Existing comment:
You can see some collapsed lava tubes here. Lava tubes form when lava is flowing like a river; the portion exposed to the atmosphere cools and becomes a hard shell while the lava inside continues to flow. Eventually, the lava inside either hardens or flows out. If the latter, a tube (more like a straw) remains. Depending on the size of the flow, the tube might be small (like these) or large enough to form caves. Those are next. |