lava tube

Basaltic, low viscosity lava would form lava tube caves.

Lava tube caves form when low viscosity lava flows beneath the hardened surface of lava flow while the volcano is active. Then when the volcano is dormant or extinct, it leaves cave like channels.

The solidified lava forming the walls of the lava tube will act to insulate the molten lava. It will therefore cool more slowly than lava at the surface and so will have a lower viscosity (it will be more runny) and so will flow faster than lava at the surface.

A lava tube cave is a natural tunnel formed by the flow of lava, creating a channel for molten lava to move through during a volcanic eruption. When the lava flow stops, the tube can cool, leaving behind a cave-like structure with unique geological features.