Grænadyngja is a steep hyaloclastite mountain west of the Sog-geosite. It has the same geolocical features as Trölladyngja mountain. It is a geothermal site with volcanic fissures. From the fissures there have been various lava flows towards the west of the peninsula, including Afstapahraun (aa type) close to Reykjanesbraut (road nr. 41) to the Keflavik International Airport.
Grænadyngja is popular for hiking. The best way to aproach the mountain is by car, exiting road nr. 41 on the crossroads towards Mt. Keilir and road nr. 420.