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Vent geometry and eruption conditions of the mixed rhyolite-basalt Námshraun lava flow, Iceland.

Research output: Contribution to Journal/MagazineJournal articlepeer-review

<mark>Journal publication date</mark>30/07/2007
<mark>Journal</mark>Journal of Volcanology and Geothermal Research
Issue number3
Number of pages15
Pages (from-to)127-141
Publication StatusPublished
<mark>Original language</mark>English


We describe the morphology and circumstances of eruption of the mixed rhyolite–basalt lava flow Námshraun in the Torfajökull–Vei∂ivötn area of central Iceland. The unusual location and exposure of the elongate fissure vent permits its length along strike (a total of 275 m) and the width of the dyke feeding it (up to 10 m) to be estimated in the field. Using analyses of the heat losses during the rise of the mixed magma through the shallow part of its conduit system, we are able to refine the absolute minimum dyke width estimate to 1.5 m. The lengths of the two main lava flow lobes, assuming that their advance was cooling-limited, imply that the volume effusion rate of the lava varied between 2.7 and 1.5 m3 s− 1 as different parts of the fissure became active. Prior to its emergence at the surface the magma had at most a small yield strength (probably significantly less than 3000–4000 Pa) and a near-Newtonian viscosity in the range 1 × 104 to 5 × 106 Pa s. After its eruption, the lava formed flows with marginal levées whose sizes imply a yield strength just less than 30 kPa. The lava in the central channels between the levées can be modeled either as a Newtonian fluid with a viscosity of between 3 × 107 and 6 × 107 Pa s or as a Bingham plastic. Estimates of the plastic viscosity from the two main flow lobes (< 104 to 6 × 105 and 1.2 × 107 to 1.8 × 107 Pa s) differ by a very large factor (at least 30) and are regarded as unreliable; however, they lead to a much smaller range of apparent viscosities, from 1.5 × 107 to 5.5 × 107 Pa s, values very similar to the viscosities found when the rheology is assumed to be Newtonian. If the field estimate of the dyke width is reliable, these results imply that the viscosity (and yield strength) of the magma averaged over the path from its source to the surface had increased by a factor close to 10 by the time that it emerged from the vent; alternatively the feeder dike may have been almost twice as wide during the eruption and relaxed to the presently exposed width as the eruption ended. The typical advance speeds of the two main flow lobes were less than 4 mm s− 1 and their emplacement times were 2.5 and 5 days. The implications for the sizes of the conduits feeding other rhyolitic and mixed lavas in central Iceland are discussed.