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

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.