Lava tube

During long-lived eruptions, lava flows tend to become "channeled" into a few main streams. Overflows of lava from these streams solidify quickly and plaster on to the channel walls, building natural levees or ramparts that allow the level of the lava to be raised. Lava streams that flow steadily in a confined channel for many hours to days may develop a solid crust or roof and thus change gradually into streams within lava tubes. Because the walls and roofs of such tubes are good thermal insulators, lava flowing through them can remain hot and fluid much longer than surface flows. Tube-fed lava can be transported for great distances from the eruption sites.^[1] Once the eruption ends, the remaining lava drains from the tube, leaving an empty cave. Lava tubes on Earth can be over 50 km long, so huge underground volumes are possible. ^[2]

Settlement

Main article: Volcanic cave settlement

Lava tubes are a proposed site for a long term settlement, providing shelter from radiation and space that can be converted into habitable areas. The disadvantages is the requirement to reinforce the ceiling so rocks do not fall from a Mars quake, and the cost of building underground rather than on the surface. A large freight elevator might be necessary, although some lava tubes open onto valleys and might be accessible from the side. Lava tubes on Mars can be quite large. There are photographs from orbit of openings in lava tubes that appear to be over 100m wide inside, and geological features, called riles, that appear to be collapsed lava tubes of up to 400m in width.