EROSION BY FLOWING LAVA - FIELD EVIDENCE

Erosion of substrate materials by melting or mechanical means has been suggested in active lava flows on Earth and other planets. Although t here are many references to lava erosion on Earth, unambiguous evidenc e is rare; geological relationships commonly cited as evidence of down cutting by lava can be explained without recourse to erosion. In order to assess possible erosion by flowing lava we carried out field studi es of tube-fed basalt flows, sheet flows of the Columbia River Basalt Province (CRB), and Precambrian komatiites. Unequivocal evidence for t hermal erosion (melted dacite substrate) was found at the Cave Basalt lava tube, Mount St. Helens, for which fluid dynamic analysis indicate s laminar flow, although erosion was enhanced in areas of locally stee p slopes, possibly as a result of localized turbulence. Other lava tub es 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 eros ion is hampered by limited and disrupted exposures. No evidence for th ermal erosion was found in the CRB. Our findings suggest that an erosi onal origin for planetary sinuous rilles and canali would be favored b y high Reynolds number flows (high mass flux, low-viscosity lava, stee p slopes) and substrates having a lower melting temperature than the l ava or low mechanical strength (e.g., regolith).