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Assignment 6 (Craters)


Assignment 6 - Craters

13) Why do you think so few meteorite craters are observed on Earth compared with other bodies
in the solar system?

Several causes conspire to erase the geological record of craters here on earth and perhaps the most insidious is that of erosion. After an impact wind, rain and the cyclic freezing and thawing of the seasons are quick, in a geological time scale, to remove every trace of all but the largest craters. Life itself can also have an eroding effect as many smaller impacts are covered in vegetation. Another cause, this time it stops the formation of craters in the first place, is our atmosphere. Many impactors fail to make it to the surface and are burnt up as they hurtle through it. Most meteors are an example of wanabe crater builders failing in their mission and burning up as they go. One very obvious cause is the fact that our earth’s surface is 70% water and impacts into an ocean environment, while devastating with the subsequent massive tsunamis etc, mean that the resulting crater if one is formed is under the ocean surface and hidden from view. By far the most final of all causes is plate tectonics. Where the surface crust is recycled at subduction zones. Craters on continental crust that experience this are propelled under overlaying crust and are forced into the mantle
where they are melted and form an indistinguishable part of the make up of the planet.

14) Many large impact craters have peaks in their centres. How do you think these central peaks
formed?
The kinetic energy in a large impact is truly vast and orders of magnitude greater than the initial resistance of the impacted rock. It ceases to have any strength and behaves like a fluid. At the moment of impact the main
pit of the crater is excavated. Almost instantly the the material on the inside of the crater walls begins to slump and move down towards the centre of the depression. As it comes together at the centre it has no other
place to go but up and the ground rebounds forming a central mass that solidifies into the central peak. A graphic illustration of this happening is in the image on the previous page of a water droplet into water where the central rebound can be clearly seen. All that is missing from this example is a method of instantly freezing the example to make it permanent. The image on the left is of perhaps one of the youngest of the Moon’s carters - Tycho with its central peak easily seen in the low attitude sunlight .

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