Author Topic: Monday 20 February 2012 - Exploding Boulders!  (Read 11631 times)

jules

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Monday 20 February 2012 - Exploding Boulders!
« on: February 20, 2012, 12:12:25 pm »
We are used to seeing boulder tracks on Moon Zoo and often come across (or actively go hunting for!) the boulder that caused them. Usually we find something like these large intact boulders having come to rest at the end of their tracks.

highlighted by placidstorm and kodemunkey

 Moon Zoo team member Dr Anthony Cook recently sent me this picture of two boulder tracks in Schiller crater:



In this case the boulders are far from intact and appear to have "exploded" at the end of their journeys. What might have caused these boulders to fracture and fragment? One theory Tony suggested was that due to being under tension the boulders might have fractured before they rolled down the slope and that the movement further weakened them. Then over time the extreme temperature variations between lunar day and night could have fragmented the weakened rocks resulting in the appearance we see in the image.

Tony said:
Quote
I'm a bit puzzled though why the one on the top left has rock debris so far away from the centre. The boulder that looks like a skull rock on the bottom right has debris a lot closer to it, that could simply be explained by bits falling off as one would expect from the explanation above.

An alternative theory is that the boulders did roll down the hill intact, but were of sufficient size, area and age to be impacted by later meteorites, and these high velocity impacts split the rocks into many pieces. However, as Tony points out, the chances of this happening to two large rocks next to each other seem a bit remote.

Here is the NAC image M109502471L and the LROC article "A Recent Journey."

In order to study this process in more detail we need more examples. So if you find any exploded (or partly exploded) boulders please post them on the forum here.
« Last Edit: February 20, 2012, 12:17:33 pm by jules »

kodemunkey

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #1 on: February 20, 2012, 12:26:14 pm »


I'll keep an eye out during my explorations.

jules

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #2 on: February 20, 2012, 12:34:17 pm »
 :D
Hoped you might!

Geoff

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #3 on: February 21, 2012, 10:38:07 am »
Great - a new project to work on! Similar features will be difficult to find as I don't remember seeing anything like these in my explorations.

jules

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #4 on: February 21, 2012, 10:44:14 am »
Nor me - that's what makes it so interesting though. Something very different to look for.

DAYO DAYO

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #5 on: March 01, 2012, 06:07:15 pm »
I wonder if the boulders from Tony are of the same rock type.  I suggest that they are very hard and not otherwise internally fractured.
I don't know if you recall the old mining method of fire setting. Baulks of timber are placed against the rock face and fired. Once the rock face is heated up cold water is poured over it and this results in the shattering of the rock due to expansion and contraction, and "Yes" it can be explosive and highly dangerous.  I think the process here is in some way similar except the rock has to be large enough not to heat all the way through when being cooked and  then it spalls off energetically due to stress from unequal heating.  It can only happen if the rock is otherwise competent and unable to accomodate the stress. At some point the rock becomes small enough and then becomes a  heat store and liable to cycles of hot and cold when it destructs through expansion and contraction. While the last  is a usual one in the geological processes on earth (usually as freeze/ thaw). I look to the first example as of being a natural fire setting process that is dynamic and the last somewhat passive  in comparison. I know firesetting is not quite the example here, but I try to point out the effects of heat /cold and maybe the effect of having a cold or hot boulder skin compared to the rest inside.
« Last Edit: March 01, 2012, 06:50:39 pm by DAYO DAYO »

jules

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #6 on: March 01, 2012, 07:40:24 pm »
Good point Dayo Dayo. It's certainly possible to find rocks of different composition in the same boulder field though how each type would behave in the lunar environment is for someone other than me to answer!

conflict01

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #7 on: March 09, 2012, 04:05:43 pm »
Wow...
Do you know if at any point during the baulders journey did they leave the ground at all or make contact with each other ??
Sorry if that is in anyway a dum question, but Im quite new to all this, and wish I had taken an interest a Lot sooner.
I dont recall actually doing any Astronomy at school, but I find it extremely fasinating now, Better late than never i guess ay ?! lol !!   ;)

kodemunkey

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #8 on: March 09, 2012, 04:33:16 pm »
If they made contact with each other, one or both of them would be smashed to bits, which i think i have seen somewhere on the moon before. I suppose it might be possible for one to skip a little as it falls down the side of a rille or a crater wall but not by much.

jules

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #9 on: March 09, 2012, 05:45:33 pm »
Welcome to Moon Zoo conflict01! It's never too late to learn - as many of us here are discovering! I doubt the boulders made contact as the 2 tracks are very separate. It would be good if we could find other examples as clear as these. Then we could retrace the journey and look for possible causes of the fragmentation. Keep you eyes peeled everybody! :o

kodemunkey

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jules

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #11 on: April 27, 2012, 10:01:22 pm »
Indeed! Are there any tracks associated with this one? Or did it explode by being hurled from somewhere else as opposed to rolling?

kodemunkey

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #12 on: April 27, 2012, 10:12:33 pm »
I think the square shaped rock has some tracks underneath it, otherwise, dunno.

ElisabethB

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #13 on: October 21, 2012, 09:52:18 pm »
Don't know if this one belongs in here !  :D
http://www.moonzoo.org/examine/AMZ4002iwx


[edited to fix bad link]
« Last Edit: October 22, 2012, 06:17:35 am by Geoff »

Geoff

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Re: Monday 20 February 2012 - Exploding Boulders!
« Reply #14 on: October 22, 2012, 06:30:32 am »
Good find Els! Can't see any tracks leading to the remains of the rock but they may have "weathered" away. Also the rock doesn't seem to fit in with the surrounding terrain so may well have come from elsewhere.
Here's a centred shot:


from NAC strip M109548636LE
« Last Edit: October 22, 2012, 06:32:33 am by Geoff »