Grænadyngja and Trolladyngja are steep hyaloclastite mountains west of the Sogin geosite. They are surrounded by young volcanic fissures, geothermal sites and beautiful colours. The two mountains are famous for various lava flows, including Afstapahraun close to the Keflavik International Airport main road.

Situated in the Odadahraun lava field, Trolladyngja is the highest of Iceland’s shield volcanoes, reaching a height of 1.468 meters (4.816 ft) above sea level, and rising almost 600 meters above the surrounding desert and lava fields.

Mt. Trolladyngja is a hyaloclastite mountain. The Trolladyngja shield volcano has two main hyaloclastite peaks, 393 and 374m high. Between this ridge and shield volcano Trolladyngja is the shield volcano Graenadyngja with the area Sog in between.

It also contains major central volcanoes to its south and north. Its lava flowed from there to create various lava fields such as Afstapahraun. There is still considerable thermal activity within this area.

It is about 10 km (6.2 miles) in diameter and its inclination is 4 to 5° in the lower slopes, but 6 to 8° at higher elevations. Additionally, its oblong crater is about 1.200 to 1.500 meters (3.397 to 4.921 feet) in length, 500 meters (1.640 feet) broad, and about 100 meters  (328 feet) deep.

Most of its lava fields have flowed in a northerly direction, with one branch of it reaching the valley of Bárðardalur, a distance of roughly 100 km (62 miles).

Trolladyngja and Graenadyngja are a part of the Reykjanes UNESCO GeoPark.