Fensfeltet

580 million years ago, there was a very unusual volcanic activity here. Today, the volcano itself has been eroded away, but we still find remnants of the volcano’s feeder pipes, as well as other traces of volcanism in the surrounding areas. In fact, evidence of this volcanic activity can be found as far away as Kragerø.

At that time, the continent of Baltica, of which “Norway” was a part, was a worn-down lowland. Geologists call this lowland the “sub-Cambrian peneplain.” This ancient plain can still be seen in landscape features in several places in Norway, including the Hardangervidda plateau128. But as has happened many times throughout Earth’s history, conditions changed, and the sea advanced over the flat plain. This change is linked to warm currents beneath the Earth’s crust at one of Baltica’s collision zones. The increase in temperature caused the crust beneath what is now Ulefoss to melt-possibly in connection with a so-called mantle diapir (hot spot)-and the Fen volcano was the result.

But what is so special about this volcano? The rocks we find today are completely atypical for volcanoes. As early as 1921, one of Norway’s leading geologists, W.C. Brøgger, claimed that the limestones at Ulefoss had a volcanic origin. He called the rocks carbonatites, but his theory was not widely accepted at the time, as everyone “knew” that limestones are sedimentary rocks formed in the sea, or at most as precipitates from hot springs. Volcanic rocks were supposed to be basalts, porphyries, and similar types. Not until the 1960s did British geologists document lime-rich lava flowing from active volcanoes in the African Rift. So Brøgger was right! The rocks he described from Nome he named after local farms, such as søvitt, melteigitt, vipetoitt, fenitt, and rauhaugitt. Damkjernitt is named after Damtjern, although Brøgger spelled the name a bit incorrectly. All the rocks were described in detail, and the Fen Complex is the type locality and an important reference site for volcanic carbonate rocks. Carbonatites are still the scientific term for this group of rocks.

The carbonatites at Fen are thus of magmatic origin, meaning they come from molten rock. Damkjernitt reached the surface from great depths in connection with explosive volcanism, and this rock can be found as dikes far outside the Fen Complex itself. Fenite is actually the result of a transformation process (fenitization). The carbonatite melt penetrated the ancient basement rocks, resulting in a rock characterized by its composition of alkali feldspar, sodium-rich amphibole, nepheline, aegirine, and phlogopite. Other minerals such as titanite and apatite may also occur.

The volcanic activity led to cracks in the Earth’s crust, and hot, aqueous solutions circulated through these fractures. This process caused oxidation of the iron present in the carbonatites, resulting in an intense red color in the iron-bearing rocks. This red color is the origin of the name “rødberg” (“red rock”), and it was in the rødberg that iron mining took place from the 1650s. The ore from Fen was delivered to Ulefos Ironworks.

Other unusual chemical elements also occur in the carbonatites. After World War II, the rare metal niobium was mined, and the occurrences of the element thorium are well known and have helped make the Fen Complex well known in Norway.