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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
Author(s) -
LARSEN GUDRÚN,
Róbertsdóttir Bryndís G.,
Óladóttir Bergrún A.,
EIRÍKSSON JÓN
Publication year - 2019
Publication title -
journal of quaternary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.142
H-Index - 94
eISSN - 1099-1417
pISSN - 0267-8179
DOI - 10.1002/jqs.3164
Subject(s) - tephra , geology , andesite , silicic , basaltic andesite , dacite , basalt , volcano , magma , geochemistry , explosive eruption , peléan eruption , mafic , volcanic rock
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.