Deposits, Sequence of Events, and the Question of Distal Ash Fallout from the AD 1362 Rhyolitic eruption of Öræfajökull, S.E. Iceland

The June AD 1362 rhyolitic eruption products of Öræfajökull, Iceland, are described from 12 proximal locations around the ice-free south side of the volcano. An initial explosive phase produced fine-grained phreatomagmatic fall layers (A). This was followed by a second, variable phreatomagmatic to dry phase of activity that deposited ash and fine lapilli beds (B), interspersed with some minor pyroclastic flow activity. One notable but locally distributed fine ash bed with accretionary lapilli was deposited south of the vent about this time. Its origin may be attributable to co-ignimbrite ash clouds associated with some of these early flows. This activity was followed by the climactic Plinian phase, the deposit of which (C) is overlain by one or two pyroclastic flow units (D). These are found in a limited area but probably had a more extensive original distribution. Deposition of the Plinian pumice- lapilli fall unit south of the volcano lasted for a minimum of 2 hours, but the whole eruption sequence probably lasted for several days. We estimate that a ~ 28 km high Plinian eruption column was generated during the phase that produced fall unit C. The magma volume of the 1362 eruption is difficult to accurately constrain because of the amount of deposits laid down over the ocean but was probably between 2 and 3 km3. Dispersal of fall units A-C in the proximal to medial area studied is to the S and SE, in contrast to the more easterly dispersal of the fines-dominated distal ash fall across Eastern Iceland, defined in the earlier classic study by Sigurdur Thorarinsson. The distal ash fall deposit was not studied in this work but its presence, carefully mapped by Thorarinsson (1958), presents a question. What is the origin of this ash layer with a different dispersal pattern to that of the main pumice lapilli fallout unit? Other Plinian eruptions such as Huaynaputina 1600 may have had a similar associated widespread fallout blanket with a dispersal different from the main lapilli deposit. Possible origins for the 1362 ash deposit, and for this phenomenon in general, will be discussed.