ABSTRACT
A second major cave formation mechanism involves melting and resolidification of materials, as in the case of lava tube caves. Though the volcanically active eras of bodies such as Mars or the Moon are long past, very large lava tubes (3-10 times sizes typical for Earth) appear to be the norm due to the lesser gravity of those bodies (Bunnell, 1991) and possibly to their magmatic composition (Keszthelyi, 1995). The morphological evidence of lava tubes is easily seen in lunar images as strings of collapse features near the tube ends, for example at the western end of Mare Imbrium. Features on the flanks of Martian volcanoes like Olympus Mons also appear as lava tube strings radiating from central volcanic sources just as we see them on Earth (see Figure). Tidal flexure volcanism on Io may have produced lava tubes (Peale, Cassen & Reynolds, 1979). The history of Venus is also dominated by massive, global volcanism of unrivalled proportions (Sakimoto, Crisp & Baloga, 1997) and Venus may prove to have lava tubes (Zimbelman, 1998). On bodies with gravity less than Earth’s, less lava flow trenching erosion will occur; thus formation of some lava tube types that we find on our planet (see Volcanic Caves) may be rare on bodies like the Moon and Mars.
