[1] The main graben forming period began in the late Carboniferous, which culminated with rift formation and volcanism, with associated rhomb porphyry lava flows.
In the Vestfold district, one lava flow was deposited on average every 250,000 years, resulting in a 3000-metre thick sequence of mainly volcanic material.
The bedrock in this area, roughly from Skien to Oslo and Mjøsa, results in soil rich in nutrients important for plant growth.
Several of the old magma plumes are now quarried, the rich black larvikite (named from Larvik, a town south of Oslo) being one.
The Särna alkaline complex in western Sweden, also of Late Carboniferous age, is thought to be related to the Oslo Graben as it is aligned to it.