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?
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.