Lappajärvi
Key facts
- Country
- Finland
- Diameter
- 23 km
- Age
- about 78 million years
- Location
- 63.20° N, 23.70° E
Lake Lappajärvi, about 23 km long, fills the eroded crater in western Finland, with the island of Kärnä near its centre.
Lake Lappajärvi, in the region of South Ostrobothnia in western Finland, fills an ancient impact crater. It is one of the largest lakes in the country formed in a crater, and it is surrounded by farmland, forests and small villages.
The impact. In the Late Cretaceous period, when dinosaurs still lived, an asteroid struck the ancient rocks of the Fennoscandian Shield. The crater it made was about 23 km across. Estimates of its age have varied, but recent precise dating suggests about 78 million years. Glaciers during the ice ages scoured the landscape, removing the original crater rim, and the eroded basin later filled with water to form the lake we see today.
Kärnäite. The island of Kärnä, near the centre of the lake, is made of impact melt rock, rock that was completely melted by the energy of the impact and then cooled. This rock has its own local name, kärnäite, after the island. It is dark and fine-grained and contains fragments of the surrounding rocks that were caught up in the melt. Similar melt rocks are found on other islands and parts of the shore.
Recognising the impact. The lake's round shape and the strange rocks of Kärnä attracted attention long before anyone suspected an impact. In the 1960s scientists found shock features in the rocks, confirming that the structure was made by an impact. Studies since then have refined its age and structure, and drilling has explored the rocks beneath the lake.
Precise dating. Lappajärvi has been important for testing dating methods. Different techniques gave quite different ages over the years, from about 73 to 78 million years. Newer measurements using uranium and lead in tiny zircon crystals that were recrystallised by the shock of the impact have given a more precise result of about 78 million years. These zircons are among the most reliable timekeepers in geology.
Finland's craters. Finland has several confirmed impact structures, including Lappajärvi, Söderfjärden, Keurusselkä and Lumparn. Their ancient bedrock and glacial history make the craters hard to spot, often hidden beneath lakes or farmland, but careful research continues to find new ones.
Visiting. Lappajärvi is a popular area for summer holidays, with lakeside cottages, swimming, fishing and boating. In winter the frozen lake is used for skiing and ice fishing. The area has information about the impact, and Kärnä island can be visited by boat. As anywhere, collecting rocks on private land requires permission.
Did you know?
Kärnäite, an impact melt rock, is named after Kärnä island in the lake.
The impact happened during the Late Cretaceous, when dinosaurs still lived.
Tiny zircon crystals from the melt rock have given a precise age of about 78 million years.