Erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on Earth and other planets. Although there are many references to lava erosion on Earth, unambiguous evidence is rare; geological relationships commonly cited as evidence of downcutting by lava can be explained without recourse to erosion. In order to assess possible erosion by flowing lava we carried out field studies of tube-fed basalt flows, sheet flows of the Columbia River Basalt Province (CRB), and Precambrian komatiites. Unequivocal evidence for thermal erosion (melted dacite substrate) was found at the Cave Basalt lava tube, Mount St. Helens, for which fluid dynamic analysis indicates laminar flow, although erosion was enhanced in areas of locally steep slopes, possibly as a result of localized turbulence. Other lava tubes in our study display strong, but inconclusive, evidence for erosion. Komatiite flows display good evidence for erosion of their substrate, possibly in a turbulent regime, but assessment of the extent of erosion is hampered by limited and disrupted exposures. No evidence for thermal erosion was found in the CRB. Our findings suggest that an erosional origin for planetary sinuous rilles and canali would be favored by high Reynolds number flows (high mass flux, low-viscosity lava, steep slopes) and substrates having a lower melting temperature than the lava or low mechanical strength (e.g., regolith). ¿ 1998 American Geophysical Union