Sten – Västmanland

I consisted of limestone from the beginning, approximately 1900 million years old. The limestone I am made of was deposited at the bottom of an ancient sea in close proximity to strong volcanism in large parts of central Sweden - the area usually called Bergslagen - at this time. Perhaps some of the lime was deposited from hot springs associated with volcanism, just as you can see limestone deposits today – so-called travertine – around hot springs on land in volcanically active areas. At that time, there wasn't much life even in the oceans, just algae and bacteria and similar single-celled organisms. But it actually seems that unicellular algae were also involved when I was formed. At Sala, they have found a kind of round decimeter-sized structures in the limestone called stromatolites, which are a kind of ancient fossil algae colonies. Similar living stromatolites are found in very shallow waters off the northern coast of Australia today.

At the same time as the volcanism, much of the ore in the Bergslagen region was also formed, such as the lead, zinc and silver ore in Sala. In the 16th century, lead and silver were mined in Sala, mostly lead, but the silver that was extracted from the lead ore was more valuable, and therefore the mine is called Sala silver mine. Later, zinc was also mined there, and mining continued through the centuries until the beginning of the 20th century. Now the mine is closed, but it has become a popular tourist destination that you can visit. Sala Silvergruva.

But why I am called dolomite marble, if I was made of limestone? Well, marble is limestone that has recrystallized due to high pressure and high temperature deep down in the earth's crust. And a few tens of millions of years after I was deposited, there was a mountain chain forming, and the sea floor where I was formed became high mountains. I ended up deep in the earth's crust at the roots of the mountain range, and was transformed from limestone to marble. Then the mountain range has eroded away over the course of millions of years and disappeared completely. Then I came up to the earth's surface again.

Some marble is white and fine, almost like snow, or like sugar that sparkles with small sugar grains. The white Italian Carrara marble is famous, and has long been used for sculptures and monuments. For being stone, limestone and marble are not that hard, and quite easy to shape and sculpt. I, the marble from Sala, am almost as white if I may say so myself without bragging. It was probably the case that the limestone from which I was formed was very pure, consisting only of the mineral calcite (which consists of calcium carbonate). But then there is also marble formed from impure limestone that contains layers of clay or volcanic ash. These layers are converted into gray or green veins of minerals other than calcite. The Kolmården marble, which is about the same age as the Sala marble, and which is found in stairs and windowsills in many residential buildings, is such a greenish marble. It is also beautiful but in a different way, with an appearance that is usually called "marbled".

Dolomite, then, you wonder. Well, dolomite is magnesium-containing limestone or marble. Otherwise, pure marble consists of the mineral calcite, which is calcium carbonate, but dolomite marble consists of the mineral dolomite, which also contains magnesium. The researchers don't really know how the magnesium got there, so it's my little secret. Incidentally, in northern Italy, in the Alps, there is a whole mountain area called the Dolomites because the mountains there are mostly made of dolomite.

Now neither silver, lead nor zinc is mined in Sala, but a short distance from the silver mine there is a dolomite quarry where dolomite marble is mined for various industrial purposes. It is from that dolomite quarry that I come from.

Geologist Åke Johansson
Swedish Museum of Natural History