Geochemistry, dispersal, volumes and chronology of Holocene from the Katlavolcanic silicic tephra layers system, Iceland

At least 12 silicic tephra layers (SILK tephras) erupted between ca. 6600 a nd ca. 1675 yr BP from the Katla volcanic system, have been identified in s outhern Iceland. In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic te phra, the SILK tephras form distinct and precise isochronous marker horizon s in a climatically sensitive location close to both the atmospheric and ma rine polar fronts. With one exception the SILK tephras have a narrow compos itional range, with SiO2 between 63 and 67%. Geochemically they are indisti nguishable from ocean transported pumice found on beaches in the North Atla ntic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO). Volumes of airborne SILK tephra range from 0.05 to 0.3 km(3). We present new isopach maps of the six large st layers and demonstrate that they originate within the Katla caldera. The apparently stable magma system conditions that produced the SILK tephras m ay have been established as a consequence of the eruption of the silicic co mponent of NAAZO (ca. 10.3 ka) and disrupted by another large-scale event, the tenth century AD Eldgja eruption (ca. 1 ka). Despite the current long r epose, silicic activity of this type may occur again in the future, present ing hitherto unknown hazards. Copyright (C) 2001 John Wiley & Sons, Ltd.