Emissivity Changes in Basalt Cooling After Eruption From PU'U O'O, Kilauea, Hawaii
Abstract
Temperature measurements of lava lakes and active flows from erupting volcanoes are important because they indicate the degree of activity and may be used to estimate flow viscosity and effusion rates, and therefore to predict ultimate flow length and width. Temperatures are measured in situ with thermocouples or radiometers, or from aircraft or spacecraft with radiometers. Remote measurement allows rapid response to eruptions in inaccessible areas and therefore is a reasonable approach to global monitoring of volcanic activity and hazard. In measuring radiometric temperatures, there are two unknowns: (1) the kinetic temperature, and (2) the emissivity, which in order to solve for the temperature is generally assumed to be the same as for the cooled lava. Kinetic and radiometric temperatures, however, appear to differ substantially for lava; for example, during the 1984 eruption of Mauna Loa we measured apparent radiometric temperatures of ~750°C near the vent, rising to ~900°C 1 km downstream. In contrast, thermocouple measurements near the vent are ~1140°C. Explanations include: (1) absorption of emitted radiance by gases evolving from the lava near the vent (SO2, H2O, CO2); and (2) emissivity rising as the lava cools. Since our ground radiometry in 1984 was made looking downwind, absorption alone cannot explain the observations. To test possibility 2, we measured the emissivity of cooling lava from Pu'u'O'o (kinetic T = 1050-400°C) using paired broadband (1.25-15 um) radiometric measurements made under clear sky (R(T)) and in a hemispheric reflector (BB(T)) on the lava. Under the hemispheric reflector the lava emits as though in a cavity (blackbody radiation) and therefore emissivity=R/BB. We also made narrow-band radiometric measurements to test the sensitivity of the recovered emissivities to the spectral shape of the blackbody spectrum alone and for better comparison to laboratory emissivity spectra. Broadband emissivities appear to rise systematically as the lava cools, from 0.55 at 1050°C to 0.85 at <500°C, and <5% of that rise can be attributed to the methodology. The emissivities of metals are known to rise with temperature, and the emissivities of dieletrics are known to decrease, consistent with our findings. We conclude that the rise in emissivity as the lava cools is real. These results suggest that emissivity must be treated as a function of temperature if accurate lava temperatures are to be recovered for active flows. In addition, accurate recovery probably requires compositional classification of the erupting lava, an unsolved problem.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.V71A1263A
- Keywords:
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- 8429 Lava rheology and morphology;
- 8494 Instruments and techniques