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In the Galle crater, there is a mountain range that is shaped like a circle segment that makes it look like a smiley face. How did this mountain range form? Is it a central ring that formed with the crater that then got partially eroded away? Or did it form after the crater? Or something else?

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Rapidly!

Galle is a large (230km) crater. In the processes that occur in the aftermath of a massive impact, deep rocks can be pushed up. Often this creates a central peak, but it can also form a ring structure, as in this lunar crater:

enter image description here
Schrödinger, a large (312km) crater, near the south lunar pole.

The Chicxulub crater also has a similar ring structure. These complex crater structures form very rapidly, in a matter of minutes, as a result of the flow of rocks. At the pressures and forces which occur in an impact, rock flows like a liquid.

The modelling of complex craters is difficult (it is hard to do an experiment!) but one reasonable model is that the ring forms as a wave generated by the collapse and terracing of the crater walls meets a second wave generated from the centre, as a central peak (which had previous formed) collapses.

The rings, as in the case of Schrodinger, may be incomplete, or reduced to just an arc and a couple of isolated peaks. : )

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  • $\begingroup$ So there were two rings, and the inner one was reduced to an arc from erosion? $\endgroup$
    – usernumber
    Dec 31, 2021 at 14:43
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    $\begingroup$ or the inner ring only partially formed, perhaps due to differences in the underlying rocks. The process of crater formation is incompletely understood. $\endgroup$
    – James K
    Dec 31, 2021 at 15:27

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