The great thing about being a geologist is that you often get to travel around the world to do fieldwork or visit a museum or attend a research conference. Sometimes the reverse situation happens – when you are on holiday there is a local geological site that is too good to miss! Recently I attended a wedding celebration in Ghana and was able to take a day trip to Lake Bosumtwi – the site of an impact collision.
Lake Bosumtwi, located in the Ashanti region of south-central Ghana, is a very well preserved impact crater. The Bosumtwi structure is important as it is only one of four impact craters across the Earth to be associated with tektite glass impact ejecta, which has been located in the Ivory Coast some 350 km to the west. Age dating of these tektites and impact glass (in a rock call suevite – named after impact rocks found in Germany at the Ries crater site) suggests that the crater was formed at 1.07 million years ago, which makes Bosumtwi one of the youngest impact craters preserved on Earth. The bolide that formed the crater is thought to have travelled from a north-east direction and hit into ancient 2 billion year old target rocks made of metamorphosed sediments and volcanics. These rocks were excavated, and a large cavity formed with a raised crater rim about 200-300 m high. The crater rim that we see today is about 11 km in diameter (the lake itself is about 8.5 km in diameter), and geophysical profiles reveal that there is a mound about 1.9 km in diameter in the crater centre, which formed when the crater floor was raised up about 130 m during the crater modification stage. This central mound indicates that the structure is a small complex crater.
Ejected rocks were mixed together to form an impact ejecta blanket that covered the surrounding area. There must have been massive devastation for hundreds of km around the impact site from airblast pressure waves, falling superheated ejecta curtains and fall-back deposits, earthquakes and wildfires. Most of the impact rocks have now been eroded away by more recent activity (rainfall, rivers, erosion), but some deposits of these rocks have been located in small regions north and south of the crater. After the crater formed, the rim walls slumped down in places to form collapsed terraces, which are now covered in lush forests and vegetation. After the crater was formed it was infilled with rain water causing a lake to develop that deposited sediments onto the lake floor.
The lake floor was studied by seismic cross sections and drilled by an International Continental Lake Drilling programme in 2004, which sank several drill sites into the lake sediments to allow scientists to study climate records. Two drill holes down to 540 m were also sunk into the deeper buried impact rocks, revealing brecciated rocks and impact melt buried at depth.
Visiting the crater site is easy if you are staying in the city of Kumasi. It is only about an hour drive away to the south-east. The lake is home to several small villages that are dotted around its shoreline. There is a small information centre in Abono village, where you can also hire a boat to take you on a tour into the lake and visit places along the banks. There is a superstition associated with the lake so that people can only fish there using planks of woods as boats. The shoreline is home to many bird species including amazing weaver birds that construct nests from weaving together reeds and grass. Ghana is a beautiful country to visit, and Bosumwti is an impressive reminder of the scale of structure that can be formed by impacting asteroids and comets.
Impact crater formation resources at the Lunar and Planetary Institute