Sveifluháls () is a mafic hyaloclastite ridge of 397 m height in the southwest of Iceland in Gullbringusýsla (Reykjanes Peninsula). It is part of Krýsuvík volcanic system and of the protected area Reykjanes Fólkvangur. Geography The mountain is situated between lake Kleifarvatn and the valley Móhálsadalur . In some parts of the ridge, there are geothermal areas, esp. at Krýsuvík and Seltún. It is located at about 5 km from the southern coast of Reykjanes Peninsula. The ridge actually consists of two parallel narrow ridge segments with a 0,5 km wide valley in between. Sveifluháls is also called Austurháls in contrast to its parallel ridge on the other side of Móhálsadalur, the Núpshlídarháls ridge, which is also called Vesturháls . Road 42 passes by directly at the foot of the mountain between the same and lake Kleifarvatn. Geology Sveifluháls is a basaltic hyaloclastite ridge, actually a "ca. 22 km long tindar complex". It consists as is usual for such subglacially formed volcanoes, of pillow lavas under a layer of phreatomagmatic tephra (hyaloclastite). Pedersen and her coworkers think that the Ice Age glacier which was covering it at time of formation had a thickness between 70 and 400 m. Formation of Sveifluháls Mercurio postulates that the ridge formation started with eruptions under a 450 – 600 m thick ice sheet between 42.000 – 12,400 years BP. The melting of glacier ice induced the formation of one or more subglacial meltwater lakes. Dropping overburden pressures lead to the eruption of vitric phreatomagmatic tuff. Over time this tuff accumulated and formed circular tuff cones, elongated tuff cones and ridges, volcanic edifices that were arranged "along 60 semi-parallel individual linear segments" (i.e. eruption fissures – in this case subglacial ones) "that vary in length between 0,25 and 1,5 km." In the following time, these edifices grew and developed further by subaqueous density currents and resuspense resp. settling of the tuff within the meltwater lakes. This was facilitated by over steepening or retreat of the ice walls around the growing volcano. Igneous Intrusions into the piles of tuff also helped destabilizing them. The tuff cones and ridges grew and merged, but they also enclosed a source of jökulhlaup in between them, inter-ridge meltwater catchments with volumes of up to 15 000 000 m3. In the whole, the eruptions of Sveifluháls may have produced 17.25 km3 of meltwater and a DRE of 2,0 km3. In the