Helgafell, a little-eroded basaltic hyaloclastite ridge in southwest Iceland, formed in a single eruption under a Pleistocene ice sheet. The ice thickness at the eruption site was at least 500 m and it was probably located some 15 km from the glacier�s snout. The eruption created a 2 km long, 0.8 km wide and initially 300 m high ridge. Gravity modelling indicates that Helgafell has a bulk density of 1800 kg m3, and that Holocene lavas around it are 40�80 m thick. This confirms that Helgafell is predominantly made of hyaloclastite, with pillow lavas and intrusions only making up a few percent of the total volume. The southeast side is made of unsorted eruption-fed hyaloclastites considered to have been piled up against an ice wall, while moderately to well-sorted watertransported material is found on the northwest side. Glacier flow and meltwater drainage was towards the northwest. The absence of basal pillow lavas suggests that magma fragmentation occurred from the onset of the eruption until its end. The lithofacies preserved indicate a fully subglacial eruption, although a final subaerial eruptive phase may have taken place through an ice chimney. The volatile contents (H2O: 0.26�0.37 wt.%) of several glass samples from the southeast side of the mountain indicate water pressures of ~1 MPa throughout the eruption. Efficient syn-eruptive drainage of meltwater coupled with rapid ice subsidence probably led to partial dynamic support of the ice, causing water pressure in the vault to be much lower than the static load of the overlying ice. Observed lack of correlation between elevation and volatile content may be a consequence of gradual reduction in dynamic support as the eruption rate declined and the edifice grew higher. Helgafell demonstrates that explosive activity may occur under ~500 m thick ice, and it may be an analogue to the ridge formed in the Gja´lp eruption in 1996.