A lava tube is an underground tunnel made by very fluid lava. According to the US Geological Survey, this low-viscosity lava tends to flow in just a few main channels during a long-lived eruption. Over time, the moving lava develops a crust at its outer edges, where it is exposed to relatively cool air or water. It may even roof over, completely enclosing the ongoing flow. After the eruption ends and the last of the molten rock has drained out, a long, tunnel-like tube or cave is left.
♦ Hawaiian lava tube caves
Many lava tubes are just a few feet high and several yards long. Some, like Nahuku, or the Thurston lava cave, in Hawaii Volcanoes National Park, can be high enough in the middle to stand in and walk through, while others are huge.
There are several other lava tube caves besides Nahuku in Hawaii, including Kazumura Cave, one of the longest and deepest lava tubes in the world. It formed on the east flank of Kilauea volcano during an eruption that ended some 500 years ago. Today, Kilauea is erupting again, and its lava still flows through tubes but in a different direction. The old lava caves are at ambient temperature levels now, with some even supporting their own micro-climate and ecosystem, like Fern Cave in Lava Beds National Monument.
♦ Lava tube features
The iron-rich basalt walls of most lava caves all over the world often take on a rusty color when they are exposed to humid air and water. These inner walls also usually have “high-lava marks” that show where the lava flowed at different times. They can also appear knobby where hot pahoehoe lava once flowed very quickly and eroded the inside of the tube, while its gasses made the surface bubble, leaving splatters that stuck to open areas on the wall and roof that were above the flow.
These “lavacicles” resemble cave stalactites but form from a volcanic process rather than dripping ground water. They can be found in many places, including Mushpot Cave, one of at least 200 lava tubes in California’s Lava Beds National Monument that were left behind after eruptions of very fluid lava from Mammoth Crater, some 30,000 years ago.
Lava pillars are the “stalagmites” of lava caves. These pipe-like features once connected the bottom and the top of the old lava flow; their height tells geologists how deep the original flow of molten material was. The lava pillar in Manjang Cave, Korea, is part a UNESCO World Heritage site. The presence of water during the lava flow contributes to pillar formation.
Lava tube caves have long served as shelters for human beings, as evidenced by cave pictures drawn by native peoples in the 190,000-year-old Undara lava tube cave system in Queensland, Australia. Indeed, Kazumura Cave in Hawaii was first documented after one of its entrances was designated as a fallout shelter in 1966.
Most importantly, though, lava tubes preserve information that scientists could never obtain during an active eruption. This shows them in more detail how volcanoes work and so helps make all of us safer, even as it opens a doorway to understanding one of Nature’s great wonders.