A shift in eruption mode of Hekla volcano,Iceland, 3000 years ago: two-coloured Hekla tephra series,characteristics, dispersal and age |
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| Authors: | GUDRÚN LARSEN Bryndís G Róbertsdóttir Bergrún A Óladóttir JÓN EIRÍKSSON |
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| Institution: | 1. Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland;2. National Energy Authority of Iceland, Reykjavík, Iceland |
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| Abstract: | Hekla volcano is a major producer of large, widespread silicic tephras. About 3000 years ago, the dominant eruption mode shifted from infrequent large (>1 km3) to more frequent moderate (<1 km3) eruptions. In the following two millennia ≥20 explosive silicic-to-intermediate eruptions occurred, and six or more basaltic. Three categories can be identified with dacite/andesite to basaltic andesite in the oldest eruptions through basaltic andesite to basalt in the youngest eruptions. Ten tephra layers of the first category have distinct field characteristics: a pale lower unit and a dark upper unit (two coloured or TC-layers). Colour separation is sharp indicating a stratified magma chamber origin. The lower unit is dominantly andesitic (61–63% SiO2), while the upper unit is basaltic andesite (53–57% SiO2). Volumes of the eight largest TC-layers range from 0.2 to 0.7 km3 as freshly fallen. Radiocarbon and soil accumulation rate dates constrain the TC-layers to between 3000 and 2200 years ago. Two of these (~2890 and ~2920 b2k) are likely to occur overseas. Low SiO2 in the last erupted tephra of the TC-layers is comparable to that of historical Hekla lavas, implying a final effusive phase. The Hekla edifice may, consequently, be younger than 3000 years. |
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| Keywords: | age Hekla tephra dispersal tephrochronology two-coloured tephra |
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