Jeløya is situated in the southeastern part of the Permo-Carboniferous Oslo graben. The oldest tectonomagmatic activity in this area was movements along the major N-S trending fault east of Jeløya.
This was followed by alternating volcanism and movement along NNW-SSE trending faults. The
volcanism resulted in a 800-1500 m thick sequence of basaltic rocks (B₁) belonging to the earliest eruptions
in the Oslo rift (starting about 300 Ma ago). These basalts are overlain by rhomb-porphyry lavas. The
basaltic sequence in northern Jeløya is dominated by pyroclastic rocks with a few lava flows, further
south by lavas, mainly alkali basalts to trachybasalts. All the lavas have a somewhat evolved character.
The basaltic rocks may be divided into three groups: (1) TiO₂-rich basalts originating from a chemically
distinct parent magma, (2) moderately TiO₂-rich lavas which crystallized under relatively reducing
conditions and show a trend from silica-undersaturation towards silica-oversaturation, and (3) moderately
TiO₂-rich basalts which crystallized under more oxidizing conditions and show no clear tendency for increasing silica activity during crystallization. The three rock groups probably represent three
contemporaneous eruption centers. One of these was situated just east of the northernmost tip of Jeløya,
and gave rise to the pyroclastic rocks and lavas of series (3) in the northern and south central parts of
the istand. Another, which gave rise to TiO₂-rich basalts, lay west of Jeløya.