Lava tubes from northern California's Lava Beds National Monument.

Lava tubes typically form within larger basalt flows. Once a sheet like lava flow covers an area and starts solidifying new lava erupted at the source starts to move through tubes that form within the lava flow. These remain open transmitting the freshly erupted lava to the lava front, bypassing the surrounding slightly older, cooler partially solidified lava flow material. This process of internal flow along interior tubes helps the flow spread much farther by delivering hot lava to the flow front area (instead of 'pushing' the previously erupting lava forward). Once the tube drains for the last time and all the flow material cools, they can leave a tube behind that people can enjoy (as witness the photos below).

Image of location of Lava Beds National Monument from USGS site http://3dparks.wr.usgs.gov/labe/ . Note all the volcanic features in this area of northern California, including Mount Shasta to the SW.

Map showing pattern and location of the lava tubes. Map taken from USGS Bulletin 1673 http://npshistory.com/publications/geology/bul/1673/contents.htm (there is a lot more detailed information and an array of photos in this report at this web site). Lava came to the surface near Mammoth Crater area in the south part, and then flowed northward down the regional slope (in places encountering and flowing into a lake that used to exist in this area).

Looking down flow direction of a solidified lava tube. The solidified remnants of the last lava to move through can be seen on the floor and small benches on the side mark the higher height of some previous flows (like bath rings), while the drips on the ceiling indicate that at times the whole tube was filled with moving lava.

Left: UNO geology prof Dr. Shuster and students crawling along a tube. Right: details of lava drip texture on the walls. The basaltic lava was quite fluid (of low viscosity).

Close up image of the lava drips that ornament the ceiling.

Swimming upstream in the tube.

Lava tube squeeze where the roof meets the floor.

Collapse entrance to the tube up on the surface of the flow. The surface cools and crystallizes first forming a crust. Continued movement then breaks up that crust, which becomes quite rough.