Please use this identifier to cite or link to this item: http://hdl.handle.net/1946/4375
The Skerin ridge on Eyjafjallajökull, south Iceland: Morphology and magma-ice interaction in an ice-confined silicic fissure eruption
Skerin ridge is a 4.5 km long and on average 100 m wide ridge, situated on a radial fissure trending NW from the summit caldera of the ice-capped central volcano Eyjafjallajökull, south Iceland. Results from previous studies on tephrochronological dating of deposits produced by jokulhlaups originating in the Skerin area indicate that the ridge was formed by an intraglacial (or within glacier) eruption in the 10th century. Recent glacial recession has exposed the Skerin ridge and thus enabled a detailed examination of the ridge structure and architecture, in particular features that indicate confinement and rapid cooling by meltwaters as expected for lava-ice interaction. In this study, the ridge and associated formations were mapped in detail, the petrographic and chemical characteristics documented and the thickness of ice surrounding the ridge measured with a mobile radio-echo sounder.
Skerin ridge is primarily constructed by trachytic lavas (volume ~0.043 km3) but also includes lesser mafic (basaltic icelandite/hawaiite) and intermediate (trachyandesite) tephra deposits (volume ~0.012 km3). Lithostratigraphy correlations indicate that the initial stage of the Skerin eruption was marked by a basaltic eruption creating a scoria cone on the NW end of the ice-free fissure with a phreatomagmatic eruption of same composition creating a tuff deposit on the SE ice-covered section of the fissure. At synchronous time an explosive eruption took place forming pumice deposits followed by an effusive eruption of trachyte lavas from a row of vents along the volcanic fissure. The lavas confined by wet cavities and canyons in the thin ice (~100 m) formed a marginal carapace of glassy columnar joints representing the area cooled by the meltwaters and a brecciated flow top being the subaerial expression of the flow as the lavas broke through the glacier. A massive microcrystalline flow interior with flow structures and fabrics formed within the crust typical for lavas cooled within an insulated environment. The eruption ended with a strombolian phase forming scoria deposits of intermediate composition on top of the trachyte lavas. Evidence is for at least one previous eruption episode to Skerin ridge of unknown age forming a glacio-confined lava flow field of basaltic composition north of the ridge. Petrographic examination shows that the erupted magma features numerous examples of disequilibrium textures confirming that magma mixing/mingling took place shortly before onset of eruption in which the silicic eruption appears to have been triggered by a mafic intrusion. The eruption products plot on the transitional line of the alkali versus silica diagram which is a characteristic trend for the south Iceland flank zone.
The products and related eruption styles at Skerin ridge provide important data for paleo-ice thicknesses as well as constraints on ice melting rates. Volume constrains on ice melting and the application of a simple modeling of meltwater generation gave meltwater volumes of 500-2500 m3/s for the initial phreatomagmatic phase (<2 hrs) and 10-100 m3/s for the effusive phase (5 to 50 days). Thus an eruption of this type although not large demands precaution by civil protection authorities since flooding may cause hazard for e.g. popular tourist routes and farmland by Markarfljót.