how can a asterisk/star like crack occur on a hard pyroclastic rock? Could it caused by a meteroite or by some other mechanism? You may see the shape of the rock on attached photo.
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1$\begingroup$ Possibly internal stress do to contracting on cooling? $\endgroup$– user824Mar 4, 2019 at 18:40
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$\begingroup$ Is there a fold close to the outcrop? I have seen similar structures at the "hinge" of a fold (the center part, I don't remember the english word for that) $\endgroup$– user12525Mar 4, 2019 at 18:52
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$\begingroup$ Cooling units show columnar jointing and the rock observed was also a volcanic rock, so thermal contraction can be expected, i think you catch the point. i'm now searching for the examples of thermal fracturing, if one shows a pattern asterisk like. There is no folding in the vicinity, so a sheath fold occurence can't be expected. $\endgroup$– Muharrem YavuzMar 4, 2019 at 21:01
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$\begingroup$ "Could it caused by a meteroite" - no. Either internal stresses during cooling, or something hit it when it wasn't completely welded yet. It is a pyroclastic setting after all with lots of stuff flying around. No need for imaginary meteorites.. $\endgroup$– GimelistMar 4, 2019 at 21:07
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3 Answers
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Radial fracture is fairly common. It can form from cooling, drying mud, chemical shrinkage, impact, or several other means. Since you are looking at volcanics you are probably looking at cooling shrinkage, The interior cools slower which means it shrinks more and that's how your get radial fractures. Fractures form perpendicular to whatever the cooling front shape is, a round shaped lava flow produces radial fractures.
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$\begingroup$ Rather than thermal contraction, I'm for concentric loading, because the surface isn't even. Its center is a hollow, shows collapsing. From center to the sides relief is rising concavely. Radial fractures may be fairly common in nature but there is no examples given in structural geology books, about thermal or impact related fracturing. $\endgroup$ Mar 6, 2019 at 9:46
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1$\begingroup$ @MuharremYavuz generally structural geology is about large scale features, You can find some mention of it here, nap.edu/read/2309/chapter/4 I have another paper I will add to the answer. $\endgroup$– JohnMar 6, 2019 at 15:01
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$\begingroup$ 'Rock Fractures and Fluid Flow', book will be definitely useful for whom will conduct an exploration for oil, gas, geothermal fluids etc. I like it, good authors & citations... For the case we have been discussing; this book have been mentioning columnar jointing, plumose structures, dike propogation... These are all can be found, at those structural geology books. But disappointingly, there is nothing about the radial fractures. $\endgroup$ Mar 6, 2019 at 22:20
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The radial pattern of the fracture is related to a blast (probably a dynamite) not an impact. The upper part of the rock had blown out by an internal force. So the surface seen on the photographs is not an impacted surface. It is the surface of the remainder part of the original rock. In the impact crater case you might see extensionally origined tangential fractures parallel to the impact crater not the radial cracks as seen on this case. Here is an link showing a similar deformation pattern origined by a blast. http://www.uoguelph.ca/geology/geol2250/glossary/HTML%20files/shattercone.html Who are interested in impact craters may want to visit the link below. http://www.impact-structures.com/ Thanks for your replies and interests.
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The features probably caused by a volcanic bomb (please see my comment above). Then one may able to learn from the shape of the deformation, the location of the eruption center.
So, how to determine the kinetic energy amount of the impact, from the dimensions of the deformation features ?
Edit: Though i gave the right answer in another post, I didn't delete this post to let anyone to see the other question in the solving process.