Kaali craters

Why this is extraordinary

At Kaali, on the island of Saaremaa, nine craters mark where a meteorite broke up and hit. The main crater is about 110 metres across and holds a pond. Eight smaller ones sit nearby. The rock is Silurian dolomite. People were already on Saaremaa when the stones fell, which is why the date matters twice: once for the impact, and once for whether anyone could have told it onward.

The clocks do not agree, and the disagreement is about 6,000 years. A 2016 study dated charcoal of spruce twigs roasted against hot ejecta, the thrown rock, in the blanket just outside the crater. Because the charcoal is in the ejecta, it belongs to the impact, and because it is wood it is not poisoned by old carbon from the dolomite. Eleven samples together give 3,237 plus or minus 10 radiocarbon years, which calibrates to 1530–1450 BCE. The authors call that a terminus post quem, the earliest the crater can have formed: the twigs were alive until then, and the impact followed soon after. An earlier study of the mud inside the main crater, on terrestrial plant remains, put the impact at 1690–1510 BCE. Those two are close. They are not the only answers. An iridium-rich layer in the Piila bog, several kilometres away, was dated 400–370 BCE and read as the impact’s fingerprint. Iridium is a marker of meteoritic material, and the layer is real. Its link to this crater is the argument, because the foreign grains were not traced grain by grain back to Kaali. At the old end, microspherules in a gravel pit were dated around 6400 BCE, and luminescence ages have been published from roughly 2300 BCE back to 7850 BCE, with one preferred age near 5150 BCE. Dates on organic matter inside the crater have also been suspected of a reservoir effect: dolomite is full of ancient carbon, and a lake that dissolves it can make a sample look too old. The ejecta charcoal was designed to avoid that trap. It has not ended the bog and the spherule arguments.

The human trace on the rim is later than the 1530 BCE charcoal. Remains of the 8th to 3rd century BCE on the rim have been read as a fortified settlement or an enclosed cult place. A stone wall around the lake, hundreds of metres long, belongs to that later attention. The impact did not, in the 2016 paper, leave a clear change in the island’s tools or in where people lived. A spectacular event and an unchanged archaeology can both be true.

A run of songs in the Kalevala tells of a spark that falls, of a new sun attempted in the sky, and of water rising in a lake, with the fireball seen off to the east of the singers’ country, toward Estonia from Karelia. One reading says that is Kaali, remembered. The songs were collected in the 19th century. They do not name Saaremaa. A spark and a risen lake are a possible description of an impact in a settled landscape, and they are also a possible description of other fires. The date you pick for the crater decides whether the songs would be remembering a recent thunder or a disaster already two thousand years behind the people who built the rim wall. No line in the epic is a radiocarbon date.