Erratics from the Aland Islands in Finland

Oost-Aland met zicht op het eiland Järsö

Aland is an archipelago consisting of thousands of large and mainly small islands and rock outcrops in the southwest of Finland. The Aland Islands are situated at the entrance to the Gulf of Bothnia in the Baltic Sea.

The region consists mainly of a 4,000 km2 rapakivi massif. Most rapakivi’s are granite rocks dating from the Precambrian Period (1640 million years ago). 

Aland showing red rapakivi granite. This reddish colour is found in places where bare granite rocks have been uncovered by glacial erosion.
Aland on the Bomarsund. The reddish colour of the eroded rocks is evidence of rapakivi granite.

Rapakivi erratics are very common in the Netherlands, above all-in places where Eastern Baltic boulder clay is present on or just under the surface. This is especially the case in the Hondsrug region of Eastern Drenthe and its continuation in the province of Groningen. 

Rapakivi’s are extremely common in Eastern Drenthe and also occur in great variety. Indeed, it is difficult to find two identical erratics. We can divide our rapakivi erratics into plutonites or deep-lying rocks, igneous magma and vulcanites. Of the latter, Aland quartz-porphyry is the best known example.

Aland rapakivi – an erratic found in Gieten (Dr.)
Aland granite-porphyry – an erratic found in Ellertshaar (Dr)
Aland quartz-porphyry – an erratic found in Emmerschans (Dr.)

Aland rapakivi is undoubtedly the best-known type of rock both in and outside the Northern Netherlands.  In a weathered condition erratics are easy to recognise by their distinctive ‘ring-structure’. A reddish to red-brown groundmass can include countless round feldspar phenocrysts, up to 2 cm in size, surrounded by a distinctive white border.

There are many different types of rock from this area including Haga granite, Pyterlite, Lemland granite and others. These are described with accompanying photos on the Dutch website

Text and photos          Harry Huisman

Translation                 Alun Harvey


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