Geo-Joint: Lava Tubes—Hot Conduits and Cool Caving

The Hawaiian Islands are built of lava. Some of it is fresh and flowing, some of it is cooled, some eroded into pebbles and soil, but it all started as molten lava. The first thing a tourist learns, especially if they are visiting Hawai’i Volcanoes National Park on the Big Island, is that there are essentially two forms of lava there. One is drier and slower-moving, called a’a. And the other is the more watery, flowing sort, called pahoehoe. It tends to leave behind the more fantastical shapes of swirly ropes and vast evenly laid-out surfaces that look almost paved. It’s the latter kind that also can produce a most remarkable structure—a lava tube. Lava tubes are essentially lava caves, but they aren’t dug out of lava bodies by creatures, nor are they routes dissolved by groundwater over millennia.

A lava tube starts to form while a lava flow is ongoing. Lava running in a linear depression will start to cool along its edges as those parts are slowed by friction. The surface will also cool faster than the main body, especially if random chunks of cooled lava begin to collect in a sort of “lava jam.” If conditions are right, this cooling creates a roof over the flow, whlch acts as an insulator and allows the hotter lava to keep running, as if in a pipe. The still-hot flow may melt previous flows that it is passing over, cutting a deeper channel. Eventually, the runny flow may drain out as the eruption slows and stops, leaving behind a cave, or lava tube. Tubes may be covered by a lava roof from a few inches to tens of feet thick, depending upon later deposition of lava or soil. The size of the passageway can be big enough to drive a big-rig though, or so small they require a claustrophobic crawl to navigate. While the flow is still going, pressure from increased volume can burst a hole in the roof, or thin spots may simply cave in. These holes in the top of the lava tube are called skylights, and can provide access for explorers.

Lava tubes have long been attractions for people. Native Hawaiians used them for shelter and for spiritual and burial purposes. For this reason, they are considered off-limits, or kapu, in the Hawaiian tradition. Outsiders are less concerned about disturbing the spirits of the ancients, and have for years explored them. The Thurston Lava Tube, known by Hawaiians as Nahuku, in Hawai’i Volcanoes National Park, has been a major draw since its “discovery” by Lorrin Thurston in 1913. It is large and easily accessible, but at 600 feet in length, it is not an extensive tube. Spelunkers have found much longer and more complicated lava tube systems, some of which go on for miles. At present, the longest mapped lava tube is a 40-mile-long passage under the east side of the Big Island, called Kazumura Cave. It has some impressive dimensions besides length—it is nearly 70 feet across at its widest, and its maximum roof height measures 60 feet!

The record for lava tube length may soon pass to a relatively recently discovered cave called Kipuka Kanohina. The braided complex of passageways that make up Kanohina have only been partially mapped, and it has already been found to be quite extensive. Caves like this are known to the hardcore cavers and researchers who get down inside them, and some are public knowledge, but the locations of many are kept as close secrets. The general public has proven themselves to be irresponsible stewards of these natural wonders. Thurston Lava Tube once had many “stalactites” on its ceiling, where lava dripped down and made long icicle-like structures, but too many people wanted a souvenir, and now those features are all gone. That was long ago, but the urge to collect pretty lava formations, leave graffiti and trash, or just mindlessly damage the fragile setting, are still all too present in today’s casual adventure-seekers.

There are, however, some lava tubes that may be well beyond the destructive little fingers of even the worst vandals on Earth….because they’re on the moon! Satellites making flyovers of the moon’s surface have sent back images that show open holes. More tellingly, other satellite instruments that measure gravity anomalies find that the areas under these holes appear to be large void spaces—the reduced mass shows itself as a weaker gravitational signal. Moon analysts theorize that the holes are skylights, similar to those in lava tubes on Earth. But these don’t appear to be human-sized or even train-sized tubes. By calculating the amount of mass that seems to be missing, they think the space inside these voids could house an entire city—a city the size of Philadelphia! The empty spaces are thought to be as much as a mile wide and perhaps 60 miles long. These large spaces are the only ones currently detectable. With stronger instruments, and radar, smaller but still useful locations could be pinpointed.

This is more than a curious astronomical quirk. If these places are as envisioned, and can be entered safely, they may one day serve as long-term housing for extraterrestial colonies. Conditions on the moon’s surface are harsh, given the lack of atmosphere. The temperature swings wildly depending upon sun or shade. Harmful radiation of other kinds is unfiltered. Micrometeorites fall and easily impact the surface—or an astronaut. A living space protected by a rock roof could protect against these hazards and make exended habitation on the moon more feasible. While such a location might seem dangerously fragile given the glassy nature of lava, gravity on the moon is much less than that on Earth, so there is less weight stress on such shell-like structures there than would be here. The stability that affords makes habitation inside these caves a more realistic option. These caves will only be of use, however, if they can be found near the polar craters which are known to contain permanently frozen water, another must-have for lunar inhabitants. So far, the evidence for tubes comes from areas distant from the poles.

So lava tubes have served as spiritual sanctuaries, tourist attractions, adventure playgrounds, and could possibly be key to those living on the moon in the study of and preparation for human colonization of distant planets. Not a bad list of accomplishments for a simple hole in the ground made by hot rocks.

caption: Pahoehoe lava flows in a more liquid way and leaves a smoother surface.

caption: A gaping skylight affords a view of a lava tube in the making.