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

摘要

We describe the morphology and circumstances of eruption of the mixed rhyolite-basalt lava flow Namshraun in the Torfajoekull-Vei∂ivoem 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 m~3 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 x 10~4 to ～5 x 10~6 Pa s. After its eruption, the lava formed flows with marginal levees whose sizes imply a yield strength just less than 30 kPa. The lava in the central channels between the levees can be modeled either as a Newtonian fluid with a viscosity of between 3 x 10~7 and 6 x 10~7 Pa s or as a Bingham plastic. Estimates of the plastic viscosity from the two main flow lobes ( < 10~4 to ～6 x 10~5 and 1.2 x 10~7 to 1.8 x 10~7 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 x 10~7 to ～5.5 x 10~7 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.