|A pahoehoe lava flow produced the lava tube on the left side of this card.
A lava tube forms when the lava on the outer surface of the flow cools much faster than the inside of the flow. The outside becomes cooled hardened lava rock while the inside stays molten and also keeps flowing. If something happens to stop the flowing lava there will be nothing to fill the void and a tube is the result.
Click on the "Next" button to learn more about what else comes out of a volcano.