Île Rouleau
Key facts
- Country
- Canada
- Diameter
- 4 km
- Age
- less than about 300 million years
- Location
- 50.68° N, 73.88° W
A small island in Lake Mistassini, Quebec, where rocks are full of breccia dykes and shatter cones. The original crater is almost entirely eroded.
Île Rouleau is a small island in Lake Mistassini, the largest natural lake in Quebec, in Canada. It is the eroded centre of an impact structure that was once about 4 km across.
What can be seen. The island is made of ancient sedimentary rocks of the Mistassini Basin, mainly dolomites and sandstones, which are steeply tilted and broken. Geologists visiting the island found shatter cones, cone-shaped fractures that form only under the shock of an impact, and veins of breccia cutting through the rock. These features show that the island is the central uplift of an impact crater, where rocks from deeper down were pushed upwards.
Erosion. The surrounding crater has been almost completely removed by erosion, including the scouring of ice sheets. What survives is only the deepest part of the structure, now standing above the lake as an island.
Age. The age is poorly known. It is thought to be younger than about 300 million years, but it has not been dated precisely.
Why it matters. Île Rouleau shows that even when a crater has disappeared, its shocked central uplift can survive and reveal the impact. The impactor is unknown.
Visiting. Lake Mistassini is large and remote. The island can be reached only by boat or floatplane, and the lake's weather can change quickly.
Did you know?
The island is the eroded central uplift of an impact crater.
It lies in Lake Mistassini, the largest natural lake in Quebec.
Shatter cones on the island revealed the impact.