On the left: the location of the Summanen impact crater in Summasjärvi in Finland among the other 11 impact structures. Right: Summasjarvi lake coastline, marked in red. Topographic heights and depth of the lake are shown in relief: brown - topographic hills, green - flat plains, and blue - deep water. Damaged rocks found on the mainland, southeast of the center of impact
Researchers managed to find an ancient impact crater in Central Finland. In diameter, it extends over 2.6 km and is covered by the Summasyarvi lake, about 9 km south-east of the nearest city of Saariyarvi and 275 km from Helsinki. The age of the impact event and the type of meteorite are still undetermined.
The discovery is based on earlier geophysical surveys of the area by the Geological Survey of Finland. A field trip in 2017 allowed us to extract evidence of an ancient asteroid strike.
For the first time, the geophysical feature of Summanen was identified in the early 2000s. This was done by geologist Joke Vann. The observation was based on aero-electromagnetic data from a low altitude, which revealed a circular electromagnetic resistant anomaly associated with the lake. The theory of meteorite impact received additional confirmation in the summer of 2017, when the Finnish-Estonian team found traces of a meteorite. In the area of the lake, cones of destruction, cracks and brecciated rocks were noticed. Microscopic studies of thin sections of shock rocks confirm the variant of meteorite impact and show a tremendous shock pressure. During the formation of the crater, the diameter was larger than the current 2.6 km, as the erosion gradually decreased in size.
Summanen is the 191st confirmed meteoric impact structure on the Earth’s surface. Most of them were found on continental territories with a small amount of oceanic impacts. Compared to world formations, it is not very large. But in Finland there are now 12 of them, which puts the country in one of the leading positions in the number of impact impacts.
The largest impact structure in Finland is Keurusselkä. It is also located in Central Finland and covers more than 30 km in diameter, and its age is 1,100 million years. The Summanen explosion is much smaller, but led to serious environmental damage.