Especially in two areas, geothermal activity at Sveifluháls is intense and connected to Krysuvík volcanic system. These are Seltún on the one hand, and the hills behind the farm and school of Krýsuvík on the other hand (so called Hveradalir). Hot ground, fumaroles, hot springs and mud pots are to be found there. Measurements in Seltún even show a thermal output of about 4 MW. Hydrothermal alteration and this heat lead to precipitation of minerals like hematite, goethite and different sulfur compounds.