Anatomy of a Lava Tube
Lava tubes form when basaltic lava drains beneath a solidified crust. The features inside record the lava's behaviour as the eruption ended — the surface levels are where lava once stood, the ceiling drips are where molten rock fell.
How tubes form
Low-viscosity basaltic lava flows on the surface. The surface chills and solidifies, insulating the molten interior. When the eruption ends, lava drains downhill, leaving an empty conduit. The walls are formed by the chilled lava margins.
Lavacicles
The lava-tube equivalent of stalactites — molten rock dripping from the hot ceiling before solidifying. Most are 10-30 cm long; some reach a metre. Pointed downward, narrow at the tip, broader at the base.
Lava benches and shelves
Horizontal ledges along tube walls record the level of flowing lava as the eruption waned. Multiple benches stack vertically where lava levels dropped in stages.
Skylights
Holes in the tube roof where the chilled crust failed or never formed. Skylights are often the natural cave entrances on visited lava tubes. They admit sunlight and surface vegetation.
Breakdown and collapse rooms
Where part of the tube ceiling collapses, the resulting room is larger than the original tube — the breakdown opens a chamber above the tube. Many lava tube visitor caves are mostly breakdown rooms.
Ages
Lava tubes form in geological instants compared to karst caves. Thurston Lava Tube on Hawaii's Big Island formed about 500 years ago; Raufarholshellir in Iceland formed about 5,200 years ago. Unlike karst caves, lava tubes do not become more elaborate over time — they simply weather.
Distribution
Where basaltic volcanoes occur: Hawaii, Iceland, Jeju Island (Korea), the Azores, Canary Islands, eastern Australia, Mexico. The Galapagos and Reunion have substantial lava-tube systems.
Visiting
Lava tubes are warm by cave standards (typical ambient surface temperature plus a few degrees). No constant temperature like karst caves. Helmet and headlamp required; tubes are dark and floors are often rough basalt.
Explore on the map
See lava tubes on the interactive map.