Manmade craters

During the last few centuries, the military learned the 'art' of creating small craters with, initially, relatively small quantities of gunpowder. As with many other aspects of technology, mankind's understanding was greatly enhanced by war, or the threat of war. The effects of cratering became more generally appreciated during World War I, but it was not until World War II, in 1940, that British scientists conducted a series of tests to determine empirical relationships that would permit estimates to be made of the size and shape of craters caused by high-explosive (HE) charges. These tests, carried out in soil and representative rock types encountered in England, permitted relationships to be established between the diameter of the crater and the energy of the charge. The range of tests carried out later by the U.S. Army Corps of Engineers in the 1950s on soils and lightly cemented soils was impressive. They too established relationships between the various parameters.

At the end of and immediately following World War II, the advent of atomic and hydrogen bombs enabled the relationship between the diameter of the crater and the size of charge (i.e. the energy of the explosion) to be extended through several orders of magnitude. Quite a number of experiments were conducted in the USA, Canada, Australia and islands in the Pacific (and, presumably, also in the former USSR and elsewhere). This work which was, of course, largely directed at purely military objectives, has now been mainly superceded by computer modelling. The geometry of the crater produced by these explosive experiments depended upon the position at which the explosion was initiated relative to the surface. However, for tests where explosions of relatively small to moderately large TNT tonnage took place at the surface, the craters which developed were always of a simple basin-like geometry. The section through a crater which developed as the result of a 20 ton TNT explosion is shown in Figure 5.5a and a natural, but very much larger structure, the Barringer or Meteor Crater, Arizona, is shown in Figure 5.5b and in vertical view in Figure 5.5c.

These structures, although termed simple craters, show a degree of evolution. It was soon realised that, initially, excavation formed a short-lived, transient crater (Figure 5.6a) which then rapidly evolved into a simple crater by slumping of the transient crater walls (Figure 5.6b). It will be noted that, as a consequence of this slumping, the simple crater is wider and shallower than the transient crater. An important feature of the transient crater is that, in section, it approximates to the nose of a parabola with the ratio of the depth to diameter of the transient crater approximating closely to 1:3.

Grieve and Pilkington (1996) show, in an elegant diagram (Figure 5.7), how, by adjusting the scale of individual craters, the sections of simple craters can be superimposed on an idealised section, so that the degree of erosion can be inferred.

Figure 5.5a Section through crater which developed in weak sediments following a 20 ton TNT explosion. Such craters are very close to circular in plan.

Figure 5.5b Diagrammatic section through Meteor Crater which is very similar to that of Figure 5.5a (after Shoemaker).

In small, 20 ton TNT tests, the surface around the crater is thinly covered with earth and debris derived from the crater. The nature of this surface cover did not become immediately clear from such small energy explosive experiments. Indeed, it was not until much larger explosions of about 100 tons of TNT, set off in the test-sites of the USA and Canada, that it was recognised that the near-surface rocks were 'blown upward and outward to form a recumbent flap' (Figure 5.8) which can also be seen in Figures 5.5b and 5.7. Indeed, it was Shoemaker's experience with the results of moderately large tonnage explosions that enabled him to demonstrate that the Barringer Crater, Arizona, was a certain impact crater. As a result of this identification, the structure was renamed Meteor Crater.

It required a further series of major explosive experiments to provide the information that permitted one to infer the evolution from the simple crater to the more complex features, that are now recognised as exhibiting the characteristics of natural major impact craters. These experiments were conducted at the Defence Research Establishment, Suffield, Alberta, Canada in the period from about 1960 to 1970. Little of the experimental work was reported except in internal reports. A brief outline of some of the important results was presented by Jones (1977, 1978). However, in his retirement, he was persuaded by the present author to prepare an extremely valuable document (entitled The Suffield Craters as Analogues of Impact Structures; Jones, 1995) which gives an overview of the range of experiments conducted in the decade 1960-70. It was only when charges in the range of 100-500 tons of TNT were exploded on poorly indurated sediments that the more complex crater geometries developed which, hitherto, had only been observed on the Moon; but whose mechanical significance had not been understood.

Figure 5.5c Oblique view of Meteor Crater reveals that the rim of the structure shows 'straight-line' sections which are probably determined by the orientation of pre-existing joint or other forms of fractures.

Transient crater

Figure 5.6a Section through a transient crater, which rapidly evolves by slumping to (see Figure 5.6b).

Figure 5.6a Section through a transient crater, which rapidly evolves by slumping to (see Figure 5.6b).

Figure 5.6b A simple crater (after Melosh, 1989).

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