Siljan
Key facts
- Country
- Sweden
- Diameter
- 52 km
- Age
- about 377 million years
- Location
- 61.03° N, 14.87° E
A ring of lakes, including Siljan and Orsasjön, about 50 km across in the Dalarna countryside, surrounding a raised central area of older rocks.
Siljan, in the heart of Sweden's Dalarna province, is the largest known impact structure in western Europe. It is also one of the most beautiful, a ring of lakes and forested hills that most visitors enjoy without ever realising they are inside a crater.
The impact. About 377 million years ago, in the Late Devonian period, an asteroid struck what was then a shallow sea over the ancient bedrock of Scandinavia. The crater it formed may originally have been 65 km or more across. Erosion has since worn away the original rim and much of the surrounding rock. Today the visible ring structure is about 52 km wide.
A ring of lakes. In the centre of the structure, ancient granite and other old crystalline rocks were pushed upwards by the impact to form a raised central plateau. Around it, in a ring-shaped depression, lie younger sedimentary rocks, limestones and shales from the Ordovician and Silurian periods, that dropped down and were protected from erosion. These softer rocks were later carved out by glaciers, and lakes filled the hollows. Lake Siljan, Orsasjön, Skattungen and others together trace a near-circle that is easy to see on a map.
The ring of sedimentary rocks is a treasure for palaeontologists. Elsewhere in central Sweden these layers have long since eroded away, but at Siljan they survived, preserving fossils of marine life such as trilobites, brachiopods and orthocone nautiloids.
Recognition. Siljan's ring shape was noted long ago, and some early geologists suggested volcanic or other causes. In the 1960s and 1970s shatter cones and shocked quartz were found, confirming an impact origin.
The search for deep gas. In the 1980s Siljan became the site of a famous experiment. The astrophysicist Thomas Gold argued that natural gas could form deep in the Earth without coming from ancient plants and animals, and that shattered rocks beneath large impact structures might trap it. Drilling at Siljan reached more than 6 km into the crystalline rocks. Small amounts of hydrocarbons were found, but nothing close to a commercial deposit, and the results did not persuade most geologists. Later exploration companies have also drilled in the area.
Visiting. Siljan is one of Sweden's best-loved regions, with towns such as Mora, Rättvik and Leksand on the lake shores. It is famous for its Midsummer celebrations, traditional crafts and the Vasaloppet ski race, which finishes in Mora. Walkers and cyclists can explore the crater's landscape on public paths, and old limestone quarries reveal the fossil-rich rocks. Sweden's right of public access allows walking in the countryside, but quarries and private land still need permission.
Did you know?
Siljan is the largest known impact structure in western Europe.
In the 1980s drillers bored more than 6 km into its rocks looking for deep natural gas.
Its ring of lakes preserves fossil-rich rocks that have eroded away elsewhere in central Sweden.