Why did a drinking glass shatter into fine powder in a circle?

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A 16 oz drinking glass half full of water shatters without any apparent help. The result is a mound of finely ground glass arranged neatly in a circle equal to the original circumference of the bottom of the drinking glass. Can physics explain how this happened? (No physics background here.)

Note: While I've heard that sound waves can shatter glass, wouldn't one suppose the glass would move about in random directions and break apart into various sizes. Is there some laboratory experiment that could shatter a drinking glass where the glass would be reduced to this fine mesh and also be contained within its original circumference?
 
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Anyone claiming this happened without a special machine to make it happen exactly
this way is lying.
 
Antiphon said:
Anyone claiming this happened without a special machine to make it happen exactly
this way is lying.
Yeah. There are lots of ways that a glass can shatter, and I suppose that safety-glass residue could be described as a powder compared to something like shards, but this particular scenario would appear to be impossible. Even if some extremely unlikely fluke of nature resulted in all of the glass falling inward to form a mound, the water that was in it would wash it away.