From feeder dykes to scoria cones: the tectonically controlled plumbing system of the Rauðhólar volcanic chain, Northern Volcanic Zone, Iceland

Abstract

The Rauðholar volcanic chain, located in the Northern Volcanic Zone of Iceland, has been variably eroded such that, in the northern part, the original scoria cones are preserved, while the central and southern parts expose their shallow feeders. The chain thus offers insight into the inner workings of the near-surface feeder system of scoria cones. The volcanic chain was mapped in 3D using GPS. The en echelon-arranged volcanic chain can be divided into three parts: The southernmost part contains only plugs and necks with a thin pyroclastic cover as well as multi-tiered lava flows. The central part combines partially eroded scoria cones, (feeder) dyke intersections, and welded scoria interbedded within rootless and clastogenic lava flows; the welded scoria is composed of different kinds of lithics and bombs. The northern part preserves almost intact, overlapping scoria cones with voluminous lapilli-sized scoriaceous deposits. The overall dyke trend is orthogonal but shows radial patterns in individual cone complexes. Feeder dykes observed to depths of about 200 m below the volcanic chain are up to 8 m thick and flare in to conduits in the uppermost 20–50 m. The exposed shallow plumbing system shows that magma pathways through the volcanic edifice are very complex with incremental, repeated intrusions. We interpret the arcuate shape to be the result of a local change in the orientation of the stress field because the Rauðholar volcanic chain is located within a major relay structure between volcanoes on the eastern Fremrinamur rift arm and a rift extension with grabens on the western periphery.