Bornholm Sun Stones May Reveal an Ancient Volcanic Eruption

Thousands of years ago, inhabitants of what is now the Danish island of Bornholm deposited hundreds of unusually carved stones in a ditch, before covering them over.

The function of these so-called ‘sun stones’, and why so many were placed in ditches at once, has long been unclear. However, ancient Greenland ice may now offer an explanation.

Around 4,900 years ago, a volcanic eruption may have been so enormous that it obscured the Sun. In response, people may have ritually sacrificed sun stones in an attempt to bring it back.

“We have known for a long time that the Sun was the focal point for the early agricultural cultures we know of in Northern Europe,” says archaeologist Rune Iversen from the University of Copenhagen.

“They farmed the land and depended on the Sun to bring home the harvest. If the Sun almost disappeared due to mist in the stratosphere for longer periods of time, it would have been extremely frightening for them.”

Bornholm sun stones at the Vasagård complex

Sun stones, known in Danish as “solsten”, have been recovered in substantial numbers from Vasagård, an archaeological site on Bornholm. Used between roughly 3500 BCE and 2700 BCE, the site is believed to have been a religious complex, and more specifically a centre of Sun worship: its entrances align with the Sun during the solstices.

In ditches beside a causeway crossing the complex, archaeologists have unearthed more than 600 complete or broken sun stones. Most are flat, rounded, palm-sized stones, marked with lines spreading out from the centre like the Sun’s rays. Their forms and engraved designs do, however, vary.

Collectively, these objects required many hours of careful engraving. Archaeologists believe such purposeful labour had a spiritual significance connected with the Sun, fertility and growth.

“Sun stones were found in large quantities at the Vasagård West site,” Iversen says, “where residents deposited them in ditches forming part of a causewayed enclosure together with the remains of ritual feasts in the form of animal bones, broken clay vessels, and flint objects around 2900 BCE. The ditches were subsequently closed.”

Evidence for a volcanic event around 2900 BCE

The fact that the stones were concentrated in the same place and period points to a particular event or purpose. Iversen and his colleagues think they may have found evidence of that event in an ice core drilled from the Greenland ice sheet, yearly sediment deposits in ancient lake beds, and tree rings formed at the same time.

A layer of the Greenland ice core laid down around 2900 BCE contains a notable quantity of sulphate. This is a recognised signature of a major volcanic eruption, when ejected material settles on an ice sheet before later layers of ice bury it.

Annual sediment layers from Germany, called varves, reveal two phases of reduced sunlight, one of which occurred notably around 2900 BCE. Meanwhile, tree-ring records from bristlecone pines in the western United States show exceptionally narrow rings from the same period, linked with very cold and dry conditions.

Large volcanic eruptions are known to cause widespread disruption lasting several years, including cooling, diminished sunlight, harvest failures and the famine that can follow. Iversen and his team consider the combined evidence to indicate a link between a volcanic event and the Vasagård sun stones.

“It is reasonable to believe that the Neolithic people on Bornholm wanted to protect themselves from further deterioration of the climate by sacrificing sun stones – or perhaps they wanted to show their gratitude that the Sun had returned again,” he says.

Changes to the Vasagård site after the sacrifice

There is another important indication. During the years after the stones were deposited, the layout of the site altered considerably. Plague was also devastating the region, while large-scale migration across Europe was driving a major cultural transformation.

If the destruction caused by a volcanic eruption had passed alongside these other major upheavals, it is plausible that local people’s changing requirements prompted them to reorganise their communal gathering place.

“After the sacrifice of the sun stones, the residents changed the structure of the site so that instead of sacrificial ditches it was provided with extensive rows of palisades and circular cult houses,” Iversen says.

“We do not know why, but it is reasonable to believe that the dramatic climatic changes they had been exposed to would have played a role in some way.”

The research has been published in Antiquity.