It is characterised by a large outlet glacier, Hoffellsjökull, and gabbro rock, which originally formed deep in the Earth's crust but is now visible due to uplift of the area and glacial erosion.

Mostly consisting of tholeiite basalt with minor amounts of hyaloclastite and rhyolite, the total thickness of the strata from this volcano is estimated to be 2,700 m. There are indications that Geitafell was a high mountain, possibly ice-capped even before the Ice Age.

Gabbro from a quarry at Geitafellstangi, on the edge of Geitafellsbjörg, has been used as cladding for the Central Bank of Iceland building in Reykjavík.

In December 1910, the Hoffell farmer, Guðmundur Jónsson, was looking for sheep in Hoffellsdalur valley when he found a large number of Iceland spar fragments in a gorge on the mountainside.

Along with a Reykjavík merchant, he started mining and exporting Iceland spar to countries like Denmark and Germany, where it was used in microscopes and other optical instruments.

Silver saxifrage grows in rock fissures and occurs only in areas of Tertiary basalt lavas, which are mostly located in the east of Iceland, but also in the west and northwest.

The walrus teeth have been dated at 7,000 years old, indicating that at the end of the last glaciation, the sea extended into a fjord where Hoffellsjökull lies now.