Dellen
Key facts
- Country
- Sweden
- Diameter
- 19 km
- Age
- about 141 million years (older estimates about 89 million)
- Location
- 61.85° N, 16.70° E
Two linked lakes, Norra and Södra Dellen, in Hälsingland, central Sweden, curving round a central island area. The round pattern is clearest from the air.
Dellen is an eroded impact structure about 19 km across in the province of Hälsingland, central Sweden. Its outline is traced by two lakes, Norra Dellen and Södra Dellen, which wrap around a central area of higher ground.
What it hit. The impactor struck ancient granites and other crystalline rocks of the Baltic Shield, nearly 1.8 billion years old. The heat of the impact melted part of the target, and this impact melt rock cooled into a fine-grained rock that was given its own name, dellenite, long before anyone knew how it had formed. Dellenite was chosen as the official provincial rock of Hälsingland.
Proof of impact. Study of the melt rock and the breccias found quartz grains with planar deformation features, microscopic sets of planes made only by shock, which established in the 1960s and 1970s that Dellen is an impact structure rather than a volcanic one.
Age. Dating has given different answers. Older argon dating suggested about 89 million years, in the Late Cretaceous. More recent and more precise dating of the melt rock gives about 141 million years, near the boundary between the Jurassic and Cretaceous periods.
Why it matters. Dellen shows how a rock type can be described and named long before its origin is understood. The impactor is unknown.
Visiting. The lakes are popular for fishing and boating. Ask landowners' permission before exploring outcrops.
Did you know?
The rock melted by the Dellen impact is called dellenite and is Hälsingland's provincial rock.
Two lakes curve around the centre of the structure.
Different dating methods have given ages more than 50 million years apart.