Shallow to emergent subaqueous basaltic explosive eruptions, here referred to as Surtseyan, are a source of pyroclastic material expelled from subaqueous vents and transferred to the water column and atmosphere. They can threaten coastal communities in different ways, and one of these is through the generation of pyroclastic density currents travelling over water once the eruption becomes subaerial; these may be preceded or accompanied by eruption-fed currents moving along the sea/lake floor. One approach for studying these eruptions is through analysis of their deposits, but these are commonly either inaccessible (under water) or poorly preserved by the time they are exposed subaerially. The site selected for this work, Pahvant Butte, UT, USA, avoids these limitations. The volcano was formed at 18,600 cal year B.P. in the giant Pleistocene Lake Bonneville, by an eruption initiated at a relatively shallow water depth (85 m). The water is now drained, and deposits of the edifice and of the ash sheet at medial distances on the old lake floor (medial-lake) are accessible and well-preserved. The ash-sheet deposits allow us to investigate dispersal of ash from a Surtseyan eruption, including the subaqueous phase of edifice-building, by comparing edifice deposits with ash-sheet deposits correlated using geochemistry and textural analysis. Results suggest that medial-deposited ash from Pahvant Butte travelled at least ~ 25 km from the vent in eruption-fed pyroclastic density currents moving along the lake floor. This transport began early in the eruption, apparently prior to the volcano’s subaerial emergence.