The weathering of Hawaiian basalts in arid and semiarid environments is accompanied by changes in their thermal infrared emittance spectra. The spectral The differences appear to be related to the degree of development, preservation, and alteration of glassy crusts; the oxidation of iron; and the accretion of silica-rich surface veneers. Because the measurements are quantitative and in image format, they are useful for estimating relative ages in geologic mapping of lava flow. In Hawaii this technique is most diagnostic for distinguishing among sparsely vegetated flow less than 1.5 ka in age. ¿ American Geophysical Union 1988