Geological heritage of The Kvarken Archipelago

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Breilin, Olli; Edén, Peter; Kotilainen, Aarno; Ojalainen, Jukka; Sipilن, Pekka
Holding Date 08 September 2008

The Kvarken Archipelago is the first World Natural Heritage site in Finland and it is a serial transboundary property with the High Coast in Sweden. The nomination is based on Natural criterion (i) Earth’s History and Geological Features. The Kvarken Archipelago is located in Western Finland, being the narrowest part of the Bothnian Bay (the Baltic Sea) between Finland and Sweden. It forms a submarine sill between the Bothnian Sea and the Bothnian Bay. The deepest strait is situated near the Holmö Island in Sweden where the maximum water depth is 24 m. The Kvarken Archipelago is in the centre of the Fennoscandian glacioisostatic land uplift area, with an overall net uplift rate of approx. 8 mm per year today. This rapid land uplift gains approximately 100 hectares of new land emerging from the Baltic Sea annually. Just after deglaciation, approx. 10 000 years ago, the water depth was nearly 300 meters. At a maintained uplift rate Finland and Sweden will become connected with by a land bridge across the Kvarken strait in about 2 000 years and the Bothnian Bay will then form the largest freshwater lake in Europe. The Quaternary deposits on top of the crystalline bedrock (1,5 - 2,0 Ma) are composed of a variety of moraine formations of glaciogenic origin formed in the end of the last Weichselian glaciation. The crystalline bedrock was eroded to a peneplain already during late Proterozoic.
The major geologic and geomorphologic features, which make the Kvarken Archipelago unique, are the De Geer-moraine fields with thousands of ridges, ribbed moraine fields, hummocky moraines, drumlins, flutings and large boulder-covered areas which form an expanding archipelago. Today the archipelago consists of approx. 7000 islands and islets and a total shoreline of approx. 3000 km. In some areas De Geer and ribbed moraines occurs in the same localities with crossing directions. Striaes from several different flow directions are evidence of the complex ice-flow pattern in the area.
Marine sediments (clay and silt), which cover deeper basins of the seafloor, are mostly eroded by currents and wave action when the land rises above the sea. These landforms of glacial origin are common also on the bottom of the sea. Glaciofluvial formations are scarce in the area and larger eolian deposits can not bee found. Because of the rapid glacioisostatic land uplift geological processes have formed and are still changing the landscape. Shingle fields and flada - glo-lake series are examples of these geological events.
The Kvarken Archipelago is a natural laboratory to represent glacial events from the Weichselian glaciation and present geological processes. The deglaciation history of this area is complex, and combined with the glacial land uplift make the Kvarken Archipelago a unique geoenvironment.