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Rootless tephra stratigraphy and emplacement processes

Rootless tephra stratigraphy and emplacement processes

Title: Rootless tephra stratigraphy and emplacement processes
Author: Hamilton, Christopher W.
Fitch, Erin P.
Fagents, Sarah A.
Thordarson, Thorvaldur   orcid.org/0000-0003-4011-7185
Date: 2017-01
Language: English
University/Institute: Háskóli Íslands
University of Iceland
School: Verkfræði- og náttúruvísindasvið (HÍ)
School of Engineering and Natural Sciences (UI)
Department: Jarðvísindadeild (HÍ)
Faculty of Earth Sciences (UI)
Series: Bulletin of Volcanology;79(1)
ISSN: 0258-8900
1432-0819 (eISSN)
DOI: 10.1007/s00445-016-1086-4
Subject: Volcanic rootless cones; Pseudocraters; Lava; Water; Phreatomagmatic; Iceland; Gervigígar; Hraun; Gjóska; Vatn
URI: https://hdl.handle.net/20.500.11815/616

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Hamilton, C. W., Fitch, E. P., Fagents, S. A., & Thordarson, T. (2017). Rootless tephra stratigraphy and emplacement processes. Bulletin of Volcanology, 79(1), 11. doi:10.1007/s00445-016-1086-4


Volcanic rootless cones are the products of thermohydraulic explosions involving rapid heat transfer from active lava (fuel) to external sources of water (coolant). Rootless eruptions are attributed to molten fuel–coolant interactions (MFCIs), but previous studies have not performed systematic investigations of rootless tephrostratigraphy and grain-size distributions to establish a baseline for evaluating relationships between environmental factors, MFCI efficiency, fragmentation, and patterns of tephra dispersal. This study examines a 13.55-m-thick vertical section through an archetypal rootless tephra sequence, which includes a rhythmic succession of 28 bed pairs. Each bed pair is interpreted to be the result of a discrete explosion cycle, with fine-grained basal material emplaced dominantly as tephra fall during an energetic opening phase, followed by the deposition of coarser-grained material mainly as ballistic ejecta during a weaker coda phase. Nine additional layers are interleaved throughout the stratigraphy and are interpreted to be dilute pyroclastic density current (PDC) deposits. Overall, the stratigraphy divides into four units: unit 1 contains the largest number of sediment-rich PDC deposits, units 2 and 3 are dominated by a rhythmic succession of bed pairs, and unit 4 includes welded layers. This pattern is consistent with a general decrease in MFCI efficiency due to the depletion of locally available coolant (i.e., groundwater or wet sediments). Changing conduit/vent geometries, mixing conditions, coolant and melt temperatures, and/or coolant impurities may also have affected MFCI efficiency, but the rhythmic nature of the bed pairs implies a periodic explosion process, which can be explained by temporary increases in the water-to-lava mass ratio during cycles of groundwater recharge.


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