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Keurusselkä

2007 Schools Wikipedia Selection. Related subjects: European Geography

   Coordinates: 62°8′N 24°36′E
   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.
   Enlarge
   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.

   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 117.30 km². Its
   average depth is 6.4 m with a maximum depth of 40 m. The surface lies
   at 105.4 m above the sea level. The lake is 27 km long and is a part of
   the Kokemäenjoki water system. Keurusselkä gained international
   publicity in 2004 when a pair of amateur geologists uncovered an
   ancient impact crater on the western shore of the lake.

   M/S Elias Lönnrot, one of the two paddle steamers in Finland, makes
   regular tourist cruises on the lake in summer.

Environmental issues

   In 1986, the Keurusselkä region was heavily contaminated (70 kBq/m²) by
   radioactive caesium ( ^137Cs) from the Chernobyl disaster fallout. In
   2003, some fish near Mänttä had still caesium concentrations several
   times as high as 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. 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. The lake is considered good for fishing, and the pike (Esox
   lucius) and perch (Perca fluviatilis) populations are especially large.

Keurusselkä impact structure

   Keurusselkä covers an ancient impact crater remnant, which was
   discovered in 2003 by amateur geologists Jarmo Moilanen and Satu
   Hietala. Shatter cones, horsetail-shaped formations in rocks
   specifically formed in meteor impacts, have been found in an 11.5 km
   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 to
   as wide as ca. 30 km in diameter. This would make Keurusselkä the
   largest impact structure in Finland surpassing the Lappajärvi crater.
   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 ca. 7–35 GPa.

   The age of the crater has not yet been established, but it must be less
   than 1.88 Ga (thousand million years; Orosirian, in the late
   Paleoproterozoic), because that is the age of the Central Finland
   Granite Complex, the local granite bedrock. Since the impact, 7–8 km of
   rock has eroded away, leaving no visible depressions. It is clear that
   the crater is very old, possibly over 1 Ga because the other Finnish
   craters of that age still have some crater depression left (see
   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.

   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.

   Only some 30 km east from the centre of the Keurusselkä impact
   structure is Karikkoselkä, a much smaller and younger impact crater.
   Ukonselkä, a roughly circular lake immediately west from Keurusselkä,
   is listed in the suspected Earth impact site database (SEIS). However,
   no evidence for an impact there has been provided so far.

   Retrieved from " http://en.wikipedia.org/wiki/Keurusselk%C3%A4"
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