HR: 08:36h
AN: V31D-04    [Abstracts]
TI: Heat Loss From Skylights in Lava Tube Systems
AU: * Witter, J B
EM: witter@higp.hawaii.edu
AF: University of Hawaii - SOEST Hawaii Institute of Geophysics and Planetology, 1680 East West Road POST 504, Honolulu, HI 96822 United States
AU: Harris, A J
EM: harris@higp.hawaii.edu
AF: University of Hawaii - SOEST Hawaii Institute of Geophysics and Planetology, 1680 East West Road POST 504, Honolulu, HI 96822 United States
AB: Skylights serve as windows into the lava distribution systems that comprise tube-fed lava flow fields. They can thus provide insights into the thermo-rheological dynamics and flow stability in such feeder systems. Skylights form when a portion of the roof of a lava tube collapses, exposing the lava flowing in the tube to the atmosphere. The radiative and convective heat loss through skylights is likely significant and has not been adequately quantified previously. The loss of radiant heat through a skylight is relatively simple to constrain and results in a transient drop in the surface temperature of the molten lava exposed at the skylight. Given a typical lava flow surface T of ~1080 $\deg$C, we calculate Q$_{rad}$ of 1.71 x 10$^{5}$ W/m$^{2}$ [Q$_{rad}$ = $\sigma$ $\epsilon$ T$^{4}$]. This gives total cooling of 0.02 - 0.07 $\deg$C over a 3 m long skylight-exposed length. This is equivalent to 5 - 22 $\deg$C/km which compares to a typical cooling rate of $\sim$1 $\deg$C/km for the tubed section. A thin crust of cooled lava forms on the surface beneath the skylight which is then entrained back into the flowing lava causing a further (entrainment-related) heat loss. Convective heat loss from skylights is more complicated [Q$_{conv}$ = h$_{c}$(T$_{lava}$ - T$_{air}$)]. Our measurements show that air can flow in or out of a skylight. This represents a balance whereby hot, buoyant air blows out of a lava tube to be replaced by an influx of cold air that is sucked into a tube. Variations in the flow of air in and out of skylights may be caused by: 1) atmospheric pressure variations (cf. breathing cave systems), 2) convective instabilities that form in the tube-contained air, 3) changes in lava mass flux in a tube, increasing or decreasing the size of the headspace of air in the tube, or 4) transient combustion of hydrogen gas. We present temperature measurements of lava and gas made at skylights on the active flow field of Kilauea volcano using thermal infrared thermometers, thermocouples, and a thermal imager. From these data we calculate the radiative and convective heat losses from lava tubes through skylights and show how heat losses and flow rates vary with time. Our aim is to fully quantify the effects of this heat loss on the rheology and cooling rate of lava flowing in tubes.
DE: 8400 VOLCANOLOGY
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting