A lava tube is a natural, tunnel-like conduit formed by a flowing stream of molten lava. These geological features are common in volcanic regions worldwide, but Hawaii provides the ideal conditions for their large-scale formation. The Hawaiian Islands, built by shield volcanoes, are characterized by highly fluid basaltic lava known as Pahoehoe. This low-viscosity lava flows easily and rapidly, allowing an outer shell to solidify while the interior remains molten. The resulting empty tunnels, or lava caves, are unique terrestrial structures that reveal the underlying processes of volcanic land formation.
The Mechanics of Formation
The creation of a lava tube begins with the eruption of low-viscosity basaltic lava, which often flows away from the vent in open channels, much like a river. As the molten rock is exposed to the cooler surface air, the edges and the top of the flow begin to cool and solidify. This process forms a continuous, hard crust over the moving stream, which acts as a roof.
The solidified crust provides a layer of insulation, efficiently trapping the intense heat within the flowing lava stream below. This thermal protection allows the molten rock to remain hot and fluid, sometimes losing only about one degree Celsius per kilometer of travel. Insulated lava streams can travel significantly further from the vent than an exposed surface flow.
As the hot lava continues to flow, it can erode the ground beneath it through thermal erosion, which deepens and narrows the conduit’s cross-section. Once the source of the lava flow is cut off, the remaining molten lava inside the insulated channel drains away. This drainage leaves behind a hollow, cylindrical void beneath the hardened surface, which is the completed lava tube.
Internal Structure and Features
The interior of a mature lava tube reveals a variety of geological formations. The cross-section of the tunnel is typically circular or oval, resulting from uniform flow and thermal erosion. The walls often display distinct horizontal markings known as flow lines or lava benches, which indicate the different levels at which the lava surface flowed before subsiding.
The ceiling and walls of the tube can also feature formations called lavacicles. These are formed when molten lava drips or oozes from the ceiling and cools into various shapes. When these drips fall to the floor, they can build up small, cone-shaped formations known as drip stalagmites.
The floor of the tube is usually composed of smooth, ropy Pahoehoe lava, though sections may be covered in breakdown. Other structures include wall linings, which are thin layers of lava coating the sides. These linings mark repeated cycles of filling and draining.
Significance in the Hawaiian Landscape
Hawaii’s geology, particularly the shield volcanoes like Kīlauea and Mauna Loa, is uniquely suited to the formation of extensive lava tube systems. The low-viscosity Pahoehoe lava allows for the creation of some of the world’s longest cave systems, such as Kazumura Cave, which stretches over 40 miles on the slopes of Kīlauea. They play a role in the island’s hydrology.
Water that filters through the porous lava rock drips into the tubes, providing a valuable source of fresh water. This water was historically collected by Native Hawaiians using gourds. The isolated and dark conditions within the tubes also create unique ecosystems for specialized cave-dwelling organisms, known as troglobites.
Beyond their ecological and hydrological roles, lava tubes hold cultural significance in the Hawaiian islands. Ancient Hawaiians utilized the caves for shelter during storms and as places to store food, benefiting from the cool, stable internal temperatures. Importantly, many tubes served as sacred burial sites for the remains of chiefs, with these areas protected from entry today.