# How does the central peak in moon craters form?

The central peak in many of the moon's large craters are visible with a telescope and they seem a little odd to me. Can someone explain how they form.

If you drop something into a pool of water, you will get a rebound effect in the middle where the object was dropped, and then waves will spread out around it. This rebound effect in the middle is the same phenomenon that causes central peaks in craters. The difference is just the scale: An impact that forms a >~15-km-diameter crater on the moon will cause the rock to act like the liquid to the point that you get the rebound effect and form a central peak.

Smaller craters on the moon will not have central peaks, and larger craters above ~120 km will form a peak-ring.

The transition diameter for these features -- a simple, bowl-shaped crater; a "complex" crater with a central peak; a peak-ring crater -- is inversely proportional to gravity. So, on Earth, the transition diameters are smaller -- you only need to get a ~3-4-km-diameter crater before you can form central peaks. On Mars, the transition diameter is around 6 km. To a lesser extent, target material strength will affect the transition diameter, as well.

But in the end, the central peaks are formed by rock rebounding, being pushed back up by the strength of the underlying rock after the initial impact event. Central peak formation happens within minutes of the impact itself, even in craters 10s-km across.

• Possibly like video of a ball dropped onto sand: youtube.com/watch?v=2An1mHRu2J4 – Stuart Woodward Nov 11 '11 at 22:21
• Hmm. That's not really a good model for the cratering process - at least not into rock. I could see that applying to ice, like on Enceladus or Europa. It's the effective explosion/vaporization of the projectile that creates the crater in the first place. Now, the splash-up in that movie is reasonable, sorta, though probably much higher than would happen in normal rock. – Stuart Robbins Nov 12 '11 at 0:24

You can actually simulate this yourself. Pour some flour on a sheet of paper, then drop a marble on it. Try different thicknesses for the flour, different marble sizes, different heights for the drop, etc.

• This is not actually going to form a central peak in any way. You'll form a crater, and it's neat, and I actually recommend doing this in a clear tub (like disposable containers you get in a 5-pack at the grocery for \$3) and use layers of different-colored things like flour, oreo crumbs, graham cracker crumbs, etc. But you will not get a central peak, and it's not a great analog because the marble will not be destroyed in the impact, which you need to do this. – Stuart Robbins Nov 10 '11 at 18:55
• @StuartRobbins but but but, in your own answer what is dropped in the water is not destroyed in the impact ( also in the picture linked by Andrew in the question's comments). Next time I bake a cake I will get a marble from my grandson and try the experiment with flour. – anna v Jun 29 '12 at 14:34
• @Florin Andrei: In your experiment you have no lithostatic pressure, which is the cause for the rebounding effect of the molten/liquid stone on a planets surface—and when something is dropped into water. Therefore you should not get a central peak in your experiment, unless you have a really big thickness of the flour and a high speed marble, but that wont fit on a sheet of paper. – erik Mar 13 '13 at 12:34
• @erik As Stuart mentions in his answer, the huge impact energy briefly causes the rock to behave like a liquid, it doesn't actually melt all the rock in the crater region (although some of the rock is melted, and even vaporized). – PM 2Ring Mar 29 '18 at 7:30