Volcanic caverns on Mars, believed to form as a result of fast-moving basaltic lava flows
Martian lava tubes are volcanic caverns on Mars that are believed to form as a result of fast-moving, basaltic lava flows associated with shield volcanism.[1] Lava tubes usually form when the external surface of the lava channels cools more quickly and forms a hardened crust over subsurface lava flows.[2] The flow eventually ceases and drains out of the tube, leaving a conduit-shaped void space which is usually several meters below the surface. Lava tubes are typically associated with extremely fluid pahoehoe lava.[3]Gravity on mars is about 38% that of Earth's, allowing Martian lava tubes to be much larger in comparison.
^Léveillé, Richard; Datta, Saugata (2010). "Lava tubes and basaltic caves as astrobiological targets on Earth and Mars: A review". Planetary and Space Science. 58 (4): 592–598. Bibcode:2010P&SS...58..592L. doi:10.1016/j.pss.2009.06.004.