We document remarkably efficient thermo-mechanical erosion by a small-volume lava flow. Downcutting by a basaltic-andesite lava flow on the steep-sided Momotombo volcano, Nicaragua, occurred at 100 times the rate commonly reported for thermal erosion in lava flow fields, even though this flow was small-volume (0.02 km3) and effused at a low rate for <1 week. The lava flow incised into the pyroclastic substrate up to 30 m, with erosion depth controlled primarily by thermal reduction of substrate hardness. We show that incision depth decreases, approximately exponentially, with distance along the flow path, until erosion stopped and the flow became constructional. This transition occurs 650 m from the vent on a slope averaging a 32∘ incline. Results indicate that syn-eruptive erosion is an important morphological process on some steep-sided volcanoes that are predominantly composed of layered pyroclasts. Rapid erosion and incision increased flow run-out for the 1905 flow, which in turn directed the flow and run-out of the 2015 lava flow. Mapping and understanding these features is critical for improving lava flow hazard assessments and provides insight into the construction and growth of composite cones.