Gradation from simple to central, or ringed, uplift craters
It will be recalled that the diameter of the Snowball Crater was approximately 87 m. Consequently, from the relationship indicated in Figure 5.6 (Melosh, 1989, p. 129), the transient crater has a diameter some 19 per cent less than the simple crater. Hence, if we take the limit of the Snowball Crater to be 1.19 times larger than that of the transient crater, from which it developed, we can infer that the transient crater had a diameter of 87*0.81=70 m. Melosh also argues that the ratio of the depth of the transient crater relative to its diameter 1:2.7, so the depth of the Snowball transient crater was likely to be about 26 m. As the average depth of the Snowball crater floor is an estimated 4.25 m, the maximum depth of infilling of the central part of the Snowball transient crater is over 21m, and this does not take into account the height of the central uplift. Jones (1995) does not actually quote the depth of the crater, because he realised that the Snowball explosion had given rise to what he described as a 'depressed rim, central uplift crater'. This description derived from the fact that, although initially flat, subsequent to the explosion, the whole of the surveyed site dipped gently inwards towards the outer limit of the crater rim. From the data presented in an E-W section (Jones, 1995), it follows that there was a gradually increasing dip from a point at the limit of the surveyed line of 2168 feet at 225 feet from GZ to 2160 feet, near the limit of the crater at 105 feet from GZ. The average slope was therefore 3.8°. Prairie Flat and Dial Pack (both 500 ton TNT experiments) exhibit a comparable inward dip of what had been, prior to the explosion, flat ground.
The change in dip of the original surface can be attributed to one of two mechanisms, or a combination of both, possibly acting in sequence. The first of these would be attributed to the intensity of the explosion, which would have caused excavation of the transient crater and would have generated high fluid pressures in the sediments surrounding the crater, by deforming the pore-spaces.
The second effect comes into play as soon as the pressure pulse has passed. The high pore-fluid pressure would reduce any cohesion with each other, which the grains may have originally possessed, to zero, thereby giving rise to an upheaval of the floor at the centre of the crater. Slumping of the walls of the crater would have occurred simultaneously, as well as a slower migration of the weaker elements of the sediments more distant from the crater walls (Figure 5.35a). As the walls slumped into the crater, they would tend to impede the upward progress of the diapir rising from the deepest part of the crater.
As regards Snowball, it is apparent that inward sliding of the wall rock completely covered the rising diapir, for a while. However, because of its liquidity, the central diapir continued to rise and soon became a prominent feature of a flooded crater (Figure 5.9). Elsewhere around the crater, the liquefied sediments from below the original water-table came to the surface to form sand and silt volcanoes and mud-flats.
Let us now consider this as a model for a quite major natural impact feature (Figure 5.35b). In this diagram, the metres have been changed to kilometres, the bars to kilobars and probable temperatures have been added. The liquefication mechanism caused by the Snowball explosion is substituted here by the flow of hot, relatively weak rock aided by the upward adjustment of the asthenosphere, which will assist in the production of the diapiric action. Concomitant slumping from the walls of the crater took place, giving rise to gravity-glide and tumbling of boulders and rubble towards the central part of the transient crater. Natural erosion and sedimentation in the crater will eventually modify and flatten the crater floor. From this section, because the mantle becomes involved with the diapiric intrusion, one may reasonably expect that such major natural impacts may well generate gravitational anomalies.
At least some multi-ringed craters are thought to be the result of the development of concentric normal faults that form within the crater, and dip inwardly towards the impact point. In other craters, however, the rings are considered to be the result of buckling of the inward-flowing debris, to cause variations in surface elevation of the crater floor. The interested reader is directed to the appropriate chapter in Melosh (1989).
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