It is a paired active central volcano with Grímsvötn, and can be classified as part of the Grímsvötn-Laki volcanic system, with common fissure swarms to the south.

[4] Thordarhyrna central volcano is 15 km (9.3 mi) in diameter,[5] and located north-east of the Síðujökull outlet glacier in a line that goes through the Háabunga ice cap feature of Vatnajokull on the way to the Grímsvötn caldera.

[4][1] It is to the south-east of a higher ice covered ridge connecting it with Grímsvötn called Háabunga which reaches a height of 1,742 m (5,715 ft).

[13] There is both a mechanical interaction encouraging dyke propagation between Thordarhyrna,[14] a stratovolcano,[15] and Grímsvötn, despite these volcanoes being relatively far apart,[16] and a close chemical affinity in their lavas.

[5] However samples from the nunataks poking through the ice have yielded basaltic andesite, dacite or rhyolite specimens consistent with more mature lavas.