Keurusselkä is a

lake A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from ...

Central Finland Central Finland (; ) is a Regions of Finland, region ( / ) in Finland. It borders the regions of Päijät-Häme, Pirkanmaa, South Ostrobothnia, Central Ostrobothnia, North Ostrobothnia, Northern Savonia, North Savo, and Southern Savonia, South S ...

between the towns of

Keuruu Keuruu (; ) is a List of cities and towns in Finland, town and municipalities of Finland, municipality of Finland. It is located in the provinces of Finland, province of Western Finland and is part of the Central Finland regions of Finland, regio ...

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 (1570–1 ...

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

impact structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an impact crater is the surface e ...

on the western shore of the lake.

Etymology

Keurusselkä was originally only the name of the largest

fjard A fjard (, ) is a large open space of water between groups of islands or mainland in archipelagos. Fjards can be found along sea coasts, in freshwater lakes or in rivers. Fjard and fjord were originally the same word, and they generally meant sa ...

of the lake, while the lake was known as ''Väärinkeuru'' or ''Keuruvesi''. The element ''keuru'' is a dialectal word meaning "crooked", in this context referring to the shape of the lake. The older names were displaced by the name Keurusselkä in the 19th century. The name of Keuruu is derived from the lake's name.

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 conside ...

caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...

, , from the

Chernobyl disaster On 26 April 1986, the no. 4 reactor of the Chernobyl Nuclear Power Plant, located near Pripyat, Ukrainian Soviet Socialist Republic, Ukrainian SSR, Soviet Union (now Ukraine), exploded. With dozens of direct casualties, it is one of only ...

fallout Nuclear fallout is residual radioactive material that is created by the reactions producing a nuclear explosion. It is initially present in the radioactive cloud created by the explosion, and "falls out" of the cloud as it is moved by the ...

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

Olkiluoto The Olkiluoto Nuclear Power Plant (, ) is one of Finland's two nuclear power plants, the other being the two-unit Loviisa Nuclear Power Plant. The plant is owned and operated by Teollisuuden Voima (TVO), and is located on Olkiluoto Island, on t ...

and

Loviisa Loviisa (; ; formerly Degerby) is a town in Finland, located on the southern coast of the country. Loviisa is situated in the eastern part of the Uusimaa region. The population of Loviisa is approximately , while the Loviisa sub-region, sub-regi ...

, which host the only two

nuclear plants A nuclear power plant (NPP), also known as a nuclear power station (NPS), nuclear generating station (NGS) or atomic power station (APS) is a thermal power station in which the heat source is a nuclear reactor. As is typical of thermal power st ...

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

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

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 estuari ...

saline water Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish wat ...

. 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 colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown lead ...

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 freshwater fish from the genus ''Perca'', which belongs to the family Percidae of the large order Perciformes. The name comes from , meaning the type species of this genus, the European perch (''P. fluviatilis'') ...

(''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 structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

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 ran ...

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's Southern Ostrobothnia region. It is located from Seinäjoki, from Kokkola and from Vaasa. The municipality has a population of () and covers an area of of which , or near ...

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 Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

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 ori ...

) in

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

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 (phanerite, phaneritic) intrusive rock, 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 coo ...

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

of the Central Finland Granite Complex is 1.88 Ga (

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

). Since the impact, 7–8 km of rock has eroded away, leaving no visible depressions (compare, in contrast,

Iso-Naakkima Iso-Naakkima is a lake in Southern Savonia, Finland, about 10 km south of the town of Pieksämäki. It is notable for having an eroded impact crater under the northwestern part of the lake. It is one of the oldest known, about 1200 million yea ...

Lumparn Lumparn (fi. ''Lumpari'') is a large bay devoid of islands in the Fasta Åland, main island of Åland, Finland, bordered by Sund, Åland, Sund to the north, Lumparland to the east, Lemland to the south and Jomala to the west. Description Most of ...

). Sparse gravity data shows a negative anomaly (an area of lower

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

) 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 In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

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 Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...

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 In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

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 via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

data.

Paleomagnetic Paleomagnetism (occasionally palaeomagnetism) is the study of prehistoric Earth's magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain ...

data will be collected to define the age of the structure. Only some east from the centre of the Keurusselkä impact structure is

Karikkoselkä Karikkoselkä is a lake formed in an impact crater in Petäjävesi, Finland. Karikkoselkä is located in Central Finland approximately 30 km east from the centre of Keurusselkä, a much older and larger impact crater. Most lakes in the regi ...

, 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

Etymology

Environmental issues

Keurusselkä impact structure

See also

References

External links