The causes of age dependent changes in the spectral response of lavas,Craters of the Moon,Idaho, U.S.A

The Craters of the Moon basaltic lavas erupted between about 15000 and 2000 years ago in short periods occurring about every 2000 years. On LANDSAT MSS imagery the appearance of pahoehoe flows with ‘Blue Dragon’ crusts, which has previously been used as an aid to mapping and correlation across the lava field, changes systematically with age. The overall brightness and the slope of the reflectance curve between band 4 and band 7 both increase with age. In situ measurements of the bidirectional reflectance factor (BRF) of natural rock surfaces in the area shows that this ageing of the remotely sensed spectral response is a combination of (1) a change in the spectral response of the rock surface due lo weathering, (2) growth of lichen and moss over the rock surface, both factors causing little further change after about 8000 years, and (3) settling of windblown sand and establishment of vascular plants (grasses and shrubs), which continues with age. The relative importance of the last factor is not directly assessable from ground based radiometer measurements; however both weathering and lichen produce similar, complementary changes in BRF which can account for much of the spectral evolution seen on MSS images.