Sweden's largest asteroid crater. Drilled 6.8 km deep to test a controversial theory that Earth's crust is soaked with 'deep' oil from space. It was not ancient life.
NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Story by Kathryn Hansen., Public domain, via Wikimedia Commons
An impact of Devonian scale
Some 377 million years ago, an asteroid estimated to be five kilometers wide struck what is now central Sweden. The collision created the Siljan Ring, the largest known impact structure in Europe, with an original estimated diameter of 52 kilometers. The event occurred during the Devonian period, and its force was so immense it deformed the 1.6-billion-year-old granite bedrock. Today, the crater is heavily eroded, its vast outline traced by the arc of Lake Siljan and several other lakes that fill the ring-shaped depression. The impact-fractured Paleozoic sedimentary rocks in the area contain many fossils, showing the geology of the era.
The structure consists of a raised central dome, about 20-30 kilometers in diameter, surrounded by the annular depression now filled by the lakes. Evidence of the extreme shock pressures from the impact is found in the local bedrock in the form of shocked quartz, a mineral transformation that only occurs under immense, instantaneous pressure. The crater's formation deeply fractured the basement rock, a feature that would later attract intense scientific interest for a very different reason.
The deep gas hypothesis
In the 1980s, the Siljan Ring became a testing ground for the controversial abiogenic petroleum theory, championed by astrophysicist Thomas Gold. Gold hypothesized that hydrocarbons are not the product of decomposed biological matter, but are instead primordial substances trapped deep within the Earth during its formation. He argued that this "deep gas" could migrate upwards through the crust. The Siljan crater presented a perfect test site: the impact would have sterilized the rock, eliminating biological precursors, and the extensive network of fractures could be a natural reservoir for upwelling hydrocarbons from the mantle.
This led to the Swedish State Power Board (Vattenfall) funding the Gravberg-1 deep drilling project, which commenced in 1986. The borehole reached a true vertical depth of 6,779 meters (about 6.8 km) into the granite bedrock. While the project did not find commercially viable quantities of gas, it did recover about 80 barrels of black, oily sludge. Gold asserted this was his predicted abiogenic oil. However, most geologists concluded the sludge was contamination from the drilling process, specifically from diesel and lubricants used in the well. The well also revealed highly saline fluids below 6 km and a nearly constant temperature gradient of 16 degrees Celsius per kilometer.
💡Fun Facts
The Siljan impact released energy equivalent to thousands of the largest nuclear weapons ever detonated.
Despite the failure to find cosmic oil, later studies of the drill cores found evidence of deep microbial life that has existed within the crater's fracture system for at least 80 million years.
The lakes that define the crater, including Lake Siljan, are a popular tourism area known for scenery and traditional Swedish culture, like the famous Dala Horse carvings.
Thomas Gold's abiogenic theory is a minority view in geology, but it spurred one of the deepest drilling projects ever into hard crystalline bedrock.
The Siljan Ring is a large, natural geological feature and is publicly accessible at all times.
Admission
Free to visit the area. Individual attractions or activities may have costs.
Accessibility
The region is accessible by car via public roads. The terrain is varied, with forests, lakes, and towns. Hiking trails like the Siljansleden offer extensive access, but terrain can be uneven.
