Glacier responses to tephra deposition are shown to be highly variable where wind-transported eruption plumes produce narrow distal fallout zones with steep lateral thickness gradients. Significant but short-lived advances of faster-flowing glaciers can be triggered by deposition from modest eruptions. The 1947 eruption of Hekla, south Iceland, covered nearby glaciers with variable thicknesses of tephra, causing dramatic spatial variations in ablation rate. Relative snowablation increased by an estimated >80% at the eastern margin of Eyjafjallajo¨kull, but decreased by >54% at the western margin. Relative ice ablation increased by only 4% in the eastbut decreased by >75% atthe western margin, only 15km distant.The effect on mass balance therefore depends on tephra distribution as well as on the nature of the glacier surface. On Gi´gjo¨kull, retardation of ablation was greatest at the terminus, associated with an anomalous advance of =328 m between 1947 and 1954. Other glaciers in the fallout zone show either no recognizable response, or accelerated retreat. Advances will be short-lived due to the rapid redistribution of supraglacial tephra in a maritime climate, and the potential for discriminating between volcanically and climatically forced advances in the glacio-geomorphological record is low.The long-term glaciological effect of volcanism is to create thin, low-albedo covers after reworking, which make conditions less favourable for glaciation.