Simple craters are circular, bowl-shaped depressions with elevated rims and an approximately parabolic interior profile. They have no major internal topographic features.
One of the most important characteristics of impact craters is their ratio of rim-to-floor depth to the rim-to-rim diameter (d/D), which has a value of about 1:5 for most simple craters on the terrestrial planets as well as on the Moon. The upper rim of a simple crater often shows stratification and evidence of mass-wasting. Most lunar craters smaller than about 15 km in diameter are simple, while on Earth the upper limit is about 4 km.
The floor of simple craters is underlain by a breccia lens consisting of rock debris and shock-melted rock with a thickness of about half the rim-to-floor depth of the crater. This lens in turn lies in a bowl of fractured country rock. It is thought that the final shape of a simple crater is formed mainly by the gravitational collapse of the rim of the transient crater immediately after it forms.
Complex craters range in diameter from a few kilometers on Earth to a huge 460 km diameter crater observed on asteroid 4 Vesta. The 15 Ma Ries crater in Bavaria, which was visited during the last AGM in Munich, is one of the best known middle-sized complex craters on Earth, with a diameter of about 24 km.
Complex craters have a d/D ratio which varies widely fom 1:5 for small fresh complex craters to 1:150 for large craters, depending upon the planet. On Earth they exhibit central stuctural uplifts, rim synclines, and outer concentric zones of normal faulting. Extraterrestrial craters have been observed with multiple central peaks and terraced rims. The central uplift consists of strata which have been uplifted above the pre-impact level, and is surrounded by a ring depression (or rim syncline) filled with fragmented material and impact melt. The uplift of the transient cavity’s floor — accompanied by subsidence of the crater rim — is thought to be the main modification mechanism for complex craters.
The transition between simple and complex craters occurs over a narrow diameter range on a particular solar system body and is thought to scale as the inverse power of the surface gravity, g. The transition occurs around 15 km diameter on the Moon, 7 km on Mars and about 4 km on Earth. As the crater size increases further, the central peak complex in a complex crater begins to break up and form an inner ring of mountains. In large craters the ring is about half the rim-to-rim diameter, and these craters are called “peak-ring” craters.
However, not all of the complex crater features appear at the same diameter. Therefore the transition diameter is often expressed as the geometric mean, Dt, of several diameter values at which particular morphological features, such as central uplifts and terraced walls, appear. Studies on Martian craters indicate that the first complex features to appear are a flat floor, a central peak and a low d/D ratio.
Interestingly, the Dt values for Earth (3.1 km) and the Moon (18.7 km) differ by a factor of six, which is exactly the ratio of their values of g. In addition, complex craters on the Moon are on average six times deeper than on Earth.
Despite the importance of gravity, the lithology of the target area influences the value of Dt as well. This is best established for terrestrial craters of course, giving a simple-to-complex crater transition diameter of 2.25 km for sedimentary rocks and 4.75 km for crystalline rocks. It is thought that at least three (interrelated) target properties influence the shape of the final crater: rock strength, stratification and volatile content. An impact of a given energy will excavate a larger cavity in soft rocks, while there is evidence that complex craters develop more readily in stratified rocks.
The simple-to-complex transition coincides with a change in the texture of the ejecta surrounding fresh martian impact craters (this is of course more difficult to observe on Earth), indicating the mechanism of ejecta emplacement is dependent on crater size.
Craters less than 4 km in diameter show typical ballistic ejecta characteristics, while between 4 and 80 km emplacement seems to occur at least partly by surface flow, and larger craters again have ballistic ejecta emplacement. This might be explained by the incorporation of subsurface volatiles in the ejecta for impacts of intermediate size, while small impacts do not excavate deep enough to tap these volatiles, and large impacts completely vaporise them.
highlights from http://ougseurope.org/rockon/surface/impactcraters.asp
amazing how may craters form hexagonal shapes. odd shaped rocks must just do that no matter what type of meteor they are, angle or speed of impact, if the planet/object has an atmosphere or not, what the body it impacts is made of, the value of G ...
. "This demonstrates that metallic meteorites having a mass on the order of 10-tons do not break up in the atmosphere, and instead explode when they reach the ground and produce a crater," says ESA's Dr Detlef Koschny, Head of Near Earth Objects segment for the SSA program.
Evidently the 60 ton Hoba meteorite did not read this article, otherwise I am sure that it would have properly exploded upon impact.This demonstrates that metallic meteorites having a mass on the order of 10-tons do not break up in the atmosphere, and instead explode when they reach the ground and produce a crater," says ESA's Dr Detlef Koschny, Head of Near Earth Objects segment for the SSA program.
They collected over 1000 kg of metallic meteorite fragments, including one 83-kg chunk thought to have split from the main meteorite body shortly before impact (it was found 200m away from the crater).
This demonstrates that metallic meteorites having a mass on the order of 10-tons do not break up in the atmosphere, and instead explode when they reach the ground and produce a crater,
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