Interactions between lava and snow/ice during the 2010 Fimmvörðuháls eruption, south-central Iceland

The 20 March–12 April basaltic effusive eruption at Fimmvörðuháls, southern Iceland, was an important opportunity to directly observe interactions between lava and snow/ice. The eruption site has local perennial snowfields and snow covered ice, and at the time of eruption it was covered with an additional ∼1–3 m of seasonal snow. Syn‐eruption observations of interactions between lava and snow/ice are grouped into four categories: (1) lava advancing directly on top of snow, (2) lava advancing on top of tephra‐covered snow, (3) snow/ice melting at lava flow margins, and (4) lava flowing beneath snow. Based on syn‐ and post‐eruption observations in 2010/11, we conclude that the features seen in the lava flow field show only limited and localized evidence for the influence of snow/ice presence during the eruption. Estimated melting rates from radiant and conductive heating at the flow fronts are too slow (on the order of 5 m/hr) to allow for complete melting of snow/ice ahead of the advancing lava flows, at least during periods of observed rapid lava advance rates (15–55 m/hr). Thus we conclude that during those periods, which largely established the aerial extent of the lava flow field, lava advanced on top of snow; that this likely was the predominant mode of lava emplacement for much of the eruption is supported by many syn‐eruption field observations. Examination of the lava flows subsequent to the eruption has so far only found subtle evidence for interactions between lava and snow/ice; for example, locally lava flows have fractured and are collapsing, or have developed marginal rubble aprons from melting of snow banks that were partly covered by lava flow margins. Key Points Lava flows mainly emplaced on top of snow Lava advance rates faster than snow melting rates Localized direct snow/ice‐lava contacts