Rock-avalanche-driven glacier advances in a volcanic mountain revealed by cosmogenic exposure dating

The Dyrfjöll ridge represents the remnant of the filling of a Neogene volcanic caldera. The volcanic succession consists of a basal ignimbrite overlain by basaltic hyaloclastites and capped by basaltic lavas. The ridge is truncated by the Dyr Pass (856 m a.s.l.), which formed when a major rock avalanche (RA), composed of hyaloclastites and lava blocks, collapsed from both flanks of the ridge. On the eastern side, the RA partly overrode the Dyrajökull glacier along the Jökuldalur valley. About 4 km downstream at the valley outlet, a well-preserved RA deposit remains, characterized by hummocky mounds of lava and hyaloclastite megablocks. Between this deposit and the present-day glacier fronts, six push-moraine complex ridges are preserved. The aim of this study is to establish the chronology of these glacier stages in Jökuldalur and to distinguish their different origins. To constrain the timing of deposition, 20 boulders were sampled for ³⁶Cl cosmogenic exposure dating from both the RA deposit and the moraine ridges. The main RA yielded an exposure age of 9.4 ± 1.1 ka. The four outermost glacial stages, represented mainly by hummocky moraine assemblages, yield ages ranging from 8.8 ± 0.9 to 6.2 ± 0.7 ka. The ages of the moraines are coeval with the Holocene Thermal Maximum, a period during which most Icelandic glaciers experienced substantial retreat or complete disappearance. The results emphasise the insulating effect of debris following RA emplacement, which appears to have promoted subsequent glacier re-advance despite prevailing peak Holocene warmth. Stage 5 yields an age of 4.0 ± 0.5 ka and is likely associated with a distinct RA event, whereas Stage 6 is younger than 0.13 ka.