Iso-Naakkima
Key facts
- Country
- Finland
- Diameter
- 3 km
- Age
- more than about 1 billion years
- Location
- 62.18° N, 27.15° E
Nothing obvious: a small hollow in the forest of Pieksämäki, eastern Finland, filled with sediment and covered with glacial deposits.
Iso-Naakkima is a buried impact crater about 3 km across near the town of Pieksämäki in eastern Finland. It is one of Finland's oldest known craters.
Discovery. The structure first showed up as a round pattern in airborne magnetic and gravity surveys. Drilling in the 1990s revealed a bowl in the crystalline bedrock, filled with sediment and broken rock. Quartz grains in the breccia show planar deformation features, produced only by shock, confirming an impact.
What it hit. The impactor struck granite and gneiss of the Baltic Shield, nearly 1.9 billion years old. The crater was filled with sandstone and siltstone, which protected it from erosion.
Age. The age is poorly known. The sediments filling the crater are thought to be of late Precambrian age, so the impact happened more than about 1 billion years ago, perhaps much earlier.
Why it matters. Iso-Naakkima is one of several small, buried Finnish craters found through geophysical surveys, a reminder that the ancient Baltic Shield may hide many more. The impactor is unknown.
Did you know?
Iso-Naakkima was found through airborne magnetic and gravity surveys.
It is filled with sandstone that protected it from erosion.
The crater may be more than a billion years old.