Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
The first stage in the excavation of any crater is the formation of a transient crater about half the size of the final crater (left). Material from the impacting object and the crust flow out along the floor of the crater and outward at high-speeds. After the shock effects pass, the result of a small impact is a simple pit up to a few kilometers across (right). Often the walls of the crater collapse to form landslides.
|A typical small pit crater on the Moon with collapsed walls.|
The physics of large impacts are mind-boggling. An impacting object a few kilometers across generates pressures of megabars, and the failure strength of rocks is typically measured in kilobars. The rocks are subjected to stresses hundreds of times their failure strengths. This means that the rocks effectively behave as if they were water. The stresses are so far beyond the strength of rocks that the impacted rocks behave as if they had no strength whatever. They flow plastically as if they were fluids.
One of the best places to see the physics of large impacts is in slow-motion photographs of droplets splashing. Every major feature of impact is represented: formation of a transient crater, ejection of jets, and collapse of the transient crater. Following the collapse of the transient crater, the floor rebounds to produce a central jet.
In craters more than a few kilometers in diameter, gravitational collapse of the crater walls and rebound of the compressed floor result in the uplift of a central peak. The sudden withdrawal of floor material toward the center causes the crater walls to collapse along concentric normal faults to create terraces. The final crater rapidly becomes wider and shallower than the transient crater.
|This small crater is about on the borderline of the central-peak class. The walls have collapsed in a landslide-like way, but there is a pronounced central peak.|
|Two central-peak craters on the Moon. Note the terraces and the much smaller central peak in the smaller crater.|
|Tycho is the youngest major crater on the Moon and its terraces and central peak are typical of its class.|
In some large craters (greater than several tens of kilometers) the rising central peak actually overshoots its stable height. It then collapses to produce a ring of peaks.
Below: The lunar crater Schrodinger is a good example of a peak-ring basin.
The transition from one crater form to the next is inversely proportional to a planet's gravity, so while only the largest craters on the moon are peak-ring craters, moderate-sized craters on the earth can be. The larger of the Clearwater Lakes craters in Quebec (below) is an example.
The very largest impacts create concentric rings of fault scarps hundreds of kilometers in diameter. These features are called Multiple-Ring Impact Basins. One theory is that the ground motion from super-impacts is so great that huge waves develop in the crust and the crust ruptures at areas of especially violent wave motion.
|Mare Orientale on the Moon is the archetypical multiple-ring impact basin. Perhaps a billion years after the impact, lava flooded the low-lying parts of the basin and also ponded against some of the scarps|
|The Caloris Basin on Mercury has a multiple-ring appearance, although the rings are ridges in a lava plain, perhaps reflecting scarps buried below.|
|The Valhalla Basin on Callisto is the champion multiple-ring impact basin in the Solar System, with perhaps 30 visible rings.|
Created 10 May 1999, Last Update 14 September 2018