TheBaltic Shield (orFennoscandian Shield) is a segment of theEarth's crust belonging to theEast European Craton, representing a large part ofFennoscandia, northwesternRussia and the northernBaltic Sea. It is composed mostly ofArchean andProterozoicgneisses andgreenstone which have undergone numerous deformations throughtectonic activity. It contains the oldest rocks of theEuropeancontinent with a thickness of 250–300 km.[citation needed]
The Baltic Shield is divided into fiveprovinces: theSvecofennian andSveconorwegian (or Southwestern gneiss) provinces in Fennoscandia, and the Karelian,Belomorian andKola provinces in Russia. The latter three are divided further into severalblocks andcomplexes and contain the oldest of the rocks, at 3100–2500Ma (million years) old. The youngest rocks belong to the Sveconorwegian province, at 1700–900 Ma old.
Thought to be formerly part of an ancient continent, the Baltic Shield grew in size through collisions with neighbouring crustal fragments. The mountains created by these tectonic processes have since been eroded to their bases, the region being largely flat today. Through five successivePleistoceneglaciations and subsequent retreats, the Baltic Shield has been scoured clean of its overlying sediments, leaving expansive areas (most within Scandinavia) exposed. It is therefore of importance togeophysicists studying the geologic history and dynamics of eastern Europe.
The scouring and compression of the Baltic Shield byglacial movements created the area's many lakes and streams, the land retaining only a thin layer of sandy sediment collected in depressions andeskers. Most soil consists ofmoraine, a grayish yellow mixture of sand and rocks, with a thin layer ofhumus on top. Vast forests, featuring almost exclusively the three species pine, spruce and birch, dominate the landscape, clearly demarcating its boundaries. The soil is acidic and has next to no carbonates such aslimestone. The scouring by the ancient glaciers and the acidity of the soil have destroyed all palaeontologically interesting materials, such as fossils.
The Baltic Shield yields important industrialminerals andores, such as those ofiron,nickel,copper andplatinum group metals. Because of its similarity to theCanadian Shield andcratons of southernAfrica andWestern Australia, the Baltic Shield had long been a suspected source ofdiamonds andgold. Currently, theCentral Lapland Greenstone Belt in the north is considered to be an unexplored area that has the potential to hold exploitable gold deposits.
Recent exploration has revealed a significant number of diamond-bearingkimberlites in theKola Peninsula, and (possibly extensive) deposits of gold inFinland.
Mountains that existed in Precambrian time were eroded into a subdued terrain already during the lateMesoproterozoic, when therapakivi granites intruded.[1] Further erosion made the terrain rather flat at the time of the deposition ofJotnian sediments.[1][2] WithProterozoic erosion amounting to tens of kilometers,[3] many of the Precambrian rocks seen today in Finland are the "roots" of ancient massifs.[1] The last major leveling event resulted in the formation of theSub-Cambrian peneplain in lateNeoproterozoic time.[1][4]
Laurentia andBalticacollided in theSilurian andDevonian, producing aHimalayas-sized mountain range named theCaledonian Mountains roughly over the same area as the present-dayScandinavian Mountains.[5][6] During theCaledonian orogeny, Finland was likely a sunkenforeland basin covered by sediments; subsequent uplift and erosion would have eroded all of these sediments.[7] While Finland has remained buried[7] or very close to sea-level since the formation of the Sub-Cambrian peneplain, some further relief was formed by a slight uplift, resulting in thecarving of valleys by rivers. The slight uplift also means that in places the uplifted peneplain can be traced assummit accordances.[1]
Denudation in the Mesozoic is counted at most in hundreds of meters.[8] Theinselberg plain ofFinnish Lapland is estimated to have formed inLate Cretaceous orPaleogene times, either bypediplanation oretchplanation. Any olderMesozoic surface in Finnish Lapland is unlikely to have survived erosion.[9] Further west, theMuddus plains and its inselbergs formed—also byetching and pediplanation—in connection to the uplift of the northern Scandinavian Mountains in the Paleogene.[10]
The northern Scandinavian Mountains had their main uplift in the Paleogene, while the southern Scandinavian Mountains and theSouth Swedish Dome were largely uplifted in theNeogene.[10][11] The uplift events were concurrent with the uplift ofEastern Greenland.[12] All of these uplifts are thought to be related to far-field stresses in Earth'slithosphere. According to this view, the Scandinavian Mountains and the South Swedish Dome can be likened to a giantanticlinal lithosphericfolds. Folding could have been caused by horizontal compression acting on a thin to thick crustal transition zone (as are all passive margins).[13][14] The uplift of the Scandinavian Mountains resulted in the progressive tilt of northern Sweden, contributing to create the paralleldrainage pattern of that region.[15] As the South Swedish Dome uplifted, this resulted in the formation of apiedmonttreppen and the obstruction of theEridanos River, diverting it to the south.[10]
While being repeatedly covered by glaciers during theQuaternary (last 2.58 million years), Fennoscandia has seen little effect on any changes in its topography from glacial erosion. Denudation during this time is geographically highly variable but averages tens of meters.[8] The southern coast of Finland,Åland and theStockholm archipelago were subject to considerable glacial erosion in the form of scraping during the Quaternary.[16] TheQuaternary ice ages resulted in the glacier's erosion of irregularly distributed weak rock,weathered rock mantles, and loose materials. When the ice massesretreated, eroded depressions turned into the many lakes seen now in Finland and Sweden.[1][17]Fractures in thebedrock were particularly affected by weathering and erosion, leaving as result straight sea and lake inlets.[1]
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