00 Porten til geologi

01 Alta’s bedrock

Alta’s bedrock consists of three layers, at the bottom is the bedrock. Age: 600-2,400 million years old. Origin: The bedrock was created when melted rock – magma – hardened, either in the earth’s interior (igneous rock) or on earth’s surface after volcano eruptions (volcanic rock), and by sediments deposited on these rock types. Condition: Greatly transformed and deformed by heavy pressure and high temperatures. Location in Alta: The inner Alta fjord and the Finnmark mountain plateau.

02 The Dividal group

On top of the bedrock is the Dividal group. Age: 400-600 million years old. Origin: Sediments such as gravel, sand, silt, and clay that have turned into stone. Condition: Sediments, such as gravel, sand, silt, and clay which has been fossilised. Location in Alta: It would have covered the whole of Norway, Sweden and Finland. Today only a narrow strip runs through Scandinavia, from Finnmark in the north to central Sweden in the south. It has eroded in the east and is covered by overthrusts in the west. In Alta the Dividal rock types can be found in Kviby, Russeluft, Kåfjord and Sautso.

03 Overthrusts

At the top are the overthrusts. Age: 510-600 million years – older than the Dividal group even if they are above them. Origin: Rock types with origin from the Baltic Shield, Laurentia, the sea floor or microcontinents, such as Iceland, between the ancient continents. Overthrusts were pushed from the northwest toward present-day Norway when the continents Baltika and Laurentia collided, creating the Caledonian mountain range 400-500 million years ago. The lower overthrusts only travelled a short distance, while the upper ones came from afar and are greatly transformed. Location in Alta: The Alta River valley and the mountains of the outer Alta fjord.

04 Geological treasure

The coastal islands Seiland, Stjernøya and Sørøya in west Finnmark are rare gems on Norway’s geological map. 550-580 million years ago, unusual rock types with beautiful crystals were created deep in the earth’s interior.

A – Albite, Seiland
B – Gypsum, Øksfjord
C – Citrine, Kvalsund
D – Quartz, Langfjord
E – Sillimanite, Sørøya
F – Diopside, Seiland
G – Sodalite/Biotite, Sørøya
H – Kyanite, Sørøya
I – Apatite/Albite, Seiland
J – Amethyst, Sørøya
K – Muscovite/Garnet, Tourmaline, Langfjord
L – Chalcopyrite, Kvalsund
M – Zircon, biotite, and albite, Seiland

05 Quick clay in Alta

In 2020, a quick clay landslide devastated Kråknes, destroying eight houses. Quick clay is common in Norway, and there have been at least three landslides around Alta since 1975. During the last ice age, clay moved from land and was deposited in the ocean. The clay originally contained salt from the seawater. Fresh water eventually rinses out the salt, making the clay less stable. Then, both natural or human intervention can make the clay collapse and become fluid.