Unique Lappajärvi – Europe’s largest impact crater lake
• the most famous and the most thoroughly studied impact crater in Finland.
• the only crater with impact melt rocks exposed in outcrops.
• one of only two craters in Finland with a preserved topographic rim.
Lappajärvi impact crater is:
• the largest impact crater lake in Europe.
• the youngest impact crater in Finland (about 76 million years old).
• the second largest impact crater in Finland (diameter about 22 km).
• the first identified impact crater in Finland (in 1968).
• the best-preserved impact crater in Finland; the southeastern rim (Lakeaharju–Pyhävuori), the central uplift (Kärnä Island), and impactites are exposed and accessible.
• the longest (since 1858) and most thoroughly studied impact crater in Finland: almost all evidence of impact discovered, including the first impact diamonds in Fennoscandia (in 1998).
The story of the impact crater lake – when the sky fell down
76 million years ago, western Finland had been eroded to a flat continental region. The dinosaurs still ruled the Earth.
One day, the orbit of an ordinary stony asteroid took it from the asteroid belt between Mars and Jupiter and put it on a collision course with the Earth. Its speed was over 60,000 km/h and it had a diameter of about 1.6 km. The impact released energy equivalent to that of over 17 million atomic bombs like the one that destroyed the city of Hiroshima. The depth of the newly-formed Lappajärvi crater was about 750 m and its diameter was about 22 km.
Picture 1: The diameter of the Lappajärvi asteroid – about 1600 m – compared to man-made landmarks around the world.
If the Lappajärvi impact occurred today, it would affect all of Finland. The towns of Lappajärvi and Vimpeli would disappear and the town of Alajärvi would be covered by 50 m of ejected rock debris. One hundred km due west, in the city of Vaasa, heat from the fireball would ignite forests and cause third degree burns. A magnitude 8.0 earthquake would collapse the buildings. About 50 cm of ejecta would cover the ruins. The air blast, arriving 5 minutes after the impact, would flatten anything that was still standing.
In Helsinki, some 340 km away, the fireball would cause second degree burns, dust-like ejecta would amount to a layer 1 cm thick, and the earthquake would cause moderate damage to buildings. The hurricane-strength air blast would arrive in 15 minutes, tearing off the roofs and toppling 30% of the trees.
Picture 2: Crushed rock debris known as the ejecta covered the region surrounding Lappajärvi in a thick blanket. Nowadays all of the ejecta have been eroded away.
A unique combination: the rim, the central uplift, and the impactites
The Lappajärvi crater is surrounded by an uplifted rim, which can still be experienced on the hiking trails of Lakeaharju and Pyhävuori. In addition to the rim, the centre of the crater was also uplifted. Nowadays this central uplift is known as Kärnä Island.
Picture 3: Kärnäite is a rock type that was created by melting during the impact.
A new melted and mixed rock type created in the impact – kärnäite – has protected Kärnä Island from subsequent erosion. Kärnäite can be found in outcrops as well as countless boulders on the island.
Another type of impact rock or impactite, known as suevite, is a fragile mixture of crushed and melted fragments of the ancient bedrock. Suevite can only be found as boulders on the southern and southeastern sides of the crater.
Lappajärvi is a magnificent place
Impact crater researchers around the world are well-aware of the exceptional nature of the Lappajärvi crater. Impact craters are so rare on the Earth that each of the about 185 identified impact craters is unique in its own way. However, most of them are so old, eroded, covered, or remote that hardly anything is known about them. Instead, much is known about the youthful Lappajärvi, although for example Kärnä Island and the crater rim still possess a number of secrets. Impact craters that provide beautiful views, such as that of Europe’s largest impact crater lake in the rim region in Lakeaharju and Pyhävuori, are rare indeed. Also, where other than in Lappajärvi can one drink ground water that originates from the crushed bedrock of the floor of an impact crater?
Globally, nature- and geotourism are on the rise. While rambling the craters of the world, I have encountered much lesser holes in the ground where an impact crater has been embraced as the unifying theme in the region and utilized in sustainable geotourism projects. I believe it’s now time to make the unique history and the natural beauty of Lappajärvi much more widely known, not only throughout Finland, but also globally.
Teemu Öhman, Ph.D.
impact crater researcher, planetary geologist