Remote sensing of the colour and temperature of volcanic lakes

Crater lakes on active volcanoes act as heat and chemical traps, and are amenable to surveillance from space. By use of all seven spectral bands, the Landsat Thematic Mapper (TM) can simultaneously measure: (i) lake surface temperature, (ii) lake surface area, and (iii) lake colour which is related to water chemistry. Use of the total instrument in this way enhances its utility for volcano surveillance. This work examines TM data for crater lakes at the following volcanoes: Ruapehu (New Zealand), Taal (Philippines), Kawah Ijen and Kelut (Indonesia), Poas (Costa Rica), and Apoyeque and Jiloa (Nicaragua). Observatory data indicate that lake surface temperatures derived by TM band 6 are typically 1-4°C less than contemporaneously measured bulk temperatures, probably due to the skin effect, the difference between water bulk and surface temperature. For Ruapehu, TM band 6 hotspots coincide approximately with known upwelling sites above volcanic vents on the lakebed. Field observations at Kawah Ijen show that the skin effect (< 3°C) is strongly correlated with windspeed: wind gusts peaking at ≈ 5m s^?1 caused rapid decreases in surface temperature of ≈ 0.5-1.0°C. These fluctuations are small compared with the magnitude of volcanogenic changes in lake temperature and do not reduce the utility of infrared surveillance. TM-derived water surface spectral reflectances indicate high concentrations of suspended chemical sediment in the most active crater lakes: Ruapehu, Poás and Ijen. For Ruapehu, imaged on two dates, the later scene reveals an upwelling slick, bright in bands 5 and 7, possibly composed of hollow sulphur spherules emitted from a subaqueous vent. The Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) and Enhanced Thematic Mapper (ETM +) both due for launch in 1998, will offer improved capabilities for remote surveillance of crater lakes.