Why are there mountains in Norway?
Norway is normally perceived as a mountainous country, but are mountains really to be expected here? The highest mountains on Earth are localized in areas where tectonic plates are pressing against each other or have done so in the recent (geological) past, like in the Himalayas, the Alps or the Andes. There are 400 million years since such processes have affected mainland Norway
Do we believe that these mountains are still standing, or were today´s mountains caused by completely different processes? If they are not 400 million years, how old are they really? Scandinavia has been uplifting since the end of the last ice age, so what was the cumulative effect of the Quaternary ice ages and subsequent meltdowns? It may come as a surprise that such fundamental questions are still hotly debated among geologists. It may be worthwhile to step back and ask what mechanisms create mountains globally, and which of these may apply to Norway.
Mountains may form in several ways. Collision between continental plates have created some of the world´s most spectacular mountain belts, and thus landmarks such as Mount Everest, K2, and Mt. Elbrus. However, mountain ranges also form along continental rifts and rift margins where the earth´s crust is extended and thinned. The unloading of large rock masses through crustal thinning and thermal pulses related to the rising of hot mantle under such areas are possible causes for this type of uplift. As shown in Antarctica, Mountains formed along rifted margins can be more than 4000 m high. Uplift normally leads to enhanced erosion, which induces removal of material and thus more uplift. Deposition of large masses of sediment on thinned crust in one area may lead to peripheral uplift in another area. In areas that have undergone glaciations and subsequent meltdown, the land surface will commonly be uplifted when the glacial load is removed. In any given area, mountains may form due to combinations of mechanisms that enhance each other, such as tectonic uplift and erosion.
Collisional tectonics give rise to mountain belts, because the crust becomes thicker than normal, and a crustal `root´ forms that will support the overlying mountains. In the case of Norway, the highest mountains are not supported by thick crust. On the contrary, geophysical investigations show that the crust is of normal or slightly reduced thickness under the highest mountains. It is therefore not likely that the 400 million years old Caledonian mountains are the direct precursors of present-day mountainous topography in Norway - even if today´s mountains contain the remnants of the collisional Caledonian mountain belt. It is more natural, perhaps, to think of them as mountains that formed along a rifted margin. However, it is a long time since the time of active rifting (55 million years or more), and geologists that work with seismic data from the Norwegian margin have argued that the rift-flank topography was eroded to a maximum height of a few hundred meters in the Cretaceous. Many will therefore argue that today´s topography is young, and probably younger than 65 million years. What was the impact of the Ice Ages which pillaged our latitudes for the last 2.8 million years? We know that Scandinavia was covered by a thick inland ice that suppressed the earth´s crust, and that the land has been rising since the inland glaciers melted. However, mountainous topography occurs also on non-glaciated margins, and geologists working on Quaternary glaciations argue that the first glaciers nucleated in high topography that was already there. If this is correct, Norway became topographically elevated sometime between 65 and 2.8 million years ago. This is referred to as the Cenozoic uplift of Scandinavia. It is a process that has been long described and much debated, but generally poorly understood.
Researchers at the NGU have identified an important relationship between the structure of Norway´s rifted margin and its onshore mountains. The highest coastal mountains and the most asymmetric topography of Scandinavia is found where the crust tapers from normal crustal thickness to a thickness of less than 10 km over a relatively short horizontal distance. Where the crust was thinned more evenly over a longer horizontal distance, the mountains are lower and the topography less asymmetric. These observations show that the crustal structure of the rifted margin (see also theme page: passive margins) exerted a fundamental influence on the formation of the Norwegian mountains. Even if the elevated rift flanks were eroded down in the Cretaceous, the distribution of normal versus strongly thinned crust, and thus the areas of erosion vs. areas of sedimentation, controlled the loci for post-rift uplift of the mainland. This may have been especially important during the intensive glacial erosion in Plio-Pleistocene time. Thermal effects of breakup and of the changing configuration of spreading ridges in the North Atlantic may also have influenced onshore uplift.
Are tectonic processes active today? Small to medium-magnitude earthquakes in Fennoscandia show that some faults are active. Identifying and understanding patterns of earthquakes helps geoscientists interpret tectonic processes. Fennoscandian earthquakes are organized into three distinct belts (SB1, SB2 and SB3 in fig.). SB1 is associated with both relatively thick Neogene sedimentary deposits and closely tracks the outer border of the margin´s necking domain, named the Taper Break (TB; see also theme page: Passive margins). SB2 is associated with the topographic escarpment and the 39 km crustal thickness contour, which are proxies for the innermost limit of extension and thus marks the landward extent of the margin's proximal domain.SB3 follows the Gulf of Bothnia, and marks the boundary between the eastwards dipping Scandinavian topography and the flat cratonic areas to the east. SB3 marks the most landward limit of the Scandinavian rifted margin.
The seismic belts tell us that principal zones in the margin are seismically active, and that processes such as erosion and deposition still influences the margin. The most seismic energy is released along the outer boundary of the necking domain, at the border to hyperthinned crust in the distal margin. Post-glacial uplift affects much of Fennoscandia, but the distribution of seismic belts shows that the underlying, inherited crustal structure still focuses earthquakes and faulting.