Summasjärvi

Summasjärvi Impact Crater

The Summasjärvi impact crater, also recognized as the crater within Lake Summanen, is the twelfth impact structure to be identified in Finland. This significant geological feature is located at the center of the lake, which lies in Saarijärvi within the Keski-Suomi region of Finland. The crater itself measures 2.6 kilometers in diameter and possesses a depth of approximately 200 meters, though it is largely obscured by later sediment deposits.

Geological Context and Discovery

The precise age of the Summasjärvi impact is not definitively known. However, it is understood to have formed prior to the Quaternary glaciation, and subsequent to the formation of the surrounding host rock during the Svecofennian orogeny, an event that occurred 1910 million years ago. While some researchers have suggested a Phanerozoic impact, no supporting evidence has been presented in the historical record.

The submerged nature of the crater contributes to its scientific intrigue. The otherwise shallow Lake Summanen (less than 10 meters deep) exhibits a distinct 41-meter central deep or trench precisely at the crater's site. This location is also characterized by a notable circular electromagnetic anomaly, indicating the presence of more conductive materials beneath the surface. This anomaly is thought to be the result of a subsurface lens, likely composed of fractured rock saturated with saline liquids, measuring between 100 to 200 meters in thickness.

The confirmation of Summasjärvi as an impact structure was achieved through geological evidence. The initial inference from the anomaly and deep trench was substantiated by the identification of characteristic impact features: shatter cones and planar deformation features. These crucial indicators were discovered in glacial erratics found adjacent to the crater, providing irrefutable proof of its extraterrestrial origin.