Keurusselkä
Encyclopedia
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 (131.2 ft). The surface lies at 105.4 m (345.8 ft) above sea level. The lake is 27 km (16.8 mi) 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.
/m²) by radioactive
caesium
, , from the Chernobyl disaster
fallout
. 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
(Esox lucius) and perch
(Perca fluviatilis) populations are especially large.
. Shatter cone
s, 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 (18.6 mi) 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 between 7 –.
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., which makes Keurusselkä 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
). 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.
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 (18.6 mi) 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
.
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
in Central
Central Finland
Central Finland is a region in Finland. It borders to the regions Päijänne Tavastia, Pirkanmaa, Southern Ostrobothnia, Central Ostrobothnia, Northern Ostrobothnia, Northern Savonia and Southern Savonia....
Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
between the towns of Keuruu
Keuruu
Keuruu 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...
to the north and Mänttä
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....
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 (131.2 ft). The surface lies at 105.4 m (345.8 ft) above sea level. The lake is 27 km (16.8 mi) long and is a part of the Kokemäenjoki
Kokemäenjoki
The Kokemäenjoki is a river and its drainage basin 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...
water system
Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
. Keurusselkä gained international publicity in 2004 when a pair of amateur geologists uncovered an ancient impact crater
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
on the western shore of the lake.
Environmental issues
In 1986, the Keurusselkä region was heavily contaminated (70 kBqBecquerel
The becquerel is the SI-derived unit of radioactivity. One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The Bq unit is therefore equivalent to an inverse second, s−1...
/m²) by radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
, , from the Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
fallout
Nuclear fallout
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 shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes...
. In 2003, some fish near Mänttä still had caesium concentrations several times higher than in Olkiluoto
Olkiluoto
The Olkiluoto Nuclear Power Plant is on Olkiluoto Island, which is on the shore of the Gulf of Bothnia in the municipality of Eurajoki in western Finland. It is one of Finland's two nuclear power plants, the other being the two-unit VVER Loviisa Nuclear Power Plant...
and Loviisa
Loviisa
Loviisa is a municipality and town of inhabitants on the southern coast of Finland. About 43 per cent of the population is Swedish-speaking.The municipality covers an area of of which is water...
, which host the nuclear plants
Nuclear power plant
A nuclear power plant is a thermal power station in which the heat source is one or more nuclear reactors. As in a conventional thermal power station the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.Nuclear power plants are usually...
of Finland. This is due to the difference between the uptake of caesium in fresh water
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
and brackish
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...
or saline water
Saline water
Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....
. 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
Northern Pike
The northern pike , is a species of carnivorous fish of the genus Esox...
(Esox lucius) and perch
European perch
The European perch, Perca fluviatilis, is a predatory species of perch found in Europe and Asia. In some areas it is known as the redfin perch or English perch, and it is often known simply as perch. The species is a popular quarry for anglers and has been widely introduced beyond its native area,...
(Perca fluviatilis) populations are especially large.
Keurusselkä impact structure
Keurusselkä covers an ancient impact crater remnant, which was discovered in 2003 by amateur geologistsGeology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
. Shatter cone
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...
s, 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 (18.6 mi) in diameter. This would make Keurusselkä the largest impact structure in Finland surpassing the Lappajärvi
Lake Lappajärvi
Lappajärvi is a lake in Finland, in the municipalities of Lappajärvi, Alajärvi and Vimpeli. It is formed in a wide meteorite crater. The age of the crater is estimated to be 73.3 million years old —the upper Cretaceous period. The lake is part of Ähtävänjoki basin together with Lake Evijärvi that...
crater. In addition to the shatter cones, microscopic studies of samples from a breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
boulder have revealed shock metamorphic
Shock metamorphism
Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorphic effects unequivocally associated with explosions...
features, (planar fractures and planar deformation features
Planar deformation features
Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals , consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain's crystal structure.PDFs are only produced...
) in quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
grains, which formed when the rock underwent extreme shock pressures of between 7 –.
Argon-argon dating
Argon-argon dating
Argon-argon dating is a radiometric dating method invented to supersede potassium-argon dating in accuracy. The older method required two samples for dating while the newer method requires only one...
of a pseudotachylitic breccia
Pseudotachylite
Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals...
from the central uplift of the Keurusselkä impact structure yielded a late Mesoproterozoic
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...
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
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
of the Central Finland Granite Complex is 1.88 Ga (Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
). Since the impact, 7–8 km of rock has eroded away, leaving no visible depressions (compare, in contrast, Iso-Naakkima
Iso-Naakkima
Iso-Naakkima is a lake in Southern Savonia, Finland, about 10 km south of the town of Pieksämäki. It is notable having an eroded Impact crater under the northwestern part of the lake. The crater is one of the oldest known, about 1200 million years old...
, Lumparn
Lumparn
Lumparn is a large bay devoid of islands in the Main Island of Åland, Finland, bordered by Sund to the north, Lumparland to the east, Lemland to the south and Jomala to the west....
).
Sparse gravity data shows a negative anomaly (an area of lower gravity
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
) 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
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
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
Magnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
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
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
instead of the impact event. No specific anomalies have been found from radiometric or electromagnetic
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
data. Paleomagnetic
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
data will be collected to define the age of the structure.
Only some 30 km (18.6 mi) east from the centre of the Keurusselkä impact structure is Karikkoselkä
Karikkoselkä
Karikkoselkä is a lake formed in an impact crater in Petäjävesi, Finland. Karikkoselkä is located approximately 30 km east from the centre of Keurusselkä, a much older and larger impact crater...
, 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 Field Studies Group
The Impact Field Studies Group is a scientific organization emphasizing geologic field research of suspected and confirmed sites of impact craters and impact structures. The group is composed of researchers, professionals and students involved in study of impact sites...
Impact Database and University of New Brunswick's Earth Impact Database
Earth Impact Database
The Earth Impact Database is the authoritative source for information on confirmed impact structures or craters on Earth. It was initiated in 1955 by the Dominion Observatory, Ottawa, under the direction of Dr. Carlyle S. Beals...
.