What are we walking on?

If you were planning on skiing, snowboarding, snowshoeing or otherwise going into the mountains this weekend, perhaps you would be interested to know what you are walking on. Would it surprise you to learn that some of the rocks on the Island of Kvaløya, in the Troms district of Norway, are 1.77-1.8 billion years old? That is 1800000000 years old.


Map of Western Troms Geology from Bergh et al., 2012.

Last weekend on a skiing trip in the south of Kvaløya, I began to wonder about the geologic history of the island. I was surprised to learn that many of the rocks at both Kvaløya and Senja were formed in the Precambrian, or more specifically the Neoarchean (2.5-2.8 billion years ago) and the Paleo-Proterozoic (1.6-2.5 billion years ago). To put this in perspective, life first appears in the fossil record at 540 million years ago, at the beginning of the Cambrian period (at the bottom of this post is a geologic timescale).

This map of the geology gives an overview of the geologic formations in Western Troms. Note that only a couple rock types are represented here (Granite, Gabbro, Diorite and Gneiss). All the rock types represented in Western Troms are either Igneous or metamorphosed igneous rocks. Igneous rocks form when molten rock cools – either when magma cools slowly within the crust, or when lava extrudes and cools quickly. The longer it takes the magma to cool down, the larger the mineral grains in the rock. It is also worthwhile to note that all the igneous rocks in Western Troms are intrusive igneous rocks, meaning that magma cooled slowly inside the earth to form them.

But how did the rocks get from forming in the interior of the earth to Western Troms? Well, that is all to do with the Caledonian Orogeny (an orogeny is an episode of mountain building). The Caledonian Orogeny occurred between the Ordovician and the Devonian (between 485-390 million years ago) and it formed the mountain belts of Scandinavia and eastern Greenland, as well as in Scotland. During the time that the Scandinavian mountain belts were forming, the continents as we know them today did not exist.


Image from Leslie et al., 2008

The two main continents that were responsible for forming the Scandinavian mountain belt are called Laurentia and Baltica. These two continents pushed together, folding and buckling the surface of the earth and forming the mountains we see today in Scandinavia (there is a third continent that may have also played a role, but lets keep it simple for now).

Interestingly, the rocks of Western Troms were actually associated with a hypothetical continent that existed before even Laurentia and Baltica. This continent was called the Rodinia Supercontinent (a clustering of all, or nearly all the continents on earth), and some of the Western Troms rocks may be a record of the break-apart of this supercontinent – although whether it existed is debated among geologists.

So, if you are taking a trip around Western Troms this weekend, now you know a little bit about the history of what you are walking on.


Geologic Time Scale from stratigraphy.org.


Bergh, S.G., Corfu, F., Myhre, P.I., Kullerud, K., Armitage, P.E.B., Zwaan, K.B., Ravna, E.J K., Holdsworth, R.E. & Chattopadhya, A. (2012) Was the Precambrian Basement of Western Troms and Lofoten-Vesterålen in Northern Norway Linked to the Lewisian of Scotland? A Comparison of Crustal Components, Tectonic Evolution and Amalgamation History. INTECH 2012 ISBN 978-953-51-0675-3.s 283 – 330.
Leslie, A.G., Smith, M. & Soper, N.J. (2008) Laurentian margin evolution and the Caledonian orogeny—A template for Scotland and East Greenland. GSA Memoirs, Vol. 12, pp. 307-343.
Cohen, K.M., Findlay, S.C., Gibbard, P.L. & Fan, J.X. (2013; updated) The ICS International Chronostratigraphy Chart. Episodes 36:199-204.



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