The Baltic Shield

2,500 million years ago Finland and the northernmost part of Sweden was part of an ancient continent called the Baltic Shield. To the southwest was an ocean. Outside the continent was an vulcanic island arc (green) and a deep-see trench (blue lines). It was somewhat like the Asian continent and the Japanese Islands today. What is now the Archipelago of Stockholm was somewere in the island arc and the trench. The greywacke rock which is typical in the archipelago is a mixture of vulcanic ashes and sands and clay. This was the debris from the erosion of the volcanic arc. The sedimentary gneisses are the metamorfosed rests of the old ocean-floor.

Between 2,000 and 1,800 million years ago the Baltic Shield collieded with another continent, first from the south-west and then from north-west and high mountain-ranges were formed, the Fennoscandic orogenes, wich today is the main part of Sweden. You can see these two directions very clearly in the directions of the cracks and faults in the bedrock of the archipelago. During the billions of years that passed these mountains was rubbed away by weathering and erosion so what we see today is the roots of these high mountains.

During the last million years there have been at least five glaciations of Scandinavia. The ices scrubbed away some centimeters of bedrock per year so several kilometers of the ground have disappeared leaving the hard cristalline ancient bedrock visible to the naked eye. The latest inland-ice melted away only about 10,000 years ago. The landmasses was pressed down by the immense ice (several kilometers high). When the ice melted most of Sweden were covered with water. But it was slowly raising again. The rate of elevation in the Archipelago is about 3-4 mm/year. But it took about 7,000 years before the first island raised above the water in the Archipelago and it is still raising so new islands emerge and the existing grow bigger and bigger. You can clearly see this on a nautical-chart. The islands seems to get smaller and smaller the farther to the east, but if you look at the underwater rocks you can see that they often lies in the extensions of the islands and that these small islands really are bigger "drowned" islands. Since they have not been exposed to the air and the sun for so long time there is not much vegetation. This makes the Archipelago so special. There is not many places in the world were you can see this very ancient bedrock exposed so clearly. The Archipelago is very old and very young, at the same time!