Dynamics, stratigraphy and proximal dispersal of supraglacial tephra during the ice-confined 2004 eruption at Grimsvotn Volcano, Iceland

The basaltic, phreatomagmatic eruption of Grimsvotn volcano, Iceland, in November 2004 (G2004) lasted for 5 days, during which time two separate vents were active. Significant deposition of tephra occurred in the first 45 h only. We have subdivided the deposit into seven units (A-G) on the basis of differences in texture, grain size and componentry, and the presence of sharp contacts between the layers. The distribution of tephra lobes was used to infer the vent of origin for each unit. The G2004 deposit is poorly sorted overall and consists of non-vesicular to highly vesicular juvenile components. Units A and B comprise almost exclusively non- to poorly vesicular glass fragments, whereas units C-G contain at least 30 vol.% highly vesicular pumice. The proportion of non-juvenile fragments increases significantly in the final unit (unit F) of the main phase; non-juvenile fragments are restricted to the coarse (> 0 I broken vertical bar) fraction of the deposit. Main phase units C and E account for 80% of the total deposit volume, including the entire distal portion, and are interpreted to represent a mixture of (1) a widely dispersed component that fell from the upper margins of a strongly inclined (45A degrees), 6-10 km high plume and (2) a locally dispersed (< 3 km from source) component originating from pyroclastic density currents and minor tephra jets.