Keurusselkä is a lake in Central Finland between the towns of Keuruu to the north and Mänttä to the south. It covers an area of 117.3 km2 (45.3 sq mi). Its average depth is 6.4 m (21 ft) with a maximum depth of 40 m (130 ft). The surface lies at 105.4 m (346 ft) above sea level. The lake is 27 km (17 mi) long and is a part of the Kokemäenjokiwater system.[1][2][3] Keurusselkä gained international publicity in 2004 when a pair of amateur geologists discovered an ancient impact crater on the western shore of the lake.[4]
Keurusselkä
Landsat 7 image of the Keurusselkä region. Area where shatter cones have been found is marked with a red ellipse and the suggested impact structure of Ukonselkä is marked with a white circle.
In 1986, the Keurusselkä region was heavily contaminated (70 kBq/m2) by radioactivecaesium, 137 Cs, from the Chernobyl disasterfallout.[2] In 2003, some fish near Mänttä still had caesium concentrations several times higher than in Olkiluoto and Loviisa, which host the nuclear plants of Finland. This is due to the difference between the uptake of caesium in fresh water and brackish or saline water.[2] 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 substances and local sewage. Despite that, the water quality is generally good, and the central part of the lake is in almost pristine condition.[2] The lake is considered good for fishing, and the pike (Esox lucius) and perch (Perca fluviatilis) populations are especially large.[5]
Keurusselkä impact structure
Keurusselkä covers an ancient impact crater remnant,[6] which was discovered in 2003 by amateur geologists.[4]Shatter cones, horsetail-shaped formations in rocks specifically formed in meteor impacts, have been found in an 11.5 km (7.1 mi) 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 10 km (6.2 mi) to as wide as about 30 km (19 mi) in diameter. This would make Keurusselkä the largest impact structure in Finland surpassing the Lappajärvi crater.[3] In addition to the shatter cones, microscopic studies of samples from a breccia boulder have revealed shock metamorphic features, (planar fractures and planar deformation features) in quartz grains, which formed when the rock underwent extreme shock pressures of between 7–35 GPa (1,000,000–5,100,000 psi).[4]
Argon-argon dating of a pseudotachylitic breccia from the central uplift of the Keurusselkä impact structure yielded a late Mesoproterozoic age of 1.14-1.15 Ga (thousand million years) for the impact.,[7] which makes Keurusselkä one of the oldest impact structures known in Europe. The age of the local granitebedrock of the Central Finland Granite Complex is 1.88 Ga (Paleoproterozoic).[3] 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 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.[3]
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 or electromagnetic data. Paleomagnetic data will be collected to define the age of the structure.[3]
Only some 30 km (19 mi) east from the centre of the Keurusselkä impact structure is Karikkoselkä, a much smaller and younger impact crater.
^. www.environment.fi. Finnish Environmental Institute. 2010. Archived from the original on 2011-05-22. Retrieved 2011-05-07.
^ abcdIlus H, Klemola S, Vartti VP, Mattila J, Ikäheimonen TK (2005). (PDF). Archived from the original (PDF) on 2011-07-21. Retrieved 2011-05-07.
^ abcdePesonen L, Hietala S, Poutanen M, Moilanen J, Lehtinen M, Ruotsalainen H (2005). "The Keurusselkä Meteorite Impact Structure, Central Finland: Geophysical Data" (PDF). Retrieved 2011-05-07.
^ abcHietala S, Moilanen J (2004). "Keurusselkä - A New Impact Structure in Central Finland" (PDF). Lunar and Planetary Science XXXV: 1619. Retrieved 2011-05-07.
^. Kalapaikka.net. Archived from the original on 2007-10-08. Retrieved 2006-05-08.
^M. Schmieder; F. Jourdan; S. Hietala; J. Moilanen; T. Öhman; E. Buchner (2009). (PDF). 40th Lunar and Planetary Science Conference: 1028. Archived from the original (PDF) on 2012-10-12. Retrieved 2011-05-07.
External links
University of Helsinki, Division of Geophysics. . Project home page. Archived from the original on 2011-07-20.
keurusselkä, lake, central, finland, between, towns, keuruu, north, mänttä, south, covers, area, average, depth, with, maximum, depth, surface, lies, above, level, lake, long, part, kokemäenjoki, water, system, gained, international, publicity, 2004, when, pai. Keurusselka is a lake in Central Finland between the towns of Keuruu to the north and Mantta to the south It covers an area of 117 3 km2 45 3 sq mi Its average depth is 6 4 m 21 ft with a maximum depth of 40 m 130 ft The surface lies at 105 4 m 346 ft above sea level The lake is 27 km 17 mi long and is a part of the Kokemaenjoki water system 1 2 3 Keurusselka gained international publicity in 2004 when a pair of amateur geologists discovered an ancient impact crater on the western shore of the lake 4 KeurusselkaLandsat 7 image of the Keurusselka region Area where shatter cones have been found is marked with a red ellipse and the suggested impact structure of Ukonselka is marked with a white circle KeurusselkaShow map of FinlandKeurusselkaShow map of EuropeLocationCentral FinlandCoordinates62 11 N 24 41 E 62 183 N 24 683 E 62 183 24 683 Coordinates 62 11 N 24 41 E 62 183 N 24 683 E 62 183 24 683Basin countriesFinlandMax length27 km 17 mi Surface area117 30 km2 45 29 sq mi Average depth6 4 m 21 ft Max depth40 m 130 ft Surface elevation105 4 m 346 ft SettlementsKeuruu Mantta Contents 1 Environmental issues 2 Keurusselka impact structure 3 See also 4 References 5 External linksEnvironmental issues EditIn 1986 the Keurusselka region was heavily contaminated 70 kBq m2 by radioactive caesium 137 Cs from the Chernobyl disaster fallout 2 In 2003 some fish near Mantta still had caesium concentrations several times higher than in Olkiluoto and Loviisa which host the nuclear plants of Finland This is due to the difference between the uptake of caesium in fresh water and brackish or saline water 2 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 substances and local sewage Despite that the water quality is generally good and the central part of the lake is in almost pristine condition 2 The lake is considered good for fishing and the pike Esox lucius and perch Perca fluviatilis populations are especially large 5 Keurusselka impact structure EditKeurusselka covers an ancient impact crater remnant 6 which was discovered in 2003 by amateur geologists 4 Shatter cones horsetail shaped formations in rocks specifically formed in meteor impacts have been found in an 11 5 km 7 1 mi 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 10 km 6 2 mi to as wide as about 30 km 19 mi in diameter This would make Keurusselka the largest impact structure in Finland surpassing the Lappajarvi crater 3 In addition to the shatter cones microscopic studies of samples from a breccia boulder have revealed shock metamorphic features planar fractures and planar deformation features in quartz grains which formed when the rock underwent extreme shock pressures of between 7 35 GPa 1 000 000 5 100 000 psi 4 Argon argon dating of a pseudotachylitic breccia from the central uplift of the Keurusselka impact structure yielded a late Mesoproterozoic age of 1 14 1 15 Ga thousand million years for the impact 7 which makes Keurusselka one of the oldest impact structures known in Europe The age of the local granite bedrock of the Central Finland Granite Complex is 1 88 Ga Paleoproterozoic 3 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 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 3 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 or electromagnetic data Paleomagnetic data will be collected to define the age of the structure 3 Only some 30 km 19 mi east from the centre of the Keurusselka impact structure is Karikkoselka a much smaller and younger impact crater See also EditImpact craters in FinlandReferences Edit Finnish lakes larger than 40 square kilometers www environment fi Finnish Environmental Institute 2010 Archived from the original on 2011 05 22 Retrieved 2011 05 07 a b c d Ilus H Klemola S Vartti VP Mattila J Ikaheimonen TK 2005 137Cs in aquatic organisms in the southern Lake Keurusselka Finland PDF Archived from the original PDF on 2011 07 21 Retrieved 2011 05 07 a b c d e Pesonen L Hietala S Poutanen M Moilanen J Lehtinen M Ruotsalainen H 2005 The Keurusselka Meteorite Impact Structure Central Finland Geophysical Data PDF Retrieved 2011 05 07 a b c Hietala S Moilanen J 2004 Keurusselka A New Impact Structure in Central Finland PDF Lunar and Planetary Science XXXV 1619 Retrieved 2011 05 07 Fishing ground of Keuruu Kalapaikka net Archived from the original on 2007 10 08 Retrieved 2006 05 08 Keurusselka Earth Impact Database Planetary and Space Science Centre University of New Brunswick Fredericton Retrieved 2009 08 17 M Schmieder F Jourdan S Hietala J Moilanen T Ohman E Buchner 2009 A HIGH PRECISION LATE MESOPROTEROZOIC 40Ar 39Ar AGE FOR THE KEURUSSELKA IMPACT STRUCTURE FINLAND PDF 40th Lunar and Planetary Science Conference 1028 Archived from the original PDF on 2012 10 12 Retrieved 2011 05 07 External links EditUniversity of Helsinki Division of Geophysics Integrated Study of Keurusselka Impact Structure Project home page Archived from the original on 2011 07 20 Geology portal Finland portal Retrieved from https en wikipedia org w index php title Keurusselka amp oldid 1061152274, wikipedia, wiki, book, books, library,
