Keurusselkä is a

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

Central Finland Central Finland ( fi, Keski-Suomi; sv, Mellersta Finland) is a region ( / ) in Finland. It borders the regions of Päijät-Häme, Pirkanmaa, South Ostrobothnia, Central Ostrobothnia, North Ostrobothnia, North Savo, and South Savo. The city o ...

between the towns of

Keuruu Keuruu (; sv, Keuru) is a town and municipality of Finland. It is located in the province of Western Finland and is part of the Central Finland region. The municipality has a population of () and covers an area of of which is water. The popul ...

to the north and

Mänttä Mänttä is a former town and municipality of Finland. It was merged with the municipality of Vilppula to form Mänttä-Vilppula on 1 January 2009. The place name ''Mänttä'' comes from an old house which Tuomas Niilonpoika Mäntsä (1570–1618 ...

to the south. It covers an area of . Its average depth is with a maximum depth of . The surface lies at above sea level. The lake is long and is a part of the

Kokemäenjoki The Kokemäenjoki ("Kokemäki River", sv, Kumo älv) is a river in southwestern Finland. Geography The river originates at Lake Liekovesi in the Pirkanmaa region, and flows to the Gulf of Bothnia at Pori in the Satakunta region. Primary tributa ...

water system A water supply network or water supply system is a system of engineered hydrologic and hydraulic components that provide water supply. A water supply system typically includes the following: # A drainage basin (see water purification – source ...

. Keurusselkä gained international publicity in 2004 when a pair of amateur geologists discovered an ancient

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

on the western shore of the lake.

Environmental issues

In 1986, the Keurusselkä region was heavily contaminated (70 kBq/m²) by

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

caesium, , from the Chernobyl disaster

fallout Nuclear fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and the shock wave has passed. It commonly refers to the radioac ...

. In 2003, some fish near Mänttä still had caesium concentrations several times higher than in

Olkiluoto The Olkiluoto Nuclear Power Plant ( fi, Olkiluodon ydinvoimalaitos, sv, Olkiluoto kärnkraftverk) is one of Finland's two nuclear power plants, the other being the two-unit VVER Loviisa Nuclear Power Plant. The plant is owned and operated by ...

and Loviisa, which host the

nuclear plants A nuclear power plant (NPP) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power stations, heat is used to generate steam that drives a steam turbine connected to a generator that produces e ...

of Finland. This is due to the difference between the uptake of caesium in fresh water and

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estu ...

or saline water. However, the concentration levels are so small that eating the fish is not considered a health risk. Apart from radioactivity, the water quality is also weakened by

humic Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...

substances and local sewage. Despite that, the water quality is generally good, and the central part of the lake is in almost pristine condition. The lake is considered good for fishing, and the

pike Pike, Pikes or The Pike may refer to: Fish * Blue pike or blue walleye, an extinct color morph of the yellow walleye ''Sander vitreus'' * Ctenoluciidae, the "pike characins", some species of which are commonly known as pikes * ''Esox'', genus of ...

(''Esox lucius'') and

perch Perch is a common name for fish of the genus ''Perca'', freshwater gamefish belonging to the family Percidae. The perch, of which three species occur in different geographical areas, lend their name to a large order of vertebrates: the Per ...

(''Perca fluviatilis'') populations are especially large.

Keurusselkä impact structure

Keurusselkä covers an ancient impact crater remnant, which was discovered in 2003 by amateur

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...

Shatter cone Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the rang ...

s, horsetail-shaped formations in rocks specifically formed in meteor impacts, have been found in an wide area, but it is possible that the area containing shatter cones may be only the central uplift of the crater. Weak traces based on digital elevation data suggest possible ring structures from to as wide as about in diameter. This would make Keurusselkä the largest impact structure in Finland surpassing the

Lappajärvi Lappajärvi is a municipalities of Finland, municipality in Finland in Southern Ostrobothnia regions of Finland, region. It is from Lappajärvi to Seinäjoki, to Kokkola and to Vaasa. The municipality has a population of () and covers an area ...

crater. In addition to the shatter cones, microscopic studies of samples from a

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

boulder have revealed shock metamorphic features, ( planar fractures and

planar deformation features Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orie ...

) in

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

grains, which formed when the rock underwent extreme shock pressures of between . Argon-argon dating of a pseudotachylitic breccia from the central uplift of the Keurusselkä impact structure yielded a late

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), ...

age of 1.14-1.15 Ga (thousand million years) for the impact., which makes Keurusselkä one of the oldest impact structures known in Europe. The age of the local

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

bedrock of the Central Finland Granite Complex is 1.88 Ga (

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

). Since the impact, 7–8 km of rock has eroded away, leaving no visible depressions (compare, in contrast, Iso-Naakkima, Lumparn). Sparse gravity data shows a negative anomaly (an area of lower

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

) in the area of the impact structure. However, there are other negative gravity anomalies nearby which are not believed to be related to the impact event. In addition, there are less dense felsic rocks in the western shore of the lake, which can contribute to the gravity low. The anomaly will be mapped in a much greater detail to determine if it is related to the impact event. Airborne magnetic data show a strikingly circular magnetic anomaly, but for a reason not yet known it is slightly east from the gravity low area as well as the region where the shatter cones are found. It is possible that the ring was created for example by a granitic intrusion instead of the impact event. No specific anomalies have been found from

radiometric Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which ch ...

electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

data.

Paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

data will be collected to define the age of the structure. Only some east from the centre of the Keurusselkä impact structure is Karikkoselkä, a much smaller and younger impact crater.

References

External links

* {{DEFAULTSORT:Keurusselka Impact craters of Finland Kokemäenjoki basin Magnetic anomalies Proterozoic impact craters Lakes of Keuruu Impact crater lakes

Environmental issues

Keurusselkä impact structure

See also

References

External links