Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland

Abstract

Thríhnúkagígur Volcano, Iceland, is a composite spatter cone and lava field characteristic of basaltic fissure eruptions. Lava drainback at the end of the eruption left ~60 m of evacuated conduit, and a 4 × 104 m3 cave formed by the erosion of unconsolidated tephra by the feeder dike. Field relationships within the shallow plumbing system provide three-dimensional insight into conduit formation in fissure systems. Petrographic estimates and the relative volumes of the cave and erupted lavas both indicate xenolithic tephra comprises 5–10 % of the erupted volume, which cannot be reproduced by geochemical mixing models. Although crustal xenolith entrainment is not geochemically significant, we posit that this process may be common in the Icelandic crust. The Thríhnúkagígur eruption illustrates how pervasive, poorly consolidated tephra or hyaloclastite can act as a mechanically weak pre-existing structure that provides a preferential pathway for magma ascent and may influence vent location.

Near-surface magma flow instability drives cyclic lava fountaining at Fagradalsfjall, Iceland

“…3. Based on an estimated surface gas emission of 4 × 10 5 m 3 per fountaining event at atmospheric pressure (see Methods), then at 1-2 MPa, where we calculate gas separation to occur in the shallow cavity, the volume of the erupted gas is on the order of 20,000-40,000 m 3 , comparable with the estimated volume of the cavity at Thríhnúkagígur 41,42 . However, since according to our model the gas derived from foam collapse only drives the initial period of fountaining when maximum fountain heights are attained, the gas contained in the foam layer is only a fraction of the total erupted gas.…”

“…Due to the proximity of these vents, which are about 100 m apart, mechanical or thermal erosion of a particularly weak lithological unit may have created a shallow cavity. A possible analog for such a feature is the Thríhnúkagigur lava cave (SW Iceland), a ~60 m deep cylindrical conduit of ~8 m diameter that narrows with depth to a lungshape hollow with dimensions of ~80 m along-strike (of the associated fissure) and ~20 m across-strike 41,42 . If a similar cavity existed beneath Fagradalsfjall during the eruption, then we envisage flow between it and the vent to be regulated by a flared-nozzle, as shown in Fig.…”