Riachão
Key facts
- Country
- Brazil
- Diameter
- 4.5 km
- Age
- less than about 200 million years
- Location
- 7.72° S, 46.65° W
A ring of low hills about 4 km across in the savanna of southern Maranhão, Brazil, with a central uplift of broken sandstone.
Riachão, sometimes called Riachão Ring, is an eroded impact structure about 4.5 km across in the state of Maranhão, north-eastern Brazil, in the savanna known as the cerrado.
What it hit. The impactor struck the Parnaíba Basin, a large area of Palaeozoic and Mesozoic sedimentary rocks, mainly sandstones and siltstones. In the centre, the rebound of the crater floor raised older layers into a small central uplift. A ring of hills marks the eroded rim.
Proof of impact. The round structure was noticed on satellite images and radar in the 1970s. Geologists later found broken and shocked rocks in the central uplift, including quartz grains with features that form only under the pressure of a shock wave.
Age. The youngest rocks affected are thought to be of Triassic or Jurassic age, so the impact happened less than about 200 million years ago. Its exact age is unknown.
Why it matters. Riachão is one of a group of impact structures in the Parnaíba Basin of northern Brazil, along with Serra da Cangalha and Santa Marta. The region's dry climate and sparse vegetation make these structures easy to see from space. The impactor is unknown.
Did you know?
Riachão was first noticed on satellite and radar images.
Two other impact structures, Serra da Cangalha and Santa Marta, lie in the same region of Brazil.
It has a small central uplift of sandstone.