Tube-in-Tube
As a lava stream moves along the floor of an existing tube, it may form an insulating crust, and then drain out beneath, just as occurs on the surface to form lava tubes. In fact, small tubes such as these are also common on surface pahoehoe lava flows, as shown in the third photo. Sometimes these are referred to as surface tubes, and they are typically only a foot or two high.
AUTHOR: Dave Bunnell