Thermal-infrared imaging of weathering and alteration changes on the surfaces of basalt flows, Hawai‘i, USA

Abstract

Basaltic pahoehoe lavas weather on time scales of decades to millennia and are chemically altered by volcanic heat and gases on time scales of days to decades following their eruption. Despite differences in reactions and rates, the end products, viewed spectrally in the thermal infrared (TIR: 8–12 μm), are similar and distinctive: initial rinds of quickly cooled, disordered glass with broad emissivity minima at 9–11 μm devitrify to yield a sharper emissivity band at 9.1 μm; opaline rinds with a similar emissivity feature accrete and gradually transition to cryptocrystalline quartz with an emissivity minimum near 8.1 μm. Ultimately, exposed surfaces are oxidized and hydrated to palagonite, with a loss of spectral contrast and character. We have collected over 600 pahoehoe samples of different ages from seven sites on Mauna Loa and the south coast of Hawai‘i in order to measure hemispheric emissivity spectra and compare them to airborne and spaceborne TIR Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) AST05 emissivity images of Hawai‘i. ASTER, with five 90 m TIR bands, has operated over the 11 year period since launch in late 1999. Images and samples from some of the Hawai‘ian sites were collected repeatedly over a decade or more to study the rate of spectral change there. Continuing eruptions of Pu‘u ‘O‘o since 1983 offered an opportunity to study the effects of a persistent dousing of lavas by acidic rain and air (‘vog’), and older flows from Mauna Loa offered an opportunity to study the effects of weathering away from active vents. Weathering changes and rates quantified spectrally are helpful in mapping and assessing long-term environments on volcanoes.