LAVA TUBES: THE VIEW FROM THE OUTSIDE LOOKING IN

Lava tubes are long, sinuous cave systems that are common features of basaltic lava flows found on Earth and other basaltic planets and moons in our Solar System. Lava tubes form in pahoehoe, ‘a‘a, inflated sheet flows, pillow basalt, and even Archean komatiites. Molten lava can be transported great distances with little loss of heat inside highly insulated lava tubes. Depending on the velocity of the lava, temperature drops may vary from 1°C per km in Hawaiian flows to as little as 1°C per 10 km in huge flood basalts. The most common method of lava tube formation is roofing-over of active lava channels. Lava tubes also form in inflated sheet flows on flat ground (<1°) by congealing of slower moving lava at the edges of the molten core of the flow. Tubes within inflated flows, including flood basalts, commonly remained filled and can be recognized by concentric layering in eroded flows. The widths of primary lava tubes on Earth vary from a few meters to about 50 meters, dimensions that appear closely related to effusion rate. Potential counterparts on Mars and the Moon may be up to 1000 m wide, suggesting either much higher effusion rates or greater stability of crust due to lower gravity.

Immature lava tubes have widths greater than heights, whereas, mature tube systems are equidimensional or have heights greater than widths. The partially molten floor of an active tube may be >50% melt and is thermally eroded by the overlying lava stream. Repeated observations in Hawaii show that the lava surface gradually drops several meters after 1-2 months due to thermal erosion. The walls and roof of the lava tubes are also riddled with interconnected vugs created by draining of partial melts (up to 30%).

One of the great paradoxes of lava tubes is that while the walls are melting, features such as stalactites and wall drips are crystallizing. Most solidification is due to crusts of metallic magnesioferrite forming by high oxygen fugacity reactions on the exposed surfaces on the roof and walls of the lava tube. The rigid outer magnesioferrite skin buckles and folds as the partially molten walls slump and drip, creating many of the wild textures found in lava tubes. Lava soda straws, driblet stalagmites, and floor volcanoes commonly form late due to depressurization of volatile-rich partial melts during draining of the tube system.