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When lightning strikes the ground in a sandy location it may fuze sand grains together to form a mineraloid known as Fulgurite. Is there a physics explanation for the tubular shape? Why is it hollow as opposed to being solid?

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Off the top of my head, maybe it creates a molten ball of sand near the surface, which is hot enough to melt the grains below it and sink downwards due to gravity. I would be interested to know the direction of a fulgurite tube in a steep sand dune or hillside. –  TheSheepMan Aug 11 '11 at 16:36
    
@TheSheepMan if lightning creates a molten ball, how can Fulgurites branch? –  JoeHobbit Aug 17 '11 at 8:19
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A lightning bolt produces hot gasses (30,000 °C). This results in very high pressures. This pushes away surrounding material. When the material cools, the empty region where the current passed is left empty for the same physical reasons that a piece of wood retains the imprint of a nail. Some of the sand is melted and solidifies before it can flow back into the hole.

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Why would one have a long hollow tube vs. a crater with a glassified surface? –  TheSheepMan Aug 11 '11 at 16:51
    
What you're asking is equivalent to the question, "how come lightning is so skinny?" which would make a good question for Physics Stack Exchange (it's because parallel currents attract each other). The current keeps flowing deep into the ground. Eventually it reaches a low impedance region where it can spread out like the water table. –  Carl Brannen Aug 11 '11 at 17:17
    
Hmm, good explanation. –  TheSheepMan Aug 11 '11 at 19:28
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