Jólnir's formation is closely linked to that of neighboring volcanic island, Surtsey, which emerged in 1963. Volcanic eruptions occurred in much of the surrounding water, but newly formed land was subject to erosion and many small islands washed away. By 1966, Jólnir had broken the surface and a volcanic cone of mafic tephra developed. The volcanic cone reached above sea level, but even this landmass eventually gave way to the eroding ocean waves.