Walking Brännö

The rocks here are rounded, weathered and abundant. Their great bodies rise and fall between the ocean and land. They are a group of rocks that have been heated and compressed, altered so that they are now metamorphic rocks. Their most recent reworking was during the Sveconorwegian orogenesis about 1.59–1.52 billion years ago. Orogenesis is a term that speaks of mountain building, of the great subterranean forces at work as Earth’s tectonic plates converge. 

Brännö is an island in the Gothenburg archipelago. There is a small ferry that will take you from Saltholmen to Brännö.

 

Lithology sourced from SGU:

Metagraywacke: greywacke (sandstone) metamorphosed.

Quartzite: quartz rich sandstone metamorphosed.

Paragneiss: sedimentary rock metamorphosed showing gneissic mineralogy and texture.

Metabasalt: metamorphosed basalt.

(1.6-1.5 billion years)

The sounds of this island have stayed with me. The falling, running water, dripping from leaves and burbling along rivulets. Pathways overflowing with water. The full moon high tide was rising as we walked.

These rocks have a complicated history. Some were sandstones, a sedimentary rock that forms as grains wash, tumble and settle while others were originally basalt, a rock formed from volcanoes. Both have been heated and altered, in some cases the heat has been so intense that the original rock has melted completely forming granite like textures.  

Now, walking across these, exposed, rounded, bodies of rock, you will find evidence of this re-working. Across one rock face are the ripples of heat and pressure, minerals gathered and puckered into ribbons of tight folds. In a few places, mylonite fabrics run in delicate linearity marking moments of dynamic recrystallisation. 

As I walked over these rocks, Orthoclase (K-feldspar) stood out in its peachy pink lightness. Its smooth faces speak to its etymology, “Greek orthos - ‘right’ in allusion to the mineral's right angle of good cleavage”. The whole landscape was harmonised with this warm colour. As the sun set, this was enhanced so that the whole island was thrumming in a soft apricot luminosity. I noticed that I was, despite the fierce westerly wind, feeling rested and at ease here. The colours, textures and forms of this outcropping rock and its grasses reflected time spent in the elements, shaped and formed together.  

For a deeper exploration of the geology take a look at this great post by Wes Gibbons on the nearby island of Vrångö.