| Summary: | The stratigraphy of Esja is described and the chronology of the succession established by means of the geomagnetic time scale. The Esja volcanic succession is about 2.4km thick, and comprises olivine tholeiites (25%), tholeiites (68%), basaltic andesites (5%), icelandites and rhyolites (2%). Volcanism was active in the Esja region for just over one million years, and during this time span, .at least ten glaciations occurred in the region. The stratigraphic succession is therefore characterized by sequences of lava flows intercalated, at intervals, by thick subglacial hyaloclastite units. Two central volcanoes were active in the Esja region; the Kjalarnes volcano was active for about 0.6 million years and was succeeded after a short interval by the Stardalur volcano, which remained active for about 0.3 million years. Flood-basalt volcanism was concomitant with the central volcanism, and most of the olivine tholeiites are considered to have been erupted in fissures and shield volcanoes unrelated to the central volcanoes. Igneous activity apparently migrated eastwards with time, reflecting the westward crustal drift away from the active volcanic zone, which is a subaerial extension of the mid-Atlantic ridge. The volcanics are tilted and downfaulted towards the east. The irregular topography created by the glaciations in Esja repeatedly prevented lavas erupted in the active volcanic zone from spreading over the tectonically less active neighbour- hood, thus producing angular unconformities in the stratigraphic succession from which the tectonic history of the region can be read. The Esja evidence suggests that tectonic activity is chiefly restricted to the active volcanic zone, and that the crust becomes tectonically inactive soon after it has drifted away from the active zone. Intrusive activity in Esja can be divided into three phases. The oldest dykes in the region trend N 25°E and contemporaneous sheets dip towards the Kjalarnes peninsula, where the intrusive activity culminated in the formation of a multiple ...
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