The pyroclastic deposits of the 1875 eruption of Askja, Iceland

Abstract

The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md$ against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.