A Lake Born from Catastrophe

To most visitors, Lake Siljan in Dalarna is simply one of Sweden's most beautiful lakes — a celebrated destination for Midsummer celebrations and traditional folk culture. But beneath the scenic surface lies the remnant of a catastrophic asteroid impact that occurred roughly 377 million years ago, during the Late Devonian period. The Siljan crater is the largest confirmed impact structure in Sweden and one of the largest in all of Europe.

The Scale of the Impact

When the impactor struck what is now central Sweden, the resulting crater was enormous. Current estimates place the original crater diameter at approximately 52–65 kilometres, though erosion over hundreds of millions of years has significantly reduced the visible surface expression. What remains today is a roughly circular ring structure, with Lake Siljan occupying the central depression.

The impacting body is estimated to have been several kilometres in diameter. The energy released would have been equivalent to billions of nuclear warheads — an event capable of triggering global environmental consequences.

Geological Evidence of Impact

How do geologists confirm that a circular lake or ring structure is actually an impact site and not a volcanic feature? At Siljan, the evidence includes:

  • Shatter cones: Striated, cone-shaped rock fractures that can only be produced by the extreme shock pressures of hypervelocity impact. These have been found in the bedrock surrounding the lake.
  • Planar deformation features (PDFs): Microscopic shock structures within quartz grains, visible under a petrographic microscope.
  • Breccia: Impact-shattered rock (suevite and other impact breccias) found within and around the structure.
  • Ring topography: The circular arrangement of geological units — uplifted basement rocks forming an annular ridge around the central depression — is consistent with a complex impact crater of this size.

Age and Timing

Radiometric dating places the Siljan impact at approximately 376–377 million years ago, coinciding with the Late Devonian mass extinction event (the Kellwasser events). Some researchers have proposed a connection between the Siljan impact and the environmental stresses of that extinction, though the scientific consensus on causation remains debated. What is clear is that the impact occurred during one of the most significant extinction crises in Earth's history.

The Siljan Oil Drilling Controversy

In the late 1980s, the Siljan crater became the site of one of the most controversial geological projects in Swedish history. Motivated by Thomas Gold's deep-hot biosphere theory — the idea that abiotic (non-biological) oil could exist deep in fractured crystalline rock — a consortium drilled the Gravberg-1 borehole to over 6 kilometres depth within the crater. The project attracted enormous media attention but ultimately found only small amounts of hydrocarbons, insufficient for commercial extraction. A second borehole (Stenberg-1) was similarly inconclusive. The episode remains a fascinating chapter in the intersection of unconventional geology and commercial ambition.

Visiting the Siljan Region

The crater ring is visible in topographic maps and satellite imagery, but from ground level the scale is too large to perceive easily. Key locations for geologically minded visitors include:

  • Mora and Rättvik: Towns situated on the crater rim, offering good lake views.
  • Bergs church area: Exposures of brecciated rock near the lake shore.
  • Local museums: The Dalarna Museum in Falun holds information on regional geology.

The best way to appreciate Siljan's cosmic origin is to combine a map with a drive around the lake's circumference — tracing the ancient ring of Sweden's most dramatic geological heritage.