Summasjärvi

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Summasjärvi, also known as Summanen, is a lake in Finland, near Saarijärvi in the Keski-Suomi region. On the island of Summassaari there is a Stone Age Museum. In the middle of the lake lies Finland’s 12th known impact crater. The crater is about 2.6 kilometers wide and up to around 200 meters deep, though most of it is buried under younger sediments. The lake around the crater is generally shallow, under 10 meters, but the crater area has a deep central trench about 41 meters deep. The age of the crater is unknown: it formed before the Quaternary glaciation, but after the bedrock formed during the Svecofennian orogeny about 1.9 billion years ago. Some scientists (Plado and colleagues) have suggested it could be Phanerozoic, but there is no solid evidence. The crater site also shows a circular electromagnetic anomaly, likely from a thick underground lens of fractured rock containing saline liquids. The impact origin is supported by shatter cones and planar deformation features found in nearby glacial rocks.