The Formation of Complex Craters
The impact craters that speckle the surfaces of all terrestrial bodies in the solar system can be divided into two morphologically distinct classes: Simple craters and Complex craters. Simple craters, as the name suggests, posses the form that most people associate with the word "crater"; a circular "bowl" shape with an uplifted rim. Complex craters, on the other hand, exhibit somewhat unintuitive structures such as central peaks, or an inner "peaked" ring, terraced rim walls and outer concentric faulted zones. Despite the dramatic physical differences between the two classes, however, the final morphology of both is now believed to be predominantly the result of the collapse of a geometrically simple, bowl-shaped "transient crater", which forms immediately after impact (Melosh & Ivanov, 1999).
Collapse of simple craters, on scales greater than 100m, is dominated by the slumping of the steep cavity walls under the action of gravity and is entirely explainable with standard debris mechanics. The transient crater´s steep walls are unstable and relax to the angle of repose by wholesale slumping of the walls. Complete understanding of the formation of complex craters, however, has eluded all who have studied it. This is due in no small part to the fact that the presence of the complicated morphologic structures, which any theory of complex crater formation must explain, appears to violate current understanding of rock and debris mechanics. In short, in order for central peaks and peak-rings to be created the sub-crater region must behave not as competent, or fractured rock, but as a fluid. Of course the collapse of impact craters cannot be entirely hydrodynamic, as the end result would inevitably be a flat surface. Evidently, the fluid collapse must be frozen or suspended in some way to produce the observed complex crater morphologies.
Of the postulated mechanisms by which this ephemeral fluidization may occur, one of the more plausible is Acoustic Fluidization. Coined by it´s propounder (Melosh, 1979), the physical basis for this phenomenon is that acoustic vibrations within a granular material could become violent enough to temporarily relieve the overburden pressure and, therefore, abrogate the internal frictional resistance of the material. Consequently, the theory predicts that if driving stresses are high enough and acoustic vibrations in the medium are strong enough the granular material will flow as though it were a fluid. A material which behaves both as a solid at low stresses and at high stresses flows like a fluid is known as a Bingham fluid. Other examples of such materials are paint, lava and clay slurries.
In our hydrocode modelling pages we show some preliminary results of crater collapse calculations using the SALE 2D hydrocode, modified to include a representation of the predicted effects of acoustic fluidization. 
