A shift in eruption mode of Hekla volcano, Iceland, 3000 years ago: two‐coloured Hekla tephra series, characteristics, dispersal and age

• Published in: Journal of Quaternary Science
• Main Authors: LARSEN, GUDRÚN, Róbertsdóttir, Bryndís G., Óladóttir, Bergrún A., EIRÍKSSON, JÓN
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2019
• Subjects: Hekla; Iceland
• Online Access: http://dx.doi.org/10.1002/jqs.3164; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fjqs.3164; https://onlinelibrary.wiley.com/doi/pdf/10.1002/jqs.3164; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/jqs.3164

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 km 3 ) to more frequent moderate (<1 km 3 ) 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% SiO 2 ), while the upper unit is basaltic andesite (53–57% SiO 2 ). Volumes of the eight largest TC‐layers range from 0.2 to 0.7 km 3 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 SiO 2 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.