Söderfjärden
Key facts
- Country
- Finland
- Diameter
- 6.6 km
- Age
- about 520 to 640 million years
- Location
- 63.00° N, 21.57° E
A strikingly round, flat field about 6 km across near Vaasa, on Finland's west coast, ringed by low forested hills. It was once a bay of the sea.
Söderfjärden is an impact structure about 6.6 km across near the town of Vaasa, on the west coast of Finland. Today it is a remarkably round, flat area of farmland surrounded by a ring of low wooded hills, one of the most visible craters in Europe.
From sea to field. After the last ice age, the land in this part of Finland has been rising as the Earth's crust rebounds from the weight of the ice, by nearly a centimetre a year. Söderfjärden was once a round bay of the sea. As the land rose, it became a shallow lake, which was drained in the twentieth century to create farmland.
What it hit. The impactor struck crystalline rocks of the Baltic Shield. The crater was then filled with Cambrian sediments, which protected it.
Proof of impact. Drilling found broken rock and sediment beneath the floor, and quartz grains with planar deformation features, produced only by shock, confirmed an impact.
Age. The Cambrian sediments filling the crater show that it formed before about 520 million years ago. It is usually thought to be roughly 600 million years old.
Visiting. A visitor centre and observation tower explain the crater. The area is also a well-known stopover for migrating cranes in spring and autumn.
Did you know?
Söderfjärden was once a bay of the sea, raised above sea level as the land rebounded after the ice age.
Its flat, round floor is farmland, drained in the twentieth century.
Thousands of migrating cranes stop at the crater each year.