Lake Lappajärvi

The research history of Lappajärvi is considered to have started in 1858 when Henrik Holmberg mentioned in his book "Materialier till Finlands geognosi" a volcanic rock found in Lappajärvi. In 1916, geologist Eero Mäkinen analyzed the rock and identified it as dacite, a type of lava rock.[16][17] After his studies on Lake Yanisyarvi in Karelia in 1920, geologist Pentti Eskola became convinced that Lappajärvi was also a volcanic lake.[18] In the summer of 1926, Eskola visited Lappajärvi and confirmed that the kärnäite rock was volcanic in origin.[19] Lappajärvi was long believed to be the remnant of an ancient volcano, and as late as 1964, Professor Ahti Simonen wrote about the lake's ancient volcanic activity.[20]

In the spring of 1967, after studying the Nördlinger Ries crater in Germany, Professor Thure Georg Sahama began to suspect that Lappajärvi was also a meteorite crater and suggested that Martti Lehtinen investigate it.[21] In 1967, Swedish researcher Nils-Bertil Svensson observed that quartz in kärnäite was fragmented in the same way as in other craters on Earth. Svensson's findings were published in the journal Nature in February 1968.[22] In 1967, Martti Lehtinen started his investigations of the crater and discovered suevite in the Hietakangas gravel pits in July 1967. In the summer of 1968, he found coesite in Lappajärvi. His licentiate thesis on the research was completed the following year.[23] In 1976, Lehtinen published an English-language doctoral dissertation on the meteorite theory, which included evidence of quartz transformations in certain rock samples that could not have resulted from volcanic activity. The dissertation is considered the final confirmation of the meteorite theory.[24] Gravity measurements conducted in the same year also supported the meteorite theory, as a gravitational anomaly was detected within a 17-kilometer diameter area.[25]

In 1980, German researchers Elmar Jessberger and Uwe Reimold used the argon–argon dating method to determine the age of the crater to be approximately 77.3 million years.[26]

From 1988 to 1990, the Geological Survey of Finland conducted drilling studies, with the first one reaching a depth of 217.75 meters in Härkäniemi, where kärnäite was found to extend from the surface to the depth of 145 m, followed by a suevite layer and a breccia layer.[27][28][29] The second drilling reached a depth of 165.75 meters in Pokela, Vimpeli.[30] The fourth drilling reached a depth of 275 meters in the intermediate island between Kärnänsaari and Matalasaari, but no kärnäite was found.[31]

In 1992, paleomagnetic methods estimated the age of the crater to be around 195 million years.[32] In 1997, small impact diamonds were discovered in suevite boulders from Lappajärvi.[33] In 2001, uranium-lead dating by the Geological Survey of Finland estimated the age of the crater to be around 73.3 million years, with a margin of error of five million years, confirming that Lappajärvi was formed during the Late Cretaceous epoch.[34] In a study published in 2013, argon–argon dating yielded an age of 76.2 million years for the crater. The study also revealed that some minerals were approximately one million years younger, suggesting that the cooling of the crater took place over a period ranging from one hundred thousand to one million years.[35] In the summer of 2018, researchers proposed an age of 77.8 million years based on uranium-lead dating.[36] After peer review, the age was revised to approximately 77.85 (± 0.78) million years.[37]