Sten – Västra Götaland

I am a block of Bohus granite from the Hallinden quarry in Bohuslän. The Bohus granite covers almost the whole of northern Bohuslän, from Lysekil and northwards, and is Bohuslän's landscape stone, but here in the stone circle I represent the entire county of Västra Götaland. You may have been on holiday in northern Bohuslän, or even come from there, and then you must have noticed the rounded outcrops of ligth red granite that dominate the landscape there. They are the Bohus granite.

I was formed about 920 million years ago, from a hot magma, a rock melt, that intruded the earth's crust and solidified, just like the other granites here in the stone circle. I am actually the youngest granite in the stone circle, and in all of Sweden, although I am not the youngest stone in the stone circle. But the stones that are younger are not granites.

The geologists believe that I have the shape of a huge flat sheet in the bedrock, which covers the whole of northern Bohuslän. I actually continue north of the Idefjorden into Norway, but there I am called Idefjord granite. Like other granites, I consist of a mixture of different minerals: light quartz, white or reddish feldspar, and black biotite and hornblende. There are small amounts of several other minerals as well, such as titanite, apatite and zircon, but these you usually have to have a microscope to see, because those mineral grains are so small and not so common. The mineral grains form a grey-white or red-white mottled mass, so that granite looks a bit like salami (“prickig korv” in Swedish). The light Bohus granite is quite fine-grained, while some of the other granites here in the stone circle are more coarse-grained, the Nordingrå granite for example.

I was formed at the end of a mountainbuilding event called the Sveconorwegian, because it included parts of Sweden and Norway (southwestern Sweden and southern Norway). It occurred between 1100 and 900 million years ago, so when I intruded, 920 million years ago, it was right at the end. Now this mountain range is completely eroded away, and that is why you can see me at the surface of the earth, since all the overlying rock has been eroded away and gone. This mountainbuilding event is thought to be due to Scandinavia and all of Baltica (northeastern Europe) colliding with eastern North America at this time. In eastern Canada and the United States there is a corresponding event of the same age. Baltica and North America and most of the other continents of the Earth then became parts of a supercontinent called Rodinia. Exactly what Rodinia looked like and where it was located is not known. After about 300 million years, Rodinia broke up, and Baltica separated from North America again.

My particular block of stone comes from a quarry - Hallinden northeast of Kungshamn - where Bohus granite is quarried. The Bohus granite is homogeneous and good and easily splits into square blocks, and is therefore well suited for quarrying and use as building stone, curbstone and paving stone. It was at the end of the 19th century that granite began to be quarried on a large scale along the coast in northern Bohuslän, when the cities grew and a lot of stone was needed to build with, and when new tools and machines made it possible to blast and saw the hard granite on a large scale. And it was good that the quarries were on the coast, so that the heavy stones could be transported by boat to where they were needed. For perhaps a hundred years, quarrying was an important industry that employed many people in northern Bohuslän, those who did not fish or were farmers. Now many quarries are closed, and those that remain, such as Hallinden, do not employ so many people anymore, because machines do most of the heavy work nowadays.

Geologist Åke Johansson
Swedish Museum of Natural History