The geothermal area of Hengill hosts various types of hot springs. Fumaroles and solfataras are typical of the rift zone which lies across the geothermal area. Mt. Hengill (804 m) itself is an exception, but hot springs abound around it, except on the northwest side. Hot springs occurring on the marginal faults of the rift zone in the east of Hengill contain a fair amount of carbon dioxide, and tufa precipitates from the water around them.
A large tufa deposit occurs on a flat at the foot of Hengill at Ölfusvatnslaugar. It is unusual as geysers have been active there in the past. The pipe of the largest is about 1,5 m wide. The water is stirred by rising bubbles of gas, but the temperature is only about 70°C and the outflow is a mere trickle. A mound of tufa has built up from the orifice, extending downslope to the east for some 50 metres. It is undercut by a small brook on the south revealing over 1 m section of the calcareous sinter.
Upslope from the mineral springs there are fumaroles depositing sulphur. The carbon dioxide is of magmatic origin and has been expelled when intrusions solidify and then mixes with ground water. This occurs in high temperature geothermal systems towards their waning stages.
Kristján Sæmundsson, 2010