November 14, 1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Lava tubes cool slowly

If the flow of lava in a tube ceased, how long would it take for the tube to cool? Does the temperature in a newly emptied tube drop by an equal amount every day, or is the heat loss irregular? We have some answers from first-hand experience with the current eruption on Kilauea's east rift zone.

The tube system of episode 53 carried lava for 10 km (6 miles) from the vent to the sea. So efficient were the tubes in containing heat that lava cooled only 10 degrees Celsius (°C) across that distance. When it reached the ocean, the lava was still a sizzling 1140°C. For as long as lava flowed, the tubes glowed like yellow-hot ovens.

Episode 53 ended abruptly on January 30, 1997, leaving the tubes drained and empty within 24 hours. But the tubes remained so hot that, for several weeks after, they emanated an incandescent glow visible at night from the skylights located sporadically along the trace of each tube. Because of the glow, many residents and tourists thought lava was still coursing through the tubes.

By way of its color, incandescent rock gives a crude estimate of temperature. For example, orange-to-yellow colors are emitted when rocks (or melt) are hotter than about 900°C. Dark-to-bright cherry red is characteristic as material cools to 630°C. Faint red glow persists down to about 480°C. The tubes, which had been cooling at a fairly steady rate of 20-30°C per day, dropped to this temperature on February 17, nearly three weeks after they drained. The skylights no longer glowed at night.

Although they were cooling, the tubes were still as hot as ovens. For comparison, a pizza oven is operated at temperatures ranging from 260 to 315°C (500-600°F). By about March 1, the tubes had cooled through this range. (As a consequence, we quit carrying frozen pizza as part of our field gear.) The rate of cooling began to diminish as well, with temperature remaining nearly constant for several days in a row.

At the more moderate temperatures, rain was able to affect the rate of cooling. Periods of heavy rainfall led to abrupt cooling, as much as 50°C in a few hours. The downward-percolating rainwater washed heat from the tube into adjacent rocks or dissipated it as steam from cracks in the surrounding lava. On March 24, seven weeks after being drained of molten lava, the tube temperature finally fell below 100°C, the boiling point of water.

The tubes remain uncomfortably warm, about 60°C (140°F), even today. It will be months before cave-dwelling insects and spiders find a hospitable abode in our recently drained tubes.

Kilauea Eruption Status--November 14, 1997

The episode 53 tubes may be cooling remnants from days gone by, but episode 55 has been erupting vigorously since March 1997. The vent inside Pu`u `O`o crater periodically overflows, spilling lava only a short distance eastward where it fills the pond within the crater. Lava from the south shield, 300 m south of Pu`u `O`o, enters tubes and travels to the coast. It enters the ocean at Waha`ula and Kamokuna benches, at the eastern edge of Hawai`i Volcanoes National Park. Conditions at the coastal entries are unstable with frequent collapses resulting in explosive activity. Lava viewers are reminded that the area is extremely hazardous.

Recent Big Island Earthquakes

There were no earthquakes reported felt during the past week.