Inside the Volcano (63°58’56.96″N 21°39’6.69″W) of the Þríhnúkagígur volcanic crater. The Þrihnúkar area is named after three peaks, that were formed in seperate volcanic eruptions and reaches an elevation of about 550 m. The three peaks are aligned and the youngest one, called Þríhnúkagígur crater, was formed in an eruption about 4,500 years ago. The Þríhnúkar area is located in the north-eastern part of an extensive system with eruptive fissures and faults, called the Brennisteinsfjöll (“Sulphur Mountains”) volcanic system, which is around 50 km long. As the fissures and faults in this region are part of the boundary system between the Eurasian and North-American tectonic plates, the chemical composition of the rocks in the Þríhnúkur area is basaltic, which is typical for rocks generated along the mid-oceanic ridge system. The cone of the Þríhnúkagígur reaches around 40 m compared to its surroundings and is build-up of spatter and scoria material. The entrance into the crater is through the drained conduit system of the scoria cone, where the magma flowed upwards during the eruption and then drained out. Alteration minerals can be seen in the cavities and the walls of Þríhnúkagígur, which where probably formed when the interior of the chamber was hot. These minerals are yellow iron sulphates and sulphur, whitish gypsum, and oxidized iron in the rock with reddish brown colour. Most of the minerals are dated to the time of formation of the chamber while the lava was degassing and cooling (Sæmundsson et al., 2015).
Sæmundsson, K., Sigmundsson, F., and Stefánsson Á. B. (2015). Geology of the Þríhnúkur area. Inside the Volcano.