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. 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 Kokemäenjokiwater 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.
Environmental issues
In 1986, the Keurusselkä region was heavily contaminated by radioactivecaesium,, from the Chernobyl disasterfallout. 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. 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 and perch populations are especially large.
Keurusselkä covers an ancient impact crater remnant, which was discovered in 2003 by amateur geologists. Shatter cones, 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 crater. In addition to the shatter cones, microscopic studies of samples from a breccia boulder have revealed shock metamorphic features, in quartz 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 age of 1.14-1.15 Ga for the impact., which makes Keurusselkä one of the oldest impact structures known in Europe. The age of the localgranitebedrock of the Central Finland Granite Complex is 1.88 Ga. Since the impact, 7–8 km of rock has eroded away, leaving no visible depressions. Sparse gravity data shows a negative anomaly 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 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 Impact Field Studies Group's Impact Database and University of New Brunswick's Earth Impact Database.
