Late Quaternary terrestrial tephrochronology of Iceland—frequency of explosive eruptions, type and volume of tephra deposits

Abstract

AbstractClose to 100 silicic tephra layers have been identified in Icelandic terrestrial soils of Holocene age. The majority of these tephras were erupted at the Hekla, Torfajökull, Öræfajökull, Askja, Snæfellsjökull, Eyjafjallajökull and Katla central volcanoes. By far the most active is Hekla with close to 50 identified silicic tephra layers in ∼8000 years, showing an inverse relationship between eruption frequency and volume of erupted tephra. Volumes of uncompacted silicic tephra layers range from <0.01 km3 to >10 km3, with nearly 50% of known tephra volumes lying between 0.1 and 0.5 km3. Seven closely spaced Hekla eruptions in the period 950–550 BC with identical major element composition illustrate the need of stratigraphic control in short and long distance correlations. A 2600 year old tephra geochemically indistinguishable from the Borrobol tephra is a reminder that no tephra is geochemically unique. The major producers of basaltic tephra are the volcanic systems that are partly covered by ice, or partly lying within areas of high groundwater or extending offshore, i.e. the Grímsvötn, Katla, Veidivötn‐Bárdarbunga, Reykjanes, Kverkfjöll and Vestmannaeyjar volcanic systems. The best known eruption record is that of the Katla system with over 170 identified basaltic tephra layers and more than 300 estimated in 8400 years. The Grímsvötn system is currently the most active volcanic system with over 70 eruptions during the last 1100 years. Volumes of uncompacted basaltic tephra layers range from <0.01 km3 to >20 km3, the majority of known tephra volumes lying between 0.1 and 1 km3. The most voluminous basaltic tephra deposit, the ∼10 200 yr old Saksunarvatn tephra, may however represent more than one eruption on the Grímsvötn system. Deposition of approximately 800 basaltic tephra layers during the last 9000 14C years is estimated but many of those erupted from volcanoes within ice caps have not been preserved. Copyright © 2007 John Wiley & Sons, Ltd.