In a comment on [this question](https://outdoors.stackexchange.com/questions/17104/how-warm-does-it-need-to-be-for-bats-to-come-out-of-their-caves) the author said he saw some bats in a lava tube. I've never heard that term and would like to know what it means, and how it happens. 

I understand that lava flows from a volcano, and affects every type of terrain in its patch, including where it lands on the ground, which can be quite a distance from the site of the eruption. 

How does it create a tube-type formation with space inside? Is it hard enough to become something permanent, or is it a temporary structure that caves in at some point? Can it be any size, height or length to qualify as a lava tube, or is there a specific set of standards it must meet?