A history written in stone (1600–100 million years)

The bedrock in the Geirangerfjord area mostly contains Precambrian gneiss from what was called the western gneiss region. Gneiss is formed by metamorphism – when rock is exposed to high pressure and high temperature. The western gneiss region is a typical example of how the earth’s crust has been forced down into the mantle – more than 150 km – and then to be lifted back up.

The bedrock in the area is the legacy of the many large tectonic periods that have built mountain ranges, swallowed and transformed rocks deep down in the mantle, lifted them back up to the surface where other forces such as air, water and ice have pushed, brushed and polished the landscape into what we can see today.

Old as the hills

Current knowledge shows that the oldest bedrock in the West of Norway must have been formed more than 1650 million years ago. But what has had most influence on how the bedrock in this part of Norway is composed today is the formation of the Caledonian mountain range about 420 million years ago. That is when a western continent (Laurentia) collided with an eastern continent (Baltica) and in the impact zone a mountain range rose up which we see the remains of here in Norway, in Great Britain, East Greenland and the East coast of North America.

Higher than the Himalayas

When the continents collided, the outermost edge of Baltica penetrated deep into the mantle below Laurentia, while the continental plate Laurentia wash pushed over and on top of the Baltic plate. In the impact zone the different rock types in the earth’s crust were folded on top of each other, and an enormous mountain range formed, which geologists think were greater and higher than the Himalaya mountain range is today. Since then this mountain range has been greatly eroded – the highest mountain in the Scandinavian caledonides is today Galdhøpiggen, which stands 2469 metres above sea level. Two thirds of the mountains in Norway are remains of the Caledonian mountain range.

The mountains above us – the rock beneath

On a regional level the Caledonian bedrock can be grouped into sheets of different rock types or rock strata. These formed enormous thrust sheets that were folded over each other and pushed hundreds of kilometres in over the Baltic plate. So the Scandinavian Caledonides are made up of thin widely extended nappes and thrust sheets. To understand how the mountains above us and the bedrock beneath us are created and formed, we have to identify the complex sequence of events leading to the various rock strata. We also have to understand how these strata are composed of rock types that are formed in very different circumstances, and therefore have their own different geological histories.

Gneiss – deep down and back up

The bottom layer of rock is called the western gneiss region. It dominates the region and represents a segment from the previous Baltic shield even if it dates from the proterozoic eon, and is thus about 900–1650 million years old, the rocks are greatly altered and transformed during the Sweconorwegian orogeny about 1000 million years ago and then the Caledonian orogeny about 420 million years ago.

The Sweconorwegian orogeny did not affect the northern part of the western gneiss region north of Geiranger.

The bedrock in the Geirangerfjord area mostly contains Precambrian gneiss from what was called the western gneiss region. Gneiss is formed by metamorphism – when rock is exposed to high pressure and high temperature. The western gneiss region is a typical example of how the earth’s crust has been forced down into the mantle – more than 150 km – and then to be lifted back up.

Spectacular rock

The highest pressure is registered in the coast region near Stad and towards the north east. Evidence of these processes is mostly based on rare finds of minerals such as diamond and coesite (a type of quartz that is formed under high pressure), which are only stable at extremely high pressure.

The bedrock further in the country also shows evidence of high pressure induced metamorphism, and the entire area is known for occurrence of eclogite, a type of rock that is only formed at high pressure. This is a spectacular and rather unusual type of rock, which mainly consists of reddish to pink granite and green pyroxene (omphacite).

Particularly interesting is the local occurrence, in the Geirangerfjord area, of microscopic residue of the mineral coesite, a type of quarts that is formed under very high pressure and moderate heat (700 °C). This has occurred 100 km depth in the earth’s crust. There are also several pockets of olivine rich peridotite over large areas of the western gneiss region.

International interest

The topography of the area combined with the fact that the bedrock contains solid rock formed under high pressure and ultrahigh pressure that are so much out in the open, gives cause for great international interest in carrying out a wide range of research to study the rocks.

Over the bedrock on the Baltic shield there are site specific remains of very thin cover sequence of quartzites and conglomerates overlain by metamorphosed shales and schists.