Aside from the attractions on Senja such as Senjatrollet (the world’s largest troll) and the traditional fishing communities (as well as a Halibut museum), the geology is also a reason to visit Senja. The northern and western coasts face out to open ocean and mountains plunge near vertically into the sea. According to some tourism websites, almost every aspect of Norwegian scenery and nature can be found on the island.
We have covered quite a lot on this blog about the Caledonian Mountain Building episode which is responsible for Scandinavian mountains in general, and talked about some of the rock types. A general rock ‘family’ that gets mentioned quite a bit are metamorphic rocks. Metamorphic rocks can form as either igneous or sedimentary rocks are altered due to either pressure (as rocks are buried in the crust) temperature (the deeper towards the mantle a rock is, the higher the temperature) or chemical alterations (i.e. when water passes through a rock chemicals in the water can interact with chemicals in the rock changing the mineral structure).
The west of the island is composed of mainly Granite and Granodiorite which has been metamorphosed to varying degrees to create Gneiss. The east of the island is composed of metamorphic rocks that were sedimentary in origin. These are rocks such as schists, and meta-sandstones. A significant portion of sedimentary rocks are deposited under water (ocean, lakes, rivers, estuaries etc.) but they can also form through on-land deposition where there is a large amount of wind activity transporting sediments, or on beaches and river valleys where sporadic high water and/or wind may transport and deposit sediment. When these sediments are buried, they are slowly subjected to higher and higher pressures and temperatures, which alters the grains. This means that a sandstone, for example, can be placed on a continuum between being fresh sandstone and greywacke (highly metamorphosed sandstone/mudstone). Among other alteration processes, minerals can become aligned in a specific direction, allowing geologists to recognize the direction in which pressure was exerted. Another process that is important in the alteration of sedimentary rock to metamorphic rock is chemical alteration.
If a sedimentary rock is deposited in a marine environment, the water that is entrained with the grains and fills pores in the rocks will be saline water. Upon burial at depth, the ‘water’ that passes by the rock – either moving up to the surface or down towards the crust – will have picked up different minerals and chemicals along the way, some of which are able to chemically alter the properties of the rock.
But what does all this have to do with Senja? Well, there is one large thrust fault that almost cuts the island in half. On one side of this fault the rocks are all metamorphic from igneous origins or igneous and on the other side of the fault the rocks are predominantly metamorphic from sedimentary origins. This means that Senja is an excellent location to try and understand how the difference in origin of a metamorphic rock can change its characteristics, and it’s easy to see. One side of the island is characterized by lots of outcropping bedrocks and steep rock faces plunging near vertically into the ocean while the other side has considerably flatter, gentler terrain. Rock type is a very large factor is the terrain change on Senja as different rock types have different hardness and therefore ability to be eroded by water, ice etc. If you are on Senja in the summer, try to spot the difference!