First item of note: Swedish geology is very different from Norwegian, and it is reflected in the geography of the neighboring country. Intrepid geologists should be aware of this, and respectful travelers should perhaps note this as a reason why these two countries are separate, very different entities.
Extra disclaimer – for those wanting to check my numbers here, I am relying on data provided by the Swedish Geological Survey (Sveriges geologiska undersökning). Their online material has been very useful and I highly recommend their introductory stuff – it’s written for the interested layperson.
Welcome to the Fennoscandian Shield!
Also referred to as the Baltic Shield, the Fennoscandian Shield stretches from eastern Norway into northwest Russia, dominating the geology of Sweden and Finland in particular (hence the name). Like its North American cousin the Canadian Shield, these rocks represent the oldest rocks in the European continent, reaching back to ~3.4 billion years ago. I didn't see any rocks that old though - Finland and Russia are currently hoarding those. Most of Sweden (north and central) is known geologically as the Svecofennian province within the shield, which is approximately 1.75-1.9 Ga and thus fairly young in comparison.
So, what did I actually see?
When I trained from Stockholm in the southeast to Åre in the northwest, I traveled through the bulk of the Svecofennian province. I spent my time in Sweden at the edge of the Svecofinnian province, with the Norwegian Caledonides to the West. My journey was over 600 km by train and took around 10 hours. While I didn’t get to thoroughly examine outcrops from my bunk on the train, I did get to see an incredible amount of terrain. My first thought was how different things were from Norway – the vast majority of Sweden is incredibly, unmistakably flat and covered in thick, impenetrable forest. According to the Swedish Environmental Protection Agency, 69% of the country is forest, trees flourishing in the hearty glacial till that forms the bulk of the country’s soil.
Contemporary Swedish geology is the result of two very different geological processes becoming intertwined – its old metamorphic and igneous rocks that form the bedrock of most of the country, and the most recent glaciation, which has left the country flatter and more waterlogged than before. What I saw was a combination of the very old and the very young (geologically speaking, of course).
Sweden has mountains, of course; north-central Sweden is famous for producing skiers, mountain bikers, and sky-runners raised on its smooth granite peaks. The mountains themselves have a very different look to them that what one might expect, however, more similar to the Texas Hill Country than the Rocky Mountains. They’re broad with relatively gentle slopes and rounded tops, like large, bald hills rising out of the forest, and they are, for the most part, made of gneiss, granite, and granodiorite. The different sections of the Fennoscandian Shield are thought to be altered sections of the earth’s crust from different points in time – mid-continental crust as well as thin sections of oceanic crust (ophiolites), for example. Scandinavian sedimentary rocks, meanwhile, are very hard to find.
Most Swedish mountains are composed of the old basement rock that makes the shield, but the Transscandinavian Intrusive Belt, which some may remember from my Norwegian geology post, is also in play here, traversing the country from Småland in the south to Dalarna in the west. Because the Fennoscandian Shield is presumed to be a surviving part of a very old continent, it has survived numerous collisions and rifts as part of its billion-year tenure on this planet. These collisions grew the edges of edge, eventually resulting in an approximation of what we see today; mountain-building occurred as a natural product of this process, though in Sweden hardly any evidence is left of these efforts.
Sweden and the Last Glacial Maximum
As I’ve already mentioned, Sweden’s current geography is primarily the result of glaciation. While the rocks that make up the bulk of the country were deposited quite a long time ago, the country’s current topographical profile can be attributed to the recent presence of a kilometers-thick ice sheet overlying most of the country, and all of the consequences that entails. The world’s most recent glaciation began around 115,000 years ago, and the infamous Last Glacial Maximum occurred about 23,000 years ago, though of course this number varies around the Earth – the average is around 24,500 for the maximum extent of the ice sheets worldwide, but in Scandinavia the number is closer to around 23,000 years ago, and thus is the number I’m using here.
Scandinavia experienced at least five major glaciations in the Pleistocene alone (2.5 mya – 11.5 kya) as a result of the northward migration of Eurasia into its current postion (as well as other factors I won’t get into here). The advance and retreat of glaciers and ice sheets across the Swedish landscape both compressed the Fennoscandian Shield and scoured its surface. The “glacial buzzsaw” – the massive force wielded by kilometers of ice moving back and forth across the country – ground mountains down to their bases, leaving the primarily flat landscape we see today. In addition, the ice sheet stripped the shield of its overlying sediments, which is why we see predominately old rocks – the ice sheets pulverized most of the younger, softer sediment.
One fun (and important) way that a country’s geology can affect its people’s development is the connection between millions of years of geologic history and contemporary soil composition. Sweden is a particularly excellent example of this, which is why I choose to discuss it in this blog post. Modern Swedish soil is glacial in origin, which should come as no surprise, and this affects forest, including what grows in it and where it exists. The sediment that coats the landscape is most often glacial till, collected in depressions that existed beneath the ice sheet, or in eskers, former meltwater channels filled with sediment that can stretch for kilometers.
Swedish forests, populated by pine, spruce, and birch, grow in the acidic glacial soil, following the boundaries of glacial till against exposed, ravaged bedrock. Because the soil is both acidic and in many cases derived from the granite and metamorphic country rocks, the Swedish Geological Survey must deal with issues such as radon risk (from the granite everywhere) as well as potential acidification of groundwater.
It can be kind of hard to notice how much material is actually glacial, just because of the scale it’s on. I hiked along an esker for 1.5 kilometers before I realized exactly what I was climbing over and why it looked the way it did. Many of the landforms are simply beyond the scale of what I’ve seen, even in the Pacific Northwest of the US. In my train rides across Sweden I saw eskers, moraines, and the curious stops and starts of the forest. It seems bizarre to think that there’s enough glacial material to form the soil that covers the majority of the country, but given the massive size of the ice that once existed here, and the duration of glaciations in the last million years, it becomes plausible.
What About the Water?
On a slightly different note, the ice sheets have not simply eroded the Swedish mountains out of existence, yielding a flat surface. The flat nature of the topography is also due to the prior existence of several kilometers’ worth of ice covering the landscape – such a huge mass of ice, which existed in the region for thousands of years, actually depresses the landscape itself, pushing down both the crust and mantle of the earth underneath Scandinavia in response to the pressure. The end result, once the ice retreated and vanished forever, is a surface once depressed that is now slowly, rebounding to its natural state over geologic time (this is known as isostatic rebound or uplift). Sweden and Finland, which were underneath the thickest part of the ice, remain somewhat flattened by their recent glacial experience, and are rising back up millimeter by millimeter – we can measure this!
Sweden’s abundance of lakes and rivers is also intimately tied to the ice sheet that once existed, and the last glacial cycle in particular. When the ice retreated it did not do so in a homogeneous fashion; rather, large chunks of ice were left behind, in addition to the numerous channels carved by meltwater underneath the ice sheet. The end result is a massive amount of lakes and rivers running through Sweden’s forests, and the eventual source of the country’s numerous hydroelectric plants. Trains often run parallel to major rives and lakes, so I got a glimpse of quite a few in my days there.
Sweden today has ~95,700 lakes whose varying morphologies can be traced to glacial influence. Many of the large northern lakes are elongate features running in parallel tracks to one another, while the clusters of smaller lakes appear to be kettle lakes, formed by the depression and subsequent melting of abandoned masses of ice. Pasternoster lakes, also known as staircase lakes, are common in southern Sweden, where they drain into the Baltic. Fun fact: for those who don’t know, paternoster lakes are a series of lakes (typically three to five) connected by a single stream, so called because they resemble the beads on a rosary. They’re all glacial in origin, formed as a glacier or ice sheet retreats backwards in intervals, leaving moraines, eroding into bedrock, and leaving successive lakes in its wake.
It was incredibly interesting to travel through a country where all of the rocks I looked at were very old, and all of the lakes and topography itself were very, very young. At the risk of sounding far too philosophical for this blog, I had the odd sensation of admiration of both the preservation and destruction recorded in the landscape. Sweden can seem pristine and untouched because of the quiet vastness of its forests – but underneath, we know that Sweden spent much of the last geologic era being scraped and scoured, and its surface is far from unaffected.
In Norway, the mountains have been preserved in something approximating their original size and shape, through they are shaved to fine points or covered in cirques. In Sweden, the mountains have been almost completely removed in some places, and the land itself is still rising back up from its experience as the bed of a glacier. Because glaciers have removed so much material, there’s a lot we don’t really know about hundreds of millions of years of Swedish geological history. What happened in between glaciations? What was the landscape like? In many ways, we really don’t know.
I’d like to go back, of course. I hope that those of you who made it through this blog post enjoyed your rambling trip through Swedish geological history! See you next time for Swedish history and culture!