Sten – Södermanland

I am a block of a type of gneiss called veined gneiss or veined sedimentary gneiss. With a slightly nicer foreign loanword, I can also be called a migmatite. Dear child has many names. It is a common rock type in Sörmland, where I come from, and locally my type of gneiss is therefore called Sörmland gneiss. Therefore, I am also Sörmland's landscape stone.

I began my journey about 1900 million years ago as sediment at the bottom of a primeval ocean, hence the name sedimentary gneiss. It was a type of sediment called greywacke, which is a mixture of sand, clay and sometimes slightly coarser gravel grains, deposited in fairly deep water. Where these greywackes are well preserved, for example on Utö in the Stockholm archipelago, you can see that they consist of centimeter-thick layers of alternating sandy and muddy material. The geologists believe that it was landslides on the sea floor that deposited these layers, first the coarser sandy material and then gradually finer-grained clay. For each new landslide, there was a new such "double layer": sand and mud, sand and mud... time after time after time.

The age of 1900 million years is a bit uncertain, it is difficult for geologists to determine the age exactly of sedimentary rocks like me which consist of a mixture of older rock material. But they think we are about as old as the volcanic rocks found in the same areas, which are about 1900 million years old. Or we could be a little bit older, since we're usually found a little below our volcanic friends, but not by much. Some of the sand and mud may have been volcanic ash from these volcanoes deposited some distance away.

Some time later, the entire bedrock in northern and central Sweden, and in Finland, was exposed to a mountainbuilding event, just as the dolomite marble from Sala told about. I had already hardened into a solid sedimentary rock, from being a loose sediment in the first place, but now I was pushed even deeper into the earth's crust, several tens of kilometers, where it was hot and sweaty and really high pressure. And everything moved so that I was kneaded here and there like dough, although I was a solid rock, and the layers in me became completely folded and wrinkled. I was filled with folds of various sizes, from centimeter-sized folds that can be seen in a single rock, to kilometer-sized folds that can be seen on the geological map patterns of different rock types. This happened between 1870 and 1780 million years ago, perhaps in a couple of different rounds.

Due to the pressure and heat, there were also a lot of chemical reactions between my minerals, some minerals could not stand the heat and broke down, and new minerals were created. This process is usually called metamorphism, a Greek word that roughly means transformation. So now I am no longer a sedimentary rock, but a metamorphic rock, a transformed rock. And the new minerals that were formed are called metamorphic minerals. Depending on which such metamorphic minerals were formed, and their composition, geologists can determine roughly what temperature and pressure I was exposed to during the metamorphism.

A common such metamorphic mineral is garnet, a mineral you may have heard of because fine and clear garnet crystals can be used as gemstones. Most often, they are brownish-red in color, and almost round, with many crystal surfaces so that they look a bit like footballs. Garnets are found a little here and there in the sedimentary gneiss, but not everywhere. They are hard, so where they are found they often stick out as small – a few millimeters in size – brownish-red knots from the rock outcrops.

In some places it got so hot – between 600 and 700 degrees Celsius – that I actually started to melt. Not all of me, but part of the minerals that had the lowest melting point. It was really sweaty, and not very nice at all. I thought I would melt completely, and turn into a magma. But then it cooled, and the molten material solidified again, becoming bright veins of quartz and feldspar, often a little winding and uneven and sometimes folded, perhaps a centimeter or so thick. Sometimes there may be larger lenses where such melted material has collected and solidified. It is because of these bright quartz-feldspar veins that my type of gneiss is called veined gneiss. Some people think veined gneiss looks like bacon, with the light veins in the gneiss as the fat in the bacon. Maybe!

In any case, if I may say so myself, I think it is a very beautiful form of gneiss, much more beautiful than the uniform gneiss granites, and I wonder why one could not mine me and use me as a building stone, much as one does with many granites. But perhaps I am too inhomogeneous and contain too many cracks to be useful as a building block, I simply break too easily. If a rock is to be used as building stone, it must be homogeneous and able to be broken into large square blocks without these cracking.

This block from Mellösa north of Flen is quite weathered on the surfaces and maybe a little dirty, so you don't see the veined gneiss structure very clearly in it, and how beautiful I can be. Maybe you should wash it, or preferably split it, so you get a completely healthy surface inside the stone. You can best see my beautiful veined gneiss structure in freshly blasted blocks, perhaps from some newly made road cutting, or in fine rock outcrops out in the archipelago right at the edge of the water where they are washed clean by the waves.

Geologist Åke Johansson
Swedish Museum of Natural History