Bosumtwi
Key facts
- Country
- Ghana
- Diameter
- 10.5 km
- Age
- about 1.07 million years
- Location
- 6.50° N, 1.42° W
A nearly circular lake about 8 km across, surrounded by steep, forested crater walls rising more than 250 m above the water, with villages around the shore.
Lake Bosumtwi, in the Ashanti Region of Ghana, fills one of the youngest and best-preserved large impact craters on Earth. It is also a sacred place for the Asante people, and a beautiful one: a round lake about 8 km across, surrounded by steep forested hills, with fishing villages scattered around the shore.
The impact. About 1.07 million years ago an asteroid perhaps 500 m to 1 km across struck the ancient crystalline rocks of West Africa. It formed a crater about 10.5 km wide. Since then rain has filled much of the crater with water. The lake has no river flowing out of it, so its level rises and falls with rainfall and evaporation. Today it is up to about 80 m deep.
Glass from the sky. The impact threw out molten rock that cooled in flight into small pieces of natural glass called tektites. These are found in neighbouring Côte d'Ivoire and are called Ivory Coast tektites. Tiny glass spherules called microtektites, from the same impact, have been found in sediments on the floor of the Atlantic Ocean off West Africa. Their age and chemistry match Bosumtwi, linking them to the crater.
A record of the past. Because the lake has no outlet, its sediments build up steadily, and they record changes in rainfall and climate over a very long time. In 2004 an international drilling project drilled into the lake floor, recovering both sediments and impact rocks. The sediments hold a record of the West African monsoon reaching back hundreds of thousands of years, while the deeper cores revealed how the crater was formed, including its central uplift and the layers of breccia beneath the lake.
A sacred lake. For the Asante people, Bosumtwi is sacred. In tradition it is a place where the souls of the dead come to bid farewell to the god Twi. Customary rules have long governed how the lake is used. Local fishermen traditionally use the padua, a wooden plank on which they sit and paddle with their hands, and for a long time the use of conventional boats on the lake was forbidden.
Visiting. Bosumtwi lies about 30 km south-east of Kumasi and is a popular place to visit, with lakeside guest houses and walking trails. Visitors should respect local customs and the sacred status of the lake. Swimming and canoe trips may be offered locally, but it is important to follow local advice.
Why it matters. Bosumtwi's young age, good preservation and link to a tektite field make it one of the most valuable craters for understanding how impacts work and how they spread material over large areas.
Did you know?
Tektites from the Bosumtwi impact are found in neighbouring Côte d'Ivoire.
Fishermen on the lake traditionally use wooden planks called padua instead of boats.
Sediments in the lake record the West African monsoon over hundreds of thousands of years.