How do lava tubes form during volcanic eruptions?

Lava tubes form primarily in highly fluid, basaltic lava, such as pahoehoe, through a two-stage process.12

The Formation Process

1. Channeling and Crusting: As lava flows downslope from a volcano, its sides congeal, confining the flow into a narrow, river-like channel. Eventually, the surface of this lava river crusts over—often aided by solid blocks of lava—creating a roof that insulates the molten material beneath. This creates a roughly cylindrical, subterranean conduit.13
2. Draining: Once the lava supply from the vent is cut off, the remaining fluid lava within the conduit continues to flow downslope and drains out. This leaves behind a hollow, tunnel-like cavity.13

During this process, high temperatures maintained by volcanic gases can fuse the conduit walls into a black glaze. As the tube drains, features such as lava stalactites, stalagmites, and ropey floor patterns may form as the final dregs of lava solidify. Over time, segments of the roof may collapse, creating "skylights" or sinkholes that provide access to the cave.134