...more about the morphology of impact craters
The morphology of impact craters changes with crater diameter. Craters are usually divided into two separated groups, called simple and complex, recognized by a growing diameter of the impactor. Furthermore, craters are usually surrounded by what is called an ejector blanket, made of material thrown away during the impact. Due to the very high temperature and pressure reached during the impact event, both the impactor and the target rocks are melted and transformed, in a mechanism that is called shock metamorphism.
|Simple craters are relatively small, with smooth bowl shapes...|
|...while complex craters are larger, with more complicated structures, like internal peaks.|
The smallest craters have a bowl shaped form and are often named simple craters. These structures, have relatively small depth to diameter ratios of about 1:5 to 1:7. This simply means that they have smooth shapes.
As the dimension of the crater increases, gravity causes the initially steep walls to collapse inward, and the rebounding of the fallen crater floor creates progressively central peaks and larger rim terracing. In this case, craters have shallower depth compared to diameter, than simple structures (ratios of about 1:10 to 1:20).
At even larger diameters, the single central peak can be replaced by one or more rings.
The diameter at which a crater becomes complex depends on the surface gravity of the planet: the greater the gravity, the smaller the diameter that produces a complex structure. On Earth, this transition diameter is 2 to 4 kilometers depending on the rock properties, while on the Moon, this transition is 15 to 20 Km. The relation between dimension and morphology is observed throughout the solar system. However, being terrestrial craters less well-preserved, this dependence can be well illustrated with well observable craters on the moon.
During the impact, very high temperature
and pressure can be reached. For this reason, the impactor as
well as the target rocks, are usually melted and mixed together,
giving birth to a new chemical composition, which is
characteristic of impact events, the impact melt. The
shock wave generated by the impact, is responsible for is the
production of irreversible chemical or physical changes in target
rocks. This phenomena is called shock
Of course the magnitude of the mechanism depends on depth: material just below the surface of the crater is significantly disrupted by the shock (near the surface, we can in fact find a layer of breccia, a type of rock composed of coarse, angular fragments of broken-up rocks). Rocks at deeper depths remain in place (and are called bedrock) but are highly fractured by the impact. The amount of fracturing decreases as the depth below the surface increases.
The energy of the impact typically causes some material to melt. In small craters, this impact melt occurs as small blobs of material within the breccia layer. In larger craters, the impact melt may occur as sheets of material. The rocks that are melted during impact, including small particles dispersed in various impact deposits and ejecta, and within the crater, are called impact melt. Impact melts are extremely uniform in their composition: they are made predominantly of the target rocks, but can contain a small amount of the impactor
Certain shock consequences have been shown to be uniquely associated with meteorite impact craters, since no other earthly mechanism, including volcans, can produce the high pressures that cause them. One of such characteristics is the formation of very rare types of quartz minerals known as coesite and stishovite, also known as shocked quartz, a particular kind of quartz that contains tiny fracture planes. Another effect is the change of rock structure from crystalline to glassy state: even diamonds can be produced in this kind of process!
An impact site is usually surrounded by an ejector blanket, a layer of material thrown away during the impact. This layer can be, in some cases, very visible.
||In particular, much of the material ejected from the crater is deposited in the area surrounding it, close to the crater, where these ejecta form a thick, continuous layer. At larger distances, the ejecta may form discontinuous clumps of material and if the ejected material is enough, it may create a new crater when it comes back down. These new craters are termed secondary craters and frequently occur as lines of craters that point back to the original crater.|
|Another characteristic of impact craters is the presence of tektites, usually found in sedimentary layers. Tektites are smooth glass-like stones with size that range from a few centimeter to microscopic. Some of these stones have drop-like shapes showing to have been shaped by aerodynamic forces.|
|These stones show clear signs of having been heated twice: the first heating process seems to have melted the entire rock to molten state, while the second heating seems to have only affected the outer layers. For this reason, tektites are believed to be formed during impacts: first, the rock hit by the impactor has been melted, then ejected into the atmosphere, where, cooling off, it has taken its regular shape. The second heating process can be produced by friction, as the stones fall back to Earth.|