Þórðarhyrna is one of seven subglacial volcanos beneath the Vatnajokull

Vatnajökull

Vatnajökull is the largest glacier in Iceland. It is located in the south-east of the island, covering more than 8% of the country.-Size:With an area of 8,100 km², Vatnajökull is the largest ice cap in Europe by volume and the second largest in area Vatnajökull is the largest glacier in...

 glacier Iceland.

Eruptions

It last erupted in 1910 and prior to that in 1903

An eruption in 3550 BC +/- 500 years poured out 1,500,000,000 cubic meters of lava in the area south of Thordarhyrna (Bergvatnsarhraun.

While an eruption 1887 to 1889 eruption had a (VEI): 2

Geology

There is a mechanical interaction between Thordarhyrna and Grimsvötn

Grímsvötn

The Grímsvötn sub-glacial lakes and the volcano of the same name are in South-East Iceland. They are in the highlands of Iceland at the northwestern side of the Vatnajökull ice-cap. The lakes are at , at an elevation of...

, despite these volcanoes being comparatively far apart so the eruption from 1902 - 1904 was combined with an eruption from Grimsvötn and had a Volcanic Explosivity Index

Volcanic Explosivity Index

The Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....

 (VEI): 4

A fault runs (N.35°W) from Thordarhyrna towards Hamarinn, and separates two, markedly-different, tectonic regimes.