Varnish, sediment, and rock controls on spectral reflectance of outcrops in arid regions

Field observations in the Meatiq Dome, Eastern Desert, Egypt, show that amphibolite weathers to form massive outcrops and dense desert pavements that are extensively varnished. Laboratory-based reflectance spectra of representative samples (covering 5 cm²) and spectra extracted from Landsat thematic mapper data (covering 2.25 x ∼10⁴m²) confirm that varnish dominates the reflectance properties of amphibolite outcrops. Field, laboratory, and Landsat data indicate that granite gneiss weathers to expose mainly bare rock, whereas granitic out-crops evolve to a mix of rock and locally generated sediment. Penetrative foliation in quartz phyllonite leads to production of slab-shaped clasts that cover the surface. The data show that mass wasting of the slabs results in a mix of rock, varnish, and sediment exposures. Outcrop reflectance properties for arid regions are thus dependent on both rock mineralogy and texture, because both parameters modulate bedrock weathering and erosion.