Sten – Skåne

I am an angular boulder of black diabase, as the Americans call me, or dolerite as the British say, from a quarry at Gylsboda in Göinge in northern Skåne (or Scania, as the southernmost part of Sweden is sometimes called in English). Skåne has the very youngest bedrock in Sweden; The Cretaceous limestone in southwestern Skåne is as young as 60 million years, and a large part of the sedimentary bedrock in Skåne is from the age of the dinosaurs, between 66 and 200 million years ago. Geologists think this is very young, at least in Sweden. But up in the forests in Göinge in northern Skåne, the bedrock is significantly older, it is similar to that in Halland and Småland. I myself, I am approximately 1220 million years old, and belong to a swarm of steep diabase dykes that stretch in a north-south direction from Skåne through Småland roughly along the E4 highway up to Lake Vättern.

The geologists have sometimes called our swarm of diabase dykes hyperite diabases, because the diabases contain a mineral called hypersthene (a kind of pyroxene), and sometimes Protogine zone diabases, because we follow a large shear zone in the bedrock called the Protogine zone, which runs from Skåne through Småland up to lake Vättern (and then it continues to Värmland, but there are no such diabases in Värmland). The stone industry, which has mined several diabase dykes in quarries in northern Skåne, refers to us as "black granite", but that is actually quite wrong. Granite is light gray or reddish and rich in the element silicon and the mineral quartz, while diabase is dark and poor in silicon and quartz.

Diabase is formed from basaltic magma - molten rock - which seeps up along cracks in the earth's crust from the underlying mantle. This occurs when a landmass, a continent, is stretched and is breaking apart. Apparently the crust in southern Sweden was rifting along a north-south zone when I was forming, because magma could push up all the way from the mantle. But luckily for the Swedes, the splitting stopped, so Halland, western Skåne and western Småland were never separated from Blekinge, eastern Skåne and eastern Småland, but they are still hanging together.

If the basalt magma solidifies as a large body down at depth, the rock is called gabbro, and becomes quite coarse-grained (because it takes a long time for the magma to cool and solidify, so the mineral grains that are formed can grow for a long time). If magma solidifies in a (usually) steep crack in the rock, the rock is called diabase. It usually becomes finer-grained, because it has solidified faster. But the diabases in our swarm are unusually coarse-grained for being diabases, and unusually dark, almost entirely black. If magma flows out onto the surface as lava during a volcanic eruption, the rock is called basalt and becomes very fine-grained.

Each of the diabase dykes in our swarm is perhaps a few tens of meters wide, or one hundred meters, and a few kilometers long. But taken as a whole, we occur within a 20-30 km wide belt with a length of nearly 300 kilometers, from southern Skåne up to Lake Vättern.

The black diabase has been popular as a building stone, and is quarried or has been quarried at several places in northern Skåne. Riksbankshuset in Stockholm has a facade of such diabase. It has also been popular with sculptors and used for various sculptures and other monuments. Not least the contrast between polished and rough-hewn surfaces can give a very special effect for such purposes.

Geologist Åke Johansson
Swedish Museum of Natural History