Sunday, April 04, 2021

Geology Through Literature - Han Christian Andersen's: The Rags

Geology Through Literature: 

Hans Christian Andersen's: The Rags (1868)

We continue on to the eighth, and penultimate entry, of Hans Christian Andersen's geological references.

For other Geology Through Literature entries, please check them out compiled on my website.

The Rags (1868)

Bedrock Geology
"I am Norse!" said the Norwegian. "And when I've said I'm Norse I guess I've said enough. I'm firm of fiber, like the ancient granite rocks of old Norway. The land up there has a constitution, like the free United States. It makes my fibers tingle to think what I am and to sound out my thoughts in words of granite!"

 Norway's bedrock is truly ancient by no exaggeration of the term. The oldest rocks in Norway are part of the Fennoscandinavian shield, which are the Scandinavian Precambrian bedrock, the oldest rocks of which can be found along the northern edges of the country in Finnmark, Troms, and Vesterålen. These oldest rocks date back almost 3 billion years old.

The oldest rock in Norway at 2.9 billion years old. Image courtesy of the Geological Survey of Norway.

As you can see in the map below, most of these truly ancient parts of the Scandinavian shield are in the Finland and Russian parts of the Fennoscandian Peninsula. 

Bedrock geology of Scandinavia. Image courtesy of the Geological Survey of Norway

The vast majority of Norway is underlain by incredibly old rocks, even those parts not in the northernmost regions. The question now is, are these rocks truly granites like the text says? Granite is often a term used as an all encompassing term for crystalline rocks. Just look at the countertop industry where everything is "granite" where much of the counter tops are not actually granite by the geological definition of the word. 

Granite is an intrusive igneous rock that formed from the cooling of magma deep within the earth where the earth insulated the magma allowing to cool slowly. The slow cooling allowed the mineral crystals within the rock the time to grow creating the crystalline rock so well known as granite. The naming of an igneous rock is dependent on how the rock formed (intrusive versus extrusive) and what the mineral composition is, essentially how much quartz is there in the rock. To identify the rocks there is a chart/scale of intrusive igneous rocks which are identified based on the silica (quartz) content. 

Basic Intrusive Igneous Rock Scale 

On the high end of the scale with a high quartz/silica content is Granite, known as a felsic rock. In the middle, with an intermediate amount of quartz/silica, is Diorite. And on the low end of the scale with no quartz/silica is Gabbro and Peridotite. Igneous rocks with no silica are known as mafic rocks.

Really old rocks also have more instances where they can get metamorphosed. This means that they had been subject to increased heat and/or pressure. When that happens, the minerals within the rock change. The rocks don't fully melt, but the increased heat allows the elements within the rock to reorganize themselves forming new rocks. When granite is metamorphosed, it produces a metamorphic rock known as a gneiss (pronounced "nice"). Gneiss is the rock most often miscategorized as a granite, since they so often resemble one another, however there is a mineralogical and a structural difference between the two. 

Portion of the Geological Map of the Fennoscandian Shield showing northern Norway. Circled rocks represent the oldest (Archaean rocks identified as "462") in Norway. Image courtesy of GigaPan.

Looking at a much more detailed geological map in the Fennoscandinavian shield, it turns out that oldest rocks within the Norwegian portion of the shield are indeed composed of granites and gneisses. The northernmost coastal rocks in Norway are identified on the geological map of the region as "462", these are Archaean granite, granodiorite, tonalite, and metamorphic equivalents. The other rocks of similar age are identified as "463", and these are migmatitic gneiss of granodioritic to dioritic composition. 

Being so old, the rocks only represent the basement of the region and have several billion years of rock history deposited on top of them. They record the history of mountains built up, eroded down, built up again, and eroded down again. There is more history in these basement rocks than most rocks will ever see in their lifetime before they are broken down by the passage of time. So, like Andersen said, the northern portions of Norway really do contain "ancient granite rocks".

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