Lake Bosumtwi

Medium sized crater

This crater was created by approximately 500 m wide chondrite (stone) meteorite, which entered the atmosphere with a speed of some 20 km per second. It approached from the east and hit the 2 billion years old rocks (quartzite, phyllite, slate, and granite) with extraordinary force. The extreme kinetic energy of the meteorite caused an explosion which most certainly was felt on the whole of Earth.

It is presumed that in those times western Africa was covered with rainforest. Forest was eliminated in a huge area, millions of lives of animals were erased in a moment and many more perished in coming months.

This explosion created a 10.5 km wide and some 750 m deep crater. This was a complex crater – the physics of the Earth’s crust caused the upheaval of the central part in the crater. This upheaval is covered with lake sediments now but it is elevated some 130 m above the original bottom of the crater.

Such a complex crater is classified as a medium-sized impact crater. Bosumtwi crater is the youngest and most likely – the best-preserved medium-sized impact crater on Earth.

Tektites

Bosumtwi Lake is surrounded by rainforest. In such conditions the soil erosion is faster than, for example, in dry deserts, and due to this crater has eroded and is covered with soil, there are no cliffs exposed. Due to this, the research of this impact crater is not that easy.

The impact origin of the crater was proposed already in 1931, but it was proved only in the 1960s. There were found tektites in Ivory Coast and later – also in Bosumtwi Lake. These pieces of melted rock formed during the impact. Coesite – a variety of silicon dioxide formed by extreme explosions – was discovered here and later were found impactites – rocks shattered by explosion.

Lake and climatic record

The bottom of the crater has been filled by a lake. Currently, Bosumtwi Lake has a diameter of some 8 km, it is up to 80 m deep. At the depth of some 15 meters, the water becomes anoxic.

Bosumtwi Lake currently does not have any inlets or outlets – it is fed only by rain.

Due to isolation here have developed endemic species of fish – a cichlid Hemichromis frempongi (possible subspecies of H.fasciatus) and subspecies of cichlids Tilapia busumana and Tilapia discolor.

The bottom of the lake is covered with up to 310 m thick layer of sediments. These sediments provide very important data about climate history in this part of Africa.

The analysis shows that this part of Africa on a regular basis experiences approximately 30 – 40 years-long droughts. The level of the lake falls per some 10 – 30 meters in such times. These droughts are caused by the changes in temperatures in the Atlantic (Atlantic Multidecadal Oscillation).

During the droughts the bed of the crater is covered with forest, only in the central part remains a smaller lake.

Sometimes though water filled the crater and was flowing out of it.

Climatic record of Lake Bosumtwi shows that approximately 70 000 years ago Africa experienced extreme drought and disappeared almost totally. It is speculated that just a handful of humans survived and in a search of a better environment they left Africa and spread in Eurasia.

The living and the souls of the dead

In some places trunks of giant trees stick out from the lake. This is a testimony of rather recent droughts when the site of the current lake was covered with a forest.

Local legends have recorded this. There is a tale that Ashanti hunter Akora Bompe in the 17th century was chasing a wounded antelope in the forest (where the lake is now). Antelope jumped in the small pool in the center of the crater as if the lake wanted to save the antelope. Since then the lake is named Bosomtwe – "antelope god".

Since then Ashanti settled here and built some 30 villages with more than 70,000 people living around the lake. Each of the villages has its own sacred grove.

Locals consider that this is a sacred lake where souls of the dead come to bid a farewell to the god Twi. Due to local beliefs, it is allowed to fish only from wooden planks (padua) here – it is taboo to touch the water with iron.

The most sacred place, a spiritual center of the lake is Abrodwum Stone. When the fish in the lake is scarce, a cow is sacrificed next to the stone and the body of the animal is thrown in the lake. Locals then rush into the lake to get a piece of the meat.

Lately this does not help anymore. Due to overpopulation and overfishing, fish is scarce in the lake.

References

1. Christopher A. Sholz, Tobias Karp, Robert P. Lyons. Structure and morphology of the Bosumtwi impact structure from seismic reflection data. Meteoritics & Planetary Science 42, Nr.4/5. p.549 – 560. (2007)

Lake Bosumtwi is included in the following article: