Stone weathering at the westernmost coast of Norway

Selja abbey ruins during heavy rain in November 2010

As probably one of only very few monuments globally, the ruined Selja medieval abbey off the westernmost coast of Norway is built from a metamorphic olivine stone (dunite) rich in talc. The combination of a hard olivine matrix and soft talc veins makes the stone weather in a peculiar way – and it’s not the soft mineral that disappears, but the hard one!

...then came a blizzard...

The little island of Selja close to Stad at the westernmost part of Norway hosted one of the earliest medieval abbeys and bishoprics in the country. It was – and still is – a key place of pilgrimage connected to the cult of St. Sunniva, one of four medieval Norwegian saints. The impressive, weather-beaten ruins have been subject to restoration since the late 19th century and currently conservation is going on in order to secure the stone masonry.

...and then the weather calmed down...

Since the abbey was abandoned in the early 16th century, rain, wind and snow have etched the olivine stone and the local gneiss in the masonry, more so as the buildings lost their roofs, the lime mortar joints crumbled and many stones fell. However, the olivine stone is only subject to heavy weathering at rather protected places rarely directly exposed to the harsh winter storms of West Norway. How can this be explained?

Typical appearance of the olivine stone at Selja. Talc is present in the light-coloured veins that weather less rapidly than the red olivine matrix

First, being very high in olivine, olivine stone or dunite naturally weathers fast (in geological terms!). Olivine is a relatively unstable mineral under atmospheric conditions, generally altering to iddingsite, a complex clay-like mineral compound including iron hydroxides, which gives dunite its characteristic red-brown weathering colour. This colour is so characteristic of dunite outcrops that many are named after it: Raudberget or “Red Mountain” is probably the most common name of such outcrops throughout Norway. The north-western part of the country is very rich in dunite, hosting the world’s largest reserves at Åheim, now exploited by the company North Cape Minerals for the global market.

Talc veins standing out from the olivine matrix

Second, contrary to olivine, talc veins are highly resistant towards natural weathering. Talc is the softest of all minerals, but it is fire-proof, not attacked by acids and bases and the veins are prone to weathering only when criss-crossed by micro cracks that can be penetrated by water which may freeze causing frost damage or carry salts which may give rise to deteriorating crystallisation.

The east wall of the tower and simultaneously the west wall of the nave

Third, at Selja abbey weathering is most pronounced at places affected by the fire that devastated the monastery in 1305. Such places include the west gable wall of St. Alban’s church, which is now part of the abbey’s tower, as well as the interior of the same tower. Although olivine, like talc, is known to be resistant towards heat, it is evident that high temperatures nevertheless caused micro-cracking and surface alteration, thus making the stone prone to subsequent weathering.

Fourth, situated almost within the Atlantic Ocean, the abbey is heavily affected by sea spray with a natural content of 3.5% sodium chloride. Thus, this salt penetrates virtually every corner of the ruin, accumulating where it cannot be washed away by rain. Such places include exactly the same ones that were affected by the 1305 fire; the gable wall of the nave and the interior of the tower. However, a big question is whether sodium chloride can crystallise causing damages in this humid climate, or if other salts, for example those derived from cement used during restorations, are more important. In-depth studies of such issues are currently going on.

Anyway, the properties of the olivine stone, the climatic conditions and the history of the abbey have led to weathering that is typically concentrated at specific parts of the ruins, leaving the olivine stone with a disintegrated matrix, but with the less weathered, more resistant talc veins standing out.

The ruins in 1867. Drawing by Schiertz

A last significant observation is that, according to old photos, the rate of weathering has greatly decreased over the last hundred years. This is probably due to restoration works that have led to a reduction in the amount of water (and thus salt) penetrating the masonry of the ruin.

The investigations at Selja were carried out in pouring rain, during a blizzard and cold, sunny days last week (early November 2010) together with the company Bakken & Magnussen A/S, which carries out the masonry conservation within the Norwegian, so-called “medieval ruins conservation programme” headed by the Directorate for Cultural Heritage. Supported by the Directorate as well as the municipality of Selje, the investigations also included provenance of olivine stone and gneiss used at the abbey. Amazing finds were made, results of which will be posted here later.

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