An extensive complex of igneous rocks is present along the eastern margin. The volcanic rocks overlie continental pre-opening rocks as a thin layer which thickens towards the continent-ocean boundary and merges with oceanic crust in the Norway Basin. The main geological units are:
A sequence of seward dipping reflectors interpreted as a pile of subaerial basaltic lava flows. The pile forms juxtaposed and partly overlapping wedge-shaped units, the inner and the outer wedge.
Complex of intrusives. A series of west-dipping reflector segments, interpreted as a dike swarm, crosscuts the stratification of the inner wedge as well as the underlying continental rocks. A number of short-horizontal reflectors, interpreted as sills, are found within the post-opening sediments.
Prior to the initial opening of the Norwegian-Greenland Sea 55 million years ago (mya), the Jan Mayen Ridge (JMR) was located in the western part of a regional sedimentary rift basin between Greenland and Norway. After the opening , the ridge gradually emerged as a separate structural entity. The geological development of the ridge may be divided into four main phases.
The eastern margin was formed as the continent broke up on the Greenland side of the regional sedimentary rift basin between Greenland and Norway. Basaltic lava flows were erupted subaerially over the continent. On the outer margin, which now constitutes the eastern flank of the ridge, they form a wedge-shaped pile thickening eastwards into the line of breakup. There the pile merges with the oldest oceanic crust in the Norway Basin. Elsewhere on the ridge the lavas are thought to form a relatively thin layer. Pre-rift sediments are imaged locally beneath the lavas. They are probably analogs to sediments preserved onshore East Greenland but their distribution is not well known. There is little evidence for rift-related faulting beneath the lava cover, the rifting was apparently concentrated on a few widely-spaced faults.
As the local provenance areas were submerged some time prior to breakup west of the ridge, the ridge complex became sediment starved. At breakup, the plains west of the ridge, which lay several hundred meters below sea level, were covered with flatlying basalts. These were either empaced as voluminous submarine lava flows or as an extensive complex of sills just beneath the seafloor. After breakup the ridge continued to subside, driven by cooling and thermal contraction. Sedimentation became increasingly pelagic, but due to the steep slopes created in the second rift phase, some gravity flow of sediments continued onto the basin plains from both the eastern and the western slopes.