Thursday, December 03, 2020

Geology Through Literature - The Tin Drum

 Geology Through Literature: The Tin Drum

Despite the main character of The Tin Drum by Günter Grass working for a headstone carver, there isn't much geology in the book. There are frequent off-handed mentions of different rocks such as granite or marble, but nothing that could be placed in context. There is one instance in the book, though, that we could delve into.

Book 2, Chapter "He Lies in Saspe"
"The (Saspe) cemetery was square with a wall running around it. We went in on the south side, through a little gate that was covered with ornamental rust and only supposed to be locked. Most of the tombstones were of black Swedish granite or diorite, rough hewn on the back and sides and polished in front."
The Saspe Cemetery is on the outskirts of Danzig, Poland, which is now known as Gdańsk. Gdańsk is a port city located on the Baltic sea in northern Poland. This means it was directly across the water from Sweden and could easily have received Swedish stones. 

Although many non-geological people will call anything that isn't marble "granite", I don't think that was the case here. Specifically, because he calls out diorite as an alternative to the black granite. Looking at the igneous rock scale (a generalized version) we can see that igneous rocks are identified by the minerals located within them.

Diagram of igneous rocks as identified by their present minerals. Image courtesy of

For the rocks in the three leftmost columns, there are two rocks each. These identify the course grained rocks (granite, diorite, and gabbro), that formed as intrusive igneous rocks. This means that they formed from a body of magma (molten rock) that cooled over a very long time underground. This allowed the crystals to grow to a point where individual crystals are easily identifiable by the naked eye. The second set of rocks are for the fine grained rocks (rhyolite, andesite, basalt). These rocks are extrusive igneous rocks, also known as volcanic. These rocks formed from lava (magma that had been erupted), and cooled very quickly. The quick cooling caused the mineral grains to be very small and mostly unobservable to the naked eye.

When you look at the rock groupings from left to right there are certain patterns. One is that the amount of silica (quartz) decreases. There is also less Orthoclase Feldspar are you move to the right. These are both generally light colored minerals. The minerals more on the right (plagioclase, pyroxene, micas, and amphibole), these are generally more darker colored minerals. So as you move from left to right, the rocks get darker in color. 

A "black granite", aka Swedish diabase from Scandinavian Stone's Gylsboda Quarry. Image courtesy of Scandinavian Stone

The use of the term "black granite" already had me questioning if this was a real thing. However, after doing some research it appears the term "black granite" is applied to rocks that "share the hardness and strength" of granitic rocks, despite them not actually being a granite. Generally, these "black granites" are what are known as a diabase (also known as dolerite), which is a microcrystalline gabbro with crystal sizes (and therefore cooling rates) falling in between gabbro and basalt.  

Globally, Sweden isn't generally known for their building stones. However, that doesn't mean that Sweden should be ignored. They actually have a significant amount of building stones that they frequently will quarry and send off to other countries, such as Italy, Spain, China, or Poland for processing. The size of the industry in Sweden is very small though, totaling ~1,200 people with three separate companies comprising the bulk of that number. 

Diabase from the Swedish Gylsboda Quarry. Image courtesy of Scandinavian Stone.

Within Sweden, there three main rocks that are quarried, among them several varieties of "black granite" AKA diabase. This includes the rock types known in the construction industry as Black Bonnacord, Black Finegrained variety, Black Ebony, Black Gylsboda, and Grey Bohus. The problem with these industry names though is that they more reference the color and texture of the rock and not any particular geological occurrence. I actually found one quarry (Black Diabase Brannhult Quarry) That has several of these types of rocks all listed within the same quarry, further emphasizing that these are likely variations on the same rock formation.

Diabase from the Swedish Hjortsjö Quarry. Image courtesy of Scandinavian Stone.

But the one thing I did find, is that many of the diabase quarries were in the south central part of Sweden, in the region called Småland (which is not only the name of the Ikea day care apparently). Looking at the geologic map below the southern portion of Sweden can be broken down into two distinct provinces, the eastern province known as the Transcandinavian Igneous Belt and the western province known simple as the Eastern Segment. 

Southern Sweden geology. Image courtesy of Salin et al., 2019.

The problem is that many of the quarry locations are falling very close to that central area between the two provinces known as the Protogine Zone, which is a major faulted shear zone. However, upon closer inspection, it appears that these quarries, which all are within a generally small region, are a part of the Transcandinavian Igneous Belt within an area known as the Småland-Värmland granitoids.

More detailed southern Sweden geologic map. Image courtesy of Högdahl et al., 2004.  

The above geological map indicates that these diabase deposits are dated to around 1.85 to 1.65 billion years old. However, diabase, is often deposited as smaller igneous bodies within a larger body, such as a sill, dike, lopolith, or a laccolith. The smaller igneous body allows for some slow cooling, but not slow enough to form a full on gabbro. Therefore the dates on these larger bodies is likely too old and the actual diabase would be younger than that. However, I don't think they would be too much younger since the magmas that formed the larger Transcandinavian Igneous Belt are likely related to the diabase producing magmas.

On the map above it also lists some other provinces of dolerite (the other name for diabase) towards the northern end of this region. The quarries that I researched are not harvesting this diabase, but that doesn't mean that another company isn't. This diabase is dated to 1.25 billion years old and I would image the diabase in the southern portion of the country would date around the same.

So in general, the "Swedish black granite" mentioned in the text was likely obtained from these southern diabase deposits within the Småland-Värmland granitoids. It also works out that since these are along the southern coast of Sweden, the transportation distance to Poland would have been much less across the Baltic Sea to the east. 


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