Lumparn
Key facts
- Country
- Finland
- Diameter
- 9 km
- Age
- about 1 billion years
- Location
- 60.15° N, 20.10° E
Lumparn, a broad bay in the main island of the Åland archipelago, south-west Finland, about 9 km across.
Lumparn is a bay in the Åland Islands, between Finland and Sweden, which fills an impact structure about 9 km across.
Discovery. The bay was long thought to be a rift, a hollow formed where the crust has pulled apart. An impact origin was proposed in 1979 and confirmed in 1993, when geologists found long shatter cones, cone-shaped fractures produced only by shock, in the south-western part of the bay. The structure has been drilled.
What it hit. The impactor struck rapakivi granite, a distinctive granite with large round crystals of feldspar, typical of Åland. The impact crushed and broke the granite.
Fossils in a crater. The crater floor is covered by Ordovician limestones, which contain fossils of sea creatures, and these are covered in turn by much younger glacial sediments. The limestones survive only because they are protected inside the crater, making Lumparn one of the few places in Finland where fossils of this age can be found.
Age. The age is poorly known. The impact happened after the granite formed, about 1.58 billion years ago, and before the limestone was deposited. It is often given as about 1 billion years.
Why it matters. Lumparn shows how a crater can act as a trap, preserving rocks that have been eroded away everywhere else. The impactor is unknown.
Did you know?
Lumparn was long thought to be a rift before shatter cones proved an impact.
Ordovician fossils survive inside the crater, unlike almost anywhere else in Finland.
The impact struck rapakivi granite, the typical rock of Åland.