-dimensional model of that state. This model was viewed with illuminating lighting which was restricted to a horizontal beam. By changing the direction of illumination, high spots on the topographic model could be readily identified and plotted on a map. It was noted that metalliferous mines were conspicuously related to these circular features, especially when the circular traces intersected.

These are simple empirical observations which, one would suppose, require at least an attempt at an explanation. However, Saul, who presented this survey, offered no explanation of these features. We consider it reasonable to suggest, from the size and frequency of these circular features, that they relate to impact phenomena in the Precambrian basement and that traces of these basement structures have been inherited by the cover rocks. (We suggest that an alternative explanation, that these circular features represent 'nests' of caldera, lacks plausibility—and requires as much, or even more, verification than the impact hypothesis.)

Norman (1980), who carried out a study of Saudi Arabia using aerial photographs and satellite images, was less inhibited. He too found evidence for an array of quite large arcuate and inferred circular features, most of which, he suggested, were traces of ancient impacts which had probably been 'inherited' by the flat-lying sediments from the underlying archean rock mass (Figure 5.27).

It will be noted that, in both these examples, we are not observing direct evidence of cratering, but the influence of structures in basement rocks of circular or arcuate fractures, which make themselves evident in much younger cover rocks. Hence, the traces in the cover rock can be related to impact events, even after the crater itself has been deeply eroded.

Figure 5.26 Showing mines in relation to ancient circular features in Arizona, USA (after Saul).

It has been shown that the major meteoritic impacts occurred on the moon prior to 3800 Ma. By far the preponderance of basement rocks on Earth yield isotopic dates less than 4000 Ma, therefore circular features shown in Figure 5.26 and Figure 5.27, are likely to have occurred when the lower or residual rate of impact existed. Nevertheless, these figures show that the Earth's basement rocks are still capable of exhibiting a quite respectable record which can be interpreted as being the result of bombardment.

The work of Russian workers (Zhukov, Murav'yev and Popsuy-Shapko) which is presented by Zaychenko et al. (1982) is shown in Figure 5.28. The original papers are not quoted, but we can reasonably assume that the figures in this paper showing circular features are accurate copies of the originals.

Zaychenko et al. suggest that the evidence which militates against the circular features being impact features (astroblemes) include the following:

Figure 5.26 Showing mines in relation to ancient circular features in Arizona, USA (after Saul).

It has been shown that the major meteoritic impacts occurred on the moon prior to 3800 Ma. By far the preponderance of basement rocks on Earth yield isotopic dates less than 4000 Ma, therefore circular features shown in Figure 5.26 and Figure 5.27, are likely to have occurred when the lower or residual rate of impact existed. Nevertheless, these figures show that the Earth's basement rocks are still capable of exhibiting a quite respectable record which can be interpreted as being the result of bombardment.

The work of Russian workers (Zhukov, Murav'yev and Popsuy-Shapko) which is presented by Zaychenko et al. (1982) is shown in Figure 5.28. The original papers are not quoted, but we can reasonably assume that the figures in this paper showing circular features are accurate copies of the originals.

Zaychenko et al. suggest that the evidence which militates against the circular features being impact features (astroblemes) include the following:

(a) their spatial location at an intersection of two deep linear faults;

(b) their occurrence at location of dome uplifts;

(c) the occurrence of annular and en echelon normal faults with normal bedding of cover rock;

(d) several formation stages of volcanic rock; and

(e) the presence of allied tuff-containing explosion pipes outside the structure.

We do not wish to go into detail regarding these points, but would refer to the results of the structures which developed in the Suffield experiments, and suggest that many of these features given in (a)-(e) could reasonably be expected in a large, old impact structure. On the basis of the evidence of circular features, presented above, we suggest that there is evidence which supports the conclusion that impacts have occurred on Earth, in sufficient numbers to resemble impact patterns on the Moon.

The question is not whether the Earth has been bombarded, but to what extent could we expect circular impact scars to be a feature of the surface morphology of the Earth. The pattern of arcuate and circular features, shown in Figures 5.26 to 5.28, we suggest, are so similar to those found on the moon that there is an a priori reason to accept that the arcs and circles on Earth, shown in the various cited figures in this chapter were mainly caused by the same mechanism that caused the features on the Moon.

Long Apcuale Features

Long Apcuale Features

Figure 5.27 Circular features in Saudi Arabia (after Norman).

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