Deep and rapid thermo-mechanical erosion by a small-volume lava flow

4 We document remarkably efficient thermo-mechanical erosion by a small5 volume lava flow. Downcutting by a basaltic-andesite lava flow on the 6 steep-sided Momotombo volcano, Nicaragua, occurred at 100 times the 7 rate commonly reported for thermal erosion in lava flow fields, even though 8 this flow was small-volume (0.02 km) and effused at a low rate for <1 9 week. The erosion depth, up to 30 m incision, is explained by reduction of 10 hardness, H, of the pyroclastic substrate into which the lava flow incised. 11 We show that incision depth decreases, approximately exponentially, with 12 distance along the flow path, until erosion stopped and the flow became 13 constructional. This transition occurs 650 m from the vent on a slope 14 averaging a 32◦ incline. Results indicate that syn-eruptive erosion is an 15 important morphological process on some steep-sided volcanoes that are 16 predominantly composed of layered pyroclasts. Rapid erosion and incision 17 increased flow run-out for the 1905 flow, which in turn directed the flow 18 and run-out of the 2015 lava flow. Mapping and understanding these fea19 tures is critical for improving lava flow hazard assessments and provides 20 insight into the construction and growth of composite cones. 21 22