Iso-Naakkima is a

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much lar ...

Southern Savonia South Savo (or Southern Savonia; fi, Etelä-Savo; sv, Södra Savolax) is a region in the south-east of Finland. It borders the regions of North Savo, North Karelia, South Karelia, Kymenlaakso, Päijät-Häme, and Central Finland. The total a ...

Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of Bo ...

, about 10 km south of the town of

Pieksämäki Pieksämäki () is a town and municipality of Finland. It is located in the Southern Savonia region, about north of Mikkeli, east of Jyväskylä and south of Kuopio. The town has a population of () and covers an area of of which is water. The ...

. It is notable for having an eroded

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

under the northwestern part of the lake. It is one of the oldest known, about 1200 million years old (900–1200 Ma,

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 ...

). Of the known craters older than a billion years old, it is also one of the smallest being only 3 km in diameter. At the time of the impact event, the continent of

Baltica Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains. The thick core of Baltica, the East European Craton, is ...

(modern northeastern

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...

) was located near the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...

. The crater is filled by recent ( Quaternary)

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

s and is not visible to the surface. Its existence was revealed by strikingly circular

geophysical Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

anomalies which are concentrated in a region 3 km in diameter; the actual remnant is about 2 km wide and 160 m deep. Its existence was later confirmed by

drilling Drilling is a cutting process where a drill bit is spun to cut a hole of circular cross-section in solid materials. The drill bit is usually a rotary cutting tool, often multi-point. The bit is pressed against the work-piece and rotated at ...

results. The discovery was made in 1989 and therefore Iso-Naakkima is the fourth known impact structure in Finland.

References

External links

Iso-Naakkima impact structureIso Naakkima impact structure — Division of Geophysics, University of Helsinki
{{Impact cratering on Earth Impact craters of Finland Kymi basin Proterozoic impact craters Lakes of Mikkeli Lakes of Pieksämäki

See also

References

External links