Thegeology of Finland is made up of a mix ofgeologically very young and very old materials. Common rock types areorthogneiss,granite,metavolcanics andmetasedimentary rocks. On top of these lies a widespread thin layer of unconsolidated deposits formed in connection to theQuaternary ice ages, for exampleeskers,till andmarine clay. Thetopographic relief is rather subdued because mountainmassifs were worn down to apeneplain long ago.
Thebedrock ofFinland belongs to theFennoscandian Shield[1] and was formed by a succession oforogenies during thePrecambrian.[2] The oldest rocks of Finland, those ofArchean age, are found in the east and north. These rocks are chieflygranitoids andmigmatiticgneiss.[1] Rocks in central and western Finland originated or were emplaced during theSvecokarelian orogeny.[1] Following this last orogeny,rapakivi granitesintruded various locations of Finland during theMesoproterozoic andNeoproterozoic, especially inÅland and in the southeast.[1]Jotnian sediments occur usually together with rapakivi granites.[3]
Mountains that existed in Precambrian time were eroded into a level terrain already during the Late Mesoproterozoic.[2][4] WithProterozoic erosion amounting to tens of kilometers,[5] many of the Precambrian rocks seen today in Finland are the "roots" of ancient massifs.[2]
As Finland is in the older part of the Fennoscandian Shield, itsbasement rocks are within three of the shield's older subdivisions known as domains: the Kola, Karelian and Svecofennian domains. This subdivision, established by Gaál and Gorbachev in 1987, is based on the differentgeological histories of the domains prior to their final amalgamation 1,800 million years ago.[6]
The extreme northeast of Finland is part of the Kola Domain because it shows considerable affinity with the geology of theKola Peninsula in Russia. AroundLake Inari there areparagneiss,orthogneiss andgreenstone belts. Rocks in this part of Finland are ofArchean andProterozoic age.[7]
To the south and west of Lake Inari lies an elongate and arcuate zone ofgranulite rock known as theLapland Granulite Belt. The belt is up to 80 km wide. The main rocks of the belt aremigmatizedgreywacke andargillites. Studies ofdetritalzircon show that the sedimentaryprotolith of the metamorphic rocks of the belt could not be more than 2900–1940 million years old.[8] The belt hasnorite andenderbiteintrusions ofcalc-alkaline chemistry.[9]
The Karelian Domain, or Karelian Block, makes up most of the bedrock of the northeastern part of Finland[10] extending into nearby areas of Russia.[6] The Karelian domain is made up of acollage of rock formed duringArchean andPaleoproterozoic times.[10][7] The boundary to the Kola Domain is made up of a gently dippingdécollement where theLapland Granulite Belt has been thrust southward over rocks of the Karelian Domain.[11]
Archean rocks in the Karelian Domain are north-southmedium gradegreenstone andmetasedimentary belts. The belts are intruded bygranitoids, usuallymonzogranite andgranodiorite. Besides these belts and intrusions there is alsometasedimentarygneiss formed atintermediate pressures.[6] Along the central part of theFinland–Russia border lies theBelomorian terrane, a subunit of the Karelian Domain thought to have formed by acollision between the Kola Domain with the Karelian Domain in thePaleoproterozoic.[6][11] This collision marked the final merger of both crustal blocks.[12] Rocks of the Belomorian terrane, like thegranitoidgneisses common there, are ofhigh grade.[11]
The Central Lapland granitoid complex covers up much of the interior of northern Finland. These rocks were formed in the final stages of the Svecofennian orogeny and are mostly made up of coarse-grained granites.[13] An alignment of granitoid intrusions southeast ofOulu likely shares the same origin.[14]
Finland's threeophiolitescrop out within the Karelian Domain. These are the Jormua, Outokumpu and Nuttio ophiolite complexes.[15] All of them wereemplaced in Paleoproterozoic times.[16] The Jormua and Outokumpu ophiolites lie parallel to and near the boundary with the Svecofennian Domain.[15] Also towards the border with the Svecovefennian Domain there is a series ofmetamorphosed Archean rocks that are stacked into animbrication pattern.[11]
The southwestern part of Finland is mainly made up of rocks of the Svecofennian Domain orSvecofennian orogen.[10] These rocks are invariably ofProterozoic age. Its boundary with the Karelian Domain (of mixedArchean andPaleoproterozoic rocks) is a northwest-southeast diagonal.[10]Plutonic rocks that formed duringaccretion of volcanic arcs or continental collisions of the Svecofennian orogeny are common in Svecofennian Domain.[17][18] Among these rocks the largest grouping is the Central Finland granitoid complex covering up much ofCentral Finland,Southern Ostrobothnia andPirkanmaa.[18] Granitoids that intruded in the aftermath of the Svercofennian orogeny are common in southern Finland occurring mostly within ca. 100 km of theGulf of Finland orLake Ladoga.[18][19] These so-called Lateorogenic granites are distinguished by usually containinggarnet andcordierite and being accompanied by rather few rocks ofmafic andintermediate composition.[19] Scattered small granitoids crop out within the same zone. Formed 1810–1770 million years ago, these are the youngest granitoids in southern Finland associated with the Svecofennian orogeny.[14]
Jotnian sediments are the oldest known sediments in the Baltic area that have not been subject to significantmetamorphism.[20][21]These sediments are typicallyquartz-richsandstones,siltstones,arkose,shale andconglomerates.[22][20] The characteristic red colour of Jotnian sediments is due to their deposition insubaerial (e.g. non-marine) conditions.[21] In Finland, Jotnian sediments occur in theMuhos Graben near Oulu at the northeastern end of theGulf of Bothnia[23][21] and further south near the coast atSatakunta.[23][24] Jotnian rocks are also found offshore between Finland and Sweden in theGulf of Bothnia and theÅland Sea includingSouth Kvarken.[23][25][26] Known Jotnian rocks at the Åland Sea are sandstones belonging to the informally defined Söderarm Formation. Above these there areUpper Riphean andVendian sandstones and shales.[27] There is evidence suggesting that Jotnian rocks, or even a Jotnianplatform, once covered much ofFennoscandia and were not restricted to a few localities like today.[26][28] The limited geographical extent of Jotnian sediments at present is indebted to theirerosion overgeological time.[26] Sedimentary rocks as old as the Jotnian sediments have a lowpreservation potential.[29]
The distribution of some Jotnian sediments is spatially associated with the occurrence of rapakivi granite.[22] Korja and co-workers (1993) claim the Jotnian sediment–rapakivi granite coincidence at the Gulf of Finland and the Gulf of Bothnia is related to the existence of thincrust at these locations.[3]
Small outcrops of alkaline rocks,carbonatites andkimberlites exist in Finland[30] including the western and southernmost outcrops of thePermian-agedKola Alkaline Province.[31][30] The Kola Alkaline Province is commonly presumed to represent anigneous hotspot created by amantle plume.[32] Carbonatites in Finland have wide range in ages but they all derive from a "well-mixed" portion of theupper mantle. The Siilinjärvi carbonatite complex of Archean age is one of the Earth's oldest carbonatites.[30] All known kimberlites are concentrated near the towns ofKuopio andKaavi. These are grouped in two clusters and includediatremes anddykes.[33]
The youngest rocks in Finland are those found nearKilpisjärvi inEnontekiö (the northwesternmost part of the country's northwesternarm).[35] These rocks belong to theScandinavian Caledonides that assembled inPaleozoic times.[2] During theCaledonian orogeny, Finland was likely a sunkenforeland basin covered by sediments; subsequent uplift and erosion would have eroded all of these sediments.[36] In Finland, Caledoniannappes overlieshield rocks of Archean age.[7] Despite occurring in about the same area the Scandinavian Caledonides and the modernScandinavian Mountains are unrelated.[37][38]
Theice sheet that covered Finland intermittently during theQuaternary grew out from the Scandinavian Mountains.[39][A] By some estimates, the Quaternary glaciers eroded away on average 25 m of rock in Finland,[2] with the degree of erosion being highly variable.[5] Some of the material eroded in Finland has ended up in Germany, Poland, Russia and theBaltic states.[2] Groundtill left by the Quaternary ice sheets is ubiquitous in Finland.[2] Relative to the rest of Finland, the southern coastal areas have a thin and patchy cover of till evidencing a more prominent role of glacial erosion in the area, whereasOstrobothnia and parts ofLapland stand out for their thick till cover.[40][B] The central parts of the Weichsel ice sheet hadcold-based conditions during the times of maximum extent. Therefore, pre-existing landforms and deposits in northern Finland escaped glacial erosion and are now particularlywell preserved.[42] Northwest to southeast movement of the ice has left a field of aligneddrumlins in central Lapland.Ribbed moraines found in the same area reflect a later west to east change in movement of the ice.[42]
During the lastdeglaciation, the first part of Finland to become ice-free was the southeastern coast; this occurred shortly before theYounger Dryas cold-spell 12,700 yearsbefore present (BP). While the ice cover continued to retreat in the southeast after Younger Dryas, retreat also occurred in the east and northeast. The retreat was fastest from the southeast resulting in the lower course of theTornio river in northwest Finland becoming the last part of the country to be ice-free. Finally, by 10,100 years BP, the ice cover had all but left Finland, retreating to Sweden and Norway before fading away.[43] Ice retreat was accompanied by the formation ofeskers and the dispersal of fine-grained sediment deposited asvarves.[2]
As the ice sheet became thinner and retreated, the land began to rise due topost-glacial rebound. Much of Finland was under water when the ice retreated and was gradually uplifted in a process that continues today.[44][C] Not all areas were drowned at the same time and it is estimated that, at one time or another, about 62% has been under water.[45] The maximum height of the ancient shoreline varied from region to region: in southern Finland 150 to 160 m, in central Finland about 200 m and in eastern Finland up to 220 m.[44] Once free of ice and water, soils have developed in Finland.Podzols with till asparent material now cover about 60% of Finland's land area.[46]
| Material | Land surface % | Cultivated soil % | |
|---|---|---|---|
| Till | 53 | 16 | |
| Peat thicker than 30 cm | 15 | 18 | |
| Bare rock | 13 | - | |
| Marine and lacustrinesilt andclay | 8 | 66 | |
| Eskers and glacifluvial material | 5 | ||
Mining for metals in Finland began in 1530 at the Ojamo iron mine[47][D] but mining in the country was minimal until the 1930s.[48] TheOutokumpuore deposit, discovered in 1910, was key to the development of mining in Finland during the 20th century. When Outokumpu opened in 1910 it was Finland's firstsulphide ore to be mined. This mine closed in 1989.[47] Another important Finnish mining resource was thenickel ofPetsamo, which was mined by CanadianINCO from the 1920s onwards. Petsamo and its mines were, however, lost to theSoviet Union in 1944 as result of theMoscow Armistice.[48]
From 2001 to 2004 the number of metallic ores being mined dropped from eleven to the following four:[48]
There are some uranium resources in Finland, but as of 2015[update] no commercially viable deposits have been identified for exclusivemining of uranium.[49] In the Karelian Domain, there are variouslayered mafic intrusions ofearly Paleoproterozoic age that have been exploited forvanadium.[50]
Most of Finland's metallic ores formed in thePaleoproterozoic during theSvecofennian orogeny or during the period of complexextensional tectonics that preceded it.[51]
Non-metallic resources Finland includephosphorus (fromapatite) that has been mined at theSiilinjärvi carbonatite since 1979, the outcrop being originally discovered in 1950.[30] The bedrock of Finland contain various types ofgemstones.[48] The Lahtojoki kimberlite hasgem qualitygarnet and diamondxenocrysts.[52]
Finland has a thriving quarrying industry. Finnishdimension stone has been used historically for buildings in Helsinki andimperial Russia'sSaint Petersburg andReval. Today, the main importers of Finnish stone are China, Germany, Italy and Sweden. The dimension stone quarried in Finland includes granites, such as the wiborgite variety of rapakivi granite, andmarble.Soapstone from Finland'sschist zone is also quarried for use in ovens.[53]
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