The Eldey and Geirfuglasker volcanic systems together form the long Eldey volcanic system on the
Reykjanes Ridge. The erupted rocks are from the
tholeiitic magma series and
picrite basalt. Eldey, and the smaller
skerries of Eldeyjardrangur, Geirfugladrangur and Geirfuglasker are the tops of historic volcanoes in the system. The Mid-Atlantic Ridge becomes at the Bight transform fault near 56.5°N the Reykjanes Ridge, which extends north to Iceland's Reykjanes Peninsula at 63.87°N As it approaches Iceland, the ridge becomes shallower, with a thickening of the oceanic crust beneath as it approaches the mantle plume associated with the
Iceland hotspot. The Reykjanes Ridge has an average spreading rate of about /year. The latest confirmed eruption of the Eldey axial volcanic ridge occurred at its northern extremity in 1926, and it may have been active in 1970. A
tephra eruption northwest of Eldey dusted the western shore of the Reykjanes Peninsula of Iceland, which occurred in June 1879. Geirfuglasker erupted in 1879 and there were 3 eruptions near Eldey between 1211 and 1422. A small geothermal area is located at depth east of Eldey. ==The last of the great auks==