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Central peaks: elastic rebound or isostasy?

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#1 kraterkid

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Posted 26 March 2014 - 08:52 PM

When reading a Wikipedia article on Complex craters, I noted it states that the central uplift is not due to elastic rebound but instead by a process of gravitational equilibrium (isostasy). Is this just another example of the inaccuracy of Wikipedia or is isostasy a good hypothesis for central uplift? Everything I have read previously stated that the central uplift responsible for central peaks is caused by elastic rebound. :question:

#2 azure1961p

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Posted 26 March 2014 - 10:06 PM

Isostasy is news to me and I have thought it to be elastic rebound. I'm wondering if gravitational equilibrium had its effect still - even though the rebound was the main cause. Id need to kno more about isostasy
before disagreeing with it but my own understanding was rebound.
Id have an easier time considering iso if these impacts didnt temporarily liquify under extreme heat and pressure. Realizing that there is a molten component then elasticity would seem to obvious to put aside but alas I'm not a lunar geologist. I'm open to new views.


Pete

#3 Bill F

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Posted 27 March 2014 - 03:25 PM

I always thought isostasy was the rather slow geological process of adjustment due to changing loads. e.g. as ice builds up in an ice age the greater load on the surface pushes the total column down into the softer mantle. Then as the ice melts it rebounds slowly. Scandinavia is still rising at a steady rate after the ice melted about 10,000 years ago.
Elastic rebound sounds more plausible.

Bill

#4 star drop

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Posted 27 March 2014 - 03:50 PM

Explanations from NASA and Lunar and Planetary Insitute.

#5 kraterkid

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Posted 27 March 2014 - 05:43 PM

Thanks for your responses Pete, Bill and Ted.

It appears that this is just another example of the inaccuracies of Wikipedia. The changes that occur due to isostasy are not the forces responsible for central peak formation. I do understand that elastic rebound results in central uplift and the creation of central peaks. I did not fully comprehend how isostasy works on the Moon until I read this Planetary Society article.

#6 star drop

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Posted 27 March 2014 - 07:59 PM

That is an excellent article. Thank you for the link, Richard.

#7 contrailmaker

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Posted 27 March 2014 - 11:55 PM

To me, Wikipedia is nothing more than *BLEEP* that people post on the internet. Why bother with it when it's just as easy to look up reliable sources?

CM

#8 kraterkid

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Posted 28 March 2014 - 11:25 AM

Even the most reliable sources can contain errors, and that includes scientific journals and websites. I think you have to start with a good source, and generally Wikipedia is not, but nothing takes the place of thoughtful analysis of any hypothesis. In other words, it's not just about looking up an answer, it involves questioning the explanation in light of your current understanding. What I learned by doing so made it all worth while, I have come away with a much better view of lunar isostasy and reinforced my understanding of central peak formation processes.

#9 Sarkikos

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Posted 28 March 2014 - 01:45 PM

Id have an easier time considering iso if these impacts didnt temporarily liquify under extreme heat and pressure. Realizing that there is a molten component then elasticity would seem to obvious to put aside but alas I'm not a lunar geologist. I'm open to new views.


Pete


Yes, I always thought the central peaks were formed by rebound of the temporarily molten lunar material. Ever seen a time lapse photo of a drop falling onto a pool of water? It forms a "central peak."

time lapse water droplet

Mike

#10 azure1961p

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Posted 28 March 2014 - 09:31 PM

Why can't we have funky central peaks though that look like:

http://www.bmumford....rops/index.html

Pete

#11 Sarkikos

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Posted 29 March 2014 - 08:16 AM

Yes, a lunar peak with a parasol would be funky.

:grin:
Mike

#12 kraterkid

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Posted 29 March 2014 - 11:41 AM

Id have an easier time considering iso if these impacts didnt temporarily liquify under extreme heat and pressure. Realizing that there is a molten component then elasticity would seem to obvious to put aside but alas I'm not a lunar geologist. I'm open to new views.


Pete


Yes, I always thought the central peaks were formed by rebound of the temporarily molten lunar material. Ever seen a time lapse photo of a drop falling onto a pool of water? It forms a "central peak."

time lapse water droplet

Mike


Mike and Pete,

The rebound that creates central peaks is not a result of molten lunar material splashing upward in the center like a water drop, rather central peaks of complex craters form as a result of the uplift of deeper lunar rock, some times as much as 10-15 km deep, rebounding and pushing up through the floor of the crater. Although there will be impact melt, it does not contribute much to the process of central peak formation. While the water drop experiment demonstrates basic elastic rebound concepts, it does not fully explain what happens when the target body is composed of rock. It is my understanding that the time required for the uplift is on the order of several minutes, yet the splash upward and subsidence of a molten column would occur in a matter of seconds, too short a time period for the central peak to solidify.

#13 Sarkikos

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Posted 29 March 2014 - 11:49 AM

So comparison between central peak formation in lunar craters and a drop falling into water is an analogy with some factors in common, others not. Both demonstrate basic elastic rebound concepts. My layman's understanding is that under extreme force and stress, some solid materials can behave somewhat like liquid ones.

Mike

#14 kraterkid

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Posted 29 March 2014 - 12:53 PM

That's true Mike, for example, rock can flow even at relatively low temperatures under the forces of pressure at depth, behaving like a thick plastic that can intrude and fold.

In the tremendous energy that an impactor imparts to the target, the impactor is almost always totally vaporized and a portion of rock in the target is fully melted when the transient crater is formed. This material does get ejected upward but when it comes back down it flows in a liquid (molten) state for a while, forming the impact melt ponds that are seen in and immediately around the crater.

#15 Raginar

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Posted 05 April 2014 - 08:28 AM

We were just observing the moon for the first time in ages and had questions about this as well. Thanks for the links above, I thought I had the concept right.

#16 kraterkid

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Posted 06 April 2014 - 11:40 AM

Glad you liked the links Chris. My brother was looking through my telescope and he had the same questions, its cool to have some basic understanding of the processes that formed the features we observe, in my mind it makes it all that much more fascinating.






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