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Lava flow trail stop 6

Lava Tubes

Image from USGS Hawaii Volcano Observatory image gallery
A lava tube with a partially collapsed roof at Kilauea in Hawaii.

Hidden beneath Sunset Crater's thick volcanic crust are elongate caves called lava tubes. Lava tubes are remnants of the Bonito Lava Flow's plumbing system. These pipes first form while the lava is actively streaming downhill. The scorching, runny basalt lava cools and hardens quickest on the surface of the flow where it contacts air. The first solid rock forms plate-like sheets that are swept along like rafts on this swiftly-flowing stream of lava. As the surface continues to cool the plates will pile up and fuse together to form a kind of crusty roof over the gushing lava river below. The solid roof insulates the still-molten lava below from the cooling effects of the air. The long, straight tubes may extend miles from the vent where the lava emerges, emptying their molten contents far downstream.

Eventually the vent exhausts its lava supply or simply becomes plugged up. Lava already in the tube drains out at the down stream end, leaving an empty lava tube behind. Sometimes part of the thin, crusty roof collapses, and an entrance to the tube opens up as it did here at Sunset Crater.

If the lava is not able to drain completely, lava within the tube solidifies to form a flat floor. The last drips of molten rock often form lava 'stalactites' that decorate the ceiling of the drained tube.

Sunset Crater's lava tube is unusual because ice is found inside it year-round. Ice is rarely found year round in large lava tubes because large tubes generally have good air circulation that eventually melts any ice that accumulates there during the Winter. The lava tube you see here has a very narrow interior compared to other lava tubes. During the winter months, ice forms from water seeping through the porous, fractured basalt. Cold, heavy air settles into the lava tube, allowing the ice to form thick deposits along the walls and floor. Although much of the ice melts each year, basalt is such an effective insulator that some ice remains in the cave even through the summer months.

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