Trolladyngja is a hyaloclastite mountain to the north of Ridge Nupshlidarhals.  Between this ridge and shield volcano Trolladyngja is the shield volcano Graenadyngja with the area Sog in-between.  Shield Volcano Trolladyngja has two main hyaloclastite peaks, 393 and 374 m. high.  It also contains major central volcanoes to the south and north.  Its lavas have run from there to create various lava fields such as Afstapahraun.  There is great thermal activity within this area.