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A five km wide celestial body created Europe’s largest impact structure

A celestial body with a diameter of five kilometres crashed into the Earth’s surface, causing the formation of the so-called Siljan Ring in Dalarna, Sweden. The original impact crater was approximately 60 kilometres in diameter and the bedrock was covered by a layer of sediments 2.5 km thick when the projectile struck, according to a doctoral thesis from Lund University in Sweden.
Shatter cone, Siljan. Photo: Sanna Alwmark
Shatter cone, Siljan. Photo: Sanna Alwmark

Lithosphere geologist Sanna Alwmark has researched the circumstances in which a celestial body impacted the Earth about 380 million years ago. She has also investigated the consequences of the collision on the bedrock.

When the extraterrestrial body collided with the Earth’s surface, it created what is now referred to as the Siljan Ring, a circle of lakes of which the largest is lake Siljan. The area is Europe’s largest impact structure.

Together with colleagues, Sanna Alwmark investigated what is known as shocked quartz. By mapping the extension of shocked quartz, the researchers were able to determine the shock pressure to which the bedrock was subjected in the impact.

“Combining the shock pressure estimates with numerical modelling, we were able to reconstruct the original impact crater, and to determine its size at 60 kilometres in diameter. Our model also shows that the crater was formed by a projectile which was five kilometres in diameter”, says Sanna Alwmark.

Furthermore, in Alwmark’s assessment, the crystalline bedrock in the Siljan area at the time of the impact was covered by a layer of sediments 2.5km thick.

Besides the impact crater in the Siljan area, Sanna Alwmark and her colleagues also located and investigated shocked quartz in the bedrock near the lakes of Målingen in Jämtland and Hummeln in Småland.

“We prove that the two lakes were also formed through an impact”, she says.

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