On the slopes of Hawaii's Mauna Loa and Kilauea volcanoes, and in the lava flows of Iceland, Mexico, and various other volcanic regions, you can find them — hollow tunnels in the rock, often extending for hundreds of metres and sometimes for kilometres. The tunnels can be tall enough to walk through and wide enough for a small truck to pass through. Their walls and ceilings are smooth basalt, often with linear grooves that show the direction of flow. Sometimes the floor is uneven, sometimes flat. The tunnels can branch and rejoin, can have side rooms and skylights opening to the surface above, can be partly collapsed in places. They are lava tubes — natural conduits formed by the cooling of the surfaces of lava flows, where the still-molten interior continued to flow and eventually drained away, leaving the hardened roof and walls behind. Lava tubes are among the most interesting features of basaltic volcanic landscapes, and they have practical importance for archaeology, paleoclimate studies, and even for proposed human habitation of other worlds.
How They Form
Lava tubes form when a flow of basaltic lava begins to develop a crust on its top surface. The cooled surface forms an insulating layer that protects the lava beneath from losing heat to the atmosphere. The lava below the crust can continue to flow at high temperature, maintained by the insulation and the steady supply of new lava from upstream. As more lava is supplied to the system, the flow continues for longer than it would if it were exposed to the air. Eventually, the upstream lava supply stops or diminishes, the flow drains downstream, and the cooled outer crust remains as a hollow tube. The tube is often partially filled with cooled lava on its floor (where the last lava flowed before stopping); the walls and ceiling are the hardened crust of the original flow. The geometry depends on the size and rate of the original flow — small lava tubes are perhaps 1-2 metres in diameter, large ones can be 10+ metres tall and 30+ metres wide.
The Size Distribution
Lava tubes range in size from small features (centimetres in diameter, metres long) to enormous ones. Kazumura Cave in Hawaii is approximately 65 kilometres long and is the longest known lava tube in the world. The Lava Beds National Monument in California has tubes up to 5 kilometres long. The Hawaii Volcanoes National Park has dozens of tubes ranging from small to substantial. Iceland's Surtshellir cave is 1,950 metres long. Korea's Manjanggul Lava Tube on Jeju Island is 13 km long. The Naica Caves in Mexico are 8 km. The size depends on the original flow scale, the duration of the eruption, and various other factors. Larger eruptions producing larger flow systems generally produce larger lava tubes; smaller eruptions produce smaller ones.
The Hawaiian System
Hawaii is the type locality for understanding lava tubes. The Big Island has extensive lava tubes from the historical eruptions of Mauna Loa and Kilauea, and many of the most-visited tubes are easily accessible. The Thurston Lava Tube (Nahuku) in Hawaii Volcanoes National Park is one of the more visited single tubes — a 600-foot section that visitors can walk through, with the eruption that formed it dated to about 500 years ago. The Kaumana Caves near Hilo are a longer system. Kazumura Cave is the longest in the world but is technically challenging and not open for casual visitors. The slopes of the Big Island's volcanoes are dotted with lava tube entrances, sometimes with collapsed sections that reveal the tube's interior. The active Kilauea volcano continues to produce new lava tubes from each major eruption, including some that have been forming actively during the 21st century.
Internal Features
Lava tubes have characteristic internal features. The walls are typically smooth basalt with horizontal "high-flow" marks showing where the lava reached its peak height during the flow. The ceiling often has "lava drips" — frozen drops or strings of basalt where the cooling lava was pulled by surface tension into hanging shapes. The floor may be flat (where the final drained lava cooled in a flat layer) or may have channels, levees, and other flow features. "Skylights" are openings where the tube's roof has collapsed, exposing the interior to the surface. "Side passages" branch off the main tube where smaller flows joined or diverged from the main flow. "Lava stalactites" hang from the ceiling, formed by lava dripping from the cooling roof. "Lava stalagmites" rise from the floor where the dripping lava accumulated. The features can be quite ornate — the inside of a large lava tube can have the visual complexity of a limestone cave, though produced by very different processes.
Lava Tubes as Archaeology
Lava tubes have substantial archaeological importance. In Hawaii and Easter Island, lava tubes were used as burial sites — the dry, stable environment of the tubes preserved human remains for thousands of years. In Hawaii, royal burials were sometimes placed in well-hidden lava tubes; the locations are often still kept secret by traditional Hawaiians. In Iceland, the Surtshellir lava tube has Viking-era artifacts dating from the 9th century, when the Vikings used the tube as a hideout (and possibly for ritual purposes). The Mexican Sistema Sac Actun underwater caves (technically marine-flooded limestone karst rather than lava tubes, but with similar archaeological value) contain Maya artifacts dating from before the Mexican coastline was flooded by post-glacial sea level rise. Lava tubes in Korea's Jeju Island contain prehistoric materials. The cool, dry, stable conditions of lava tubes preserve organic and inorganic materials for very long periods.
Lava Tubes as Paleoclimate Archives
Lava tubes preserve indirect climate records that can be valuable for paleoclimate research. Speleothems — the cave decorations that form when seepage water deposits minerals — can grow in lava tubes (where seepage water from the surface deposits calcium carbonate or other minerals). These speleothems can be dated and analysed for stable isotopes to reconstruct past climate conditions. Lava tubes can also contain organic materials (bat guano, plant remains, animal remains) that record past biological conditions. Pollen preserved in cave sediments can reconstruct past vegetation. The Lava Beds National Monument in California has yielded important paleoclimate data from its speleothems and from organic remains in its tubes. The cool, dry conditions of lava tubes are excellent for preservation, so the records they contain are sometimes more complete than those available from surface deposits.
Lava Tubes as Habitat
Lava tubes provide unique habitat for cave-adapted organisms. Most lava tubes are too dry to support extensive cave fauna, but those with stable water and organic input can host distinctive ecosystems. The Hawaiian lava tubes contain endemic cave-dwelling spiders, beetles, crickets, and other invertebrates that have evolved in the dark, stable environment over millennia. Some Hawaiian cave-adapted species are eyeless — having lost their visual systems through evolution in the dark environment. The Wai Cave (Lapakahi Cave) on the Big Island has a particularly rich cave fauna. Smaller invertebrates including various spider species and beetle species are found in many of the larger Hawaiian lava tubes. The Lava Beds National Monument in California has cave-adapted bats and various invertebrates. Lava tube ecosystems are typically fragile because they depend on the stable cave environment, which can be disrupted by air movement (from visitor entry), water flow changes, or other perturbations.
Lunar and Martian Lava Tubes
One of the more exciting recent developments in lava tube science is the discovery of lava tubes on the Moon and Mars. The Moon was volcanically active billions of years ago, and the basaltic mare basalts of the lunar surface contain extensive lava tube systems. Detection of skylights — collapsed sections revealing tube interiors — has identified several major lunar lava tubes that are estimated to be hundreds of metres in diameter. Mars similarly has extensive lava tube systems, particularly in the Tharsis volcanic region. Both the lunar and Martian lava tubes have been proposed as potential sites for human habitation — they could provide natural shelter from radiation, micrometeorites, and temperature extremes, with internal volumes large enough to accommodate substantial bases. The reduced gravity on the Moon and Mars means that the underlying rocks could support much larger lava tubes than are stable on Earth. Lunar lava tubes might be as much as 1 kilometre in diameter. This has produced substantial interest in lava tube science for space exploration applications.
Conservation
Lava tubes are often protected as part of national parks and similar protected areas. Hawaii Volcanoes National Park, Lava Beds National Monument, Manjanggul on Jeju, Surtshellir in Iceland, and many other lava tubes are within protected areas. The protection is needed because lava tubes are fragile — visitor traffic can damage delicate features, alter cave climates, and disturb wildlife. Some major lava tubes are closed to public access entirely, allowing them to be preserved for scientific study without disturbance. Others are open with restrictions on lighting, group sizes, and other factors. The continued integrity of lava tube environments is one of the more interesting conservation challenges, balancing public access and scientific value against the need to preserve unique geological features.
The Continuing Role
New lava tubes are still forming in active volcanic regions. Hawaii's Kilauea has been producing new lava tubes essentially continuously during its current eruptive period (which has been ongoing since 1983 with various interruptions). Iceland's recent eruptions have produced new lava tubes. The 2021 Cumbre Vieja eruption on La Palma created new tube systems. Each new eruption that produces basaltic lava with high enough volume and steady enough flow can produce new lava tubes. The continued formation of new tubes is one of the persistent expressions of Earth's volcanism, a reminder that the geological processes that shape our planet are still active. The combination of new tubes being formed and old tubes being slowly modified by erosion and collapse produces the rich variety of lava tube features that exists across the world's basaltic volcanic landscapes.