Rochechouart
Key facts
- Country
- France
- Diameter
- 23 km
- Age
- about 207 million years
- Location
- 45.83° N, 0.93° E
No crater shape remains. Rolling countryside near Limoges hides impact breccias, which appear in quarries, roadside outcrops and the stone of local buildings, including the Château de Rochechouart.
Rochechouart, in the Limousin region of western France, is an impact structure that you cannot see as a crater at all. Erosion has worn the landscape flat, removing the rim and the bowl. Yet the evidence is all around: in the rocks of the fields, the walls of churches and the stones of a medieval castle.
The impact. About 207 million years ago, in the Late Triassic period, an asteroid struck the ancient crystalline rocks of what is now central France. The crater is thought to have been about 23 km wide, although some studies suggest it may have been larger. Erosion over 200 million years has removed the surface features, leaving only the rocks that formed beneath the crater floor.
Rocks made by an impact. The area contains a remarkable variety of impactites, rocks created or transformed by the impact. There are breccias of shattered granite and gneiss, red and green rocks that partly melted, and glassy impact melt rocks. Some are named after local villages, such as the Rochechouart, Chassenon and Babaudus types. The impactor seems to have been rich in iron and nickel, because some of the breccias are enriched in these metals.
Building with breccia. For centuries people used the local stone without knowing its origin. The Château de Rochechouart, a castle dating back to the Middle Ages, is built partly from impact breccia, as are many churches and houses. The Roman baths at Chassenon, an important archaeological site, also used impact rocks. To geologists, the whole district is an open-air museum of impact rocks.
Recognition. The unusual breccias puzzled geologists for a long time, and many thought they were volcanic. In 1969 the French scientist François Kraut proposed that they were caused by a meteorite impact, after recognising shock features in the rocks. Later work confirmed this, including the discovery of shatter cones and shocked quartz.
An extinction suspect? Rochechouart's age is close to the end of the Triassic period, when a mass extinction took place around 201 million years ago. Recent precise dating shows it is several million years older than that extinction, so it is not thought to be the cause.
Visiting. Part of the area is protected as a national nature reserve, created in 2008 to safeguard the impact rocks. The Espace Météorite Paul Pellas in the village of Rochechouart is a small museum that explains the impact and displays local rocks and meteorites. Collecting within the nature reserve is forbidden, and elsewhere the landowner's permission is needed.
Did you know?
Parts of the Château de Rochechouart are built from rock created by the impact.
The crater has been completely flattened by erosion, but its rocks remain.
Part of the impact structure is protected as a national nature reserve.