Mt. Trolladyngja is a hyaloclastite mountain. The shield Volcano Trolladyngja 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.
Situated in the Odadahraun lava field, Trolladyngja is the highest of the Icelandic 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. It also contains major central volcanoes to the south and north. Its lava have run from there to create various lava fields such as Afstapahraun. There is great thermal activity within this area.
It is about 10 kilometers in diameter and its inclination is 4 to 5° in the lower slopes, but 6 to 8° at higher elevations. Its oblong crater is about 1,200 to 1,500 meters in length, 500 meters broad, and about 100 meters 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.