Tubular lava stalactites are often found in lava tubes. Field observations, sample analysis, and comparative studies indicate that these are segregations extruded during cooling from partially crystallized lava al about 1,070 - 1,000 °C. Retrograde boiling (gas pressure) within the lava provides a mechanism to expel the interstitial liquid. In addition to tubular lava stalactites, a variety of other lava features can also result, such as lava helictites, lava coralloids, barnacle-like stretched lava, runners, runner channels, and some lava blisters and squeeze-ups.
Allred, Kevin and Carlene Allred.
The origin of tubular lava stalactites and other related forms.
International Journal of Speleology,
Available at: http://scholarcommons.usf.edu/ijs/vol27/iss1/14