Explosive lava–water interactions I: architecture and emplacement chronology of volcanic rootless cone groups in the 1783–1784 Laki lava flow, Iceland |
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Authors: | Christopher W Hamilton Thorvaldur Thordarson Sarah A Fagents |
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Institution: | (1) Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i, 1680 East-West Road, Honolulu, HI 96822, USA;(2) School of Geosciences, University of Edinburgh, Edinburgh, Scotland, UK |
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Abstract: | To determine the relationships between rootless cone emplacement mechanisms, morphology, and spatial distribution, we mapped
the Hnúta and Hrossatungur groups of the 1783–1784 Laki lava flow in Iceland. We based our facies maps on Differential Global
Positioning System (DGPS) measurements, photogeological interpretations, and supporting field observations. The study area
covers 2.77 km2 and includes 2216 explosion sites. To establish the timing of rootless cone formation we incorporated tephrochronological
constraints from eighty-eight stratigraphic sections and determined that the Hnúta and Hrossatungur groups are composite structures
formed by the emplacement of six geographically and chronologically discrete domains. Rootless eruptions initiated in domain
1 on the first day of the Laki eruption (June 8, 1783) and lasted 1–2 days. The second episode of rootless activity began
in domain 2 on June 11 and lasted 1–3 days. The four domains of the Hrossatungur group dominantly formed after June 14 and
exhibit a complex emplacement sequence that reflects interactions between the Laki lava, contemporaneously emplaced rootless
cones, and an existing topographic ridge. In the study area, we identify three distinct rootless cone archetypes (i.e., recurring
morphological forms) that are related to tube-, channel-, and broad sheet lobe-fed eruptions. We assert that emplacement of
lava above compressible substrates (e.g., unconsolidated sediments) may trigger rootless eruptions by causing subsidence-induced flexure and failure of the basal crust,
thereby allowing molten lava (fuel) to come into direct contact with groundwater (coolant) and initiating analogs to explosive
molten fuel–coolant interactions (MFCIs). |
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