Permafrost aggradation caused by tephra accumulation over snow‐covered surfaces: examples from the Hekla‐2000 eruption in Iceland

The relationship between explosive volcanic eruptions and permafrost aggradation is discussed at a regional (Hekla volcano, Iceland, 63°59′N 19°40′W) and a global scale. During the most recent Hekla eruption in winter 2000, tephra buried the adjacent winter snow cover. Observations of denivation landforms, excavated sections and active‐layer thickness were carried out 15 to 77 months after the eruption. Results show that the combination of climate that favours permafrost and a thin layer of tephra is sufficient to reduce the sub‐tephra snow ablation substantially, possibly even to zero, causing aggradation of the surface and preserving massive ground ice and permafrost. Based on these observations, a conceptual model for permafrost aggradation related to tephra accumulation was developed. On a global scale, about one third of all active volcanoes seems to be located in permafrost‐favourable areas, suggesting that explosive volcanic eruptions may be significant for permafrost aggradation. Copyright © 2007 John Wiley & Sons, Ltd.