It was long thought that the circular Lake Mien in southern Småland was the remains of a volcano. However, in 1910 the geologist Arvid Högbom put forward a ground-breaking idea that the lake could instead be the result of a meteoric impact some 120 million years ago – a theory that has predominated in the research community since the 1960s. Despite a range of discoveries that support this theory, there has long been stubborn resistance to it, mainly from the public. In a new study published in the scientific journal Meteoritics and Planetary Science, however, a team of researchers from Lund University has found decisive proof in support of the meteorite theory. The researchers have discovered a mineral known as reidite that can only be created amidst the extreme pressure of a meteoric impact.
“We investigated hair strand-thin parts of the mineral zircon in rock samples from Mien. It was inside these shocked grains of zircon that we managed to find traces of the high-pressure polymorph reidite, which acts as a kind of time capsule dating back to the impact,” explains Josefin Martell, doctoral student in geology at Lund University.
Reidite is formed under extremely high pressure, around 30 gigapascals. This is comparable to the pressure found 1 000 kilometres below the Earth’s surface. The only natural process that can cause this in the space of a few seconds is a meteoric impact.
“The celestial object caused the bedrock to melt. The crater that was formed was originally some 9 kilometres in diameter. The impact required to form such a crater would be the equivalent of 1 000 atomic bombs,” explains Martell.
The research team at Lund University hopes that the discovery will now draw a line under the discussion about the formation of the pike-filled Lake Mien. Martell thinks it is time to establish Mien once and for all as an “impact structure”.
“The inhabitants of Tingsryd and environs should be proud. There are not many people who have the chance to bathe in an ancient impact structure,” notes Martell.
The study is presented in the scientific journal Meteoritics and Planetary Science.
Study: Shock deformation in zircon grains from the Mien impact structure, Sweden – wiley.com