4.1 Overview of the Bosumtwi Impact Crater
The Bosumtwi Impact Crater is 1.07 Myr old and, therefore, one of the youngest moderately sized impact craters on Earth. The impact occurred in the 2.1 to 2.2 Gyr Proterozoic Birimian Supergroup, which is intruded by granitic and granodioritic bodies. The crater is classified as one of the best-preserved impact structures on Earth. The crater (06°30‘N, 01°25‘W) is 30 km southeast of Kumasi, the second-largest city in Ghana. It is a complex impact crater with three distinct rings. The 10.5 km diameter inner crater is filled with a lake 8 km in diameter and 75 m deep at its centre (Boamah and Koeberl, 2007 ). The inner rim of the crater rises 200 to 300 m above the lake level and is covered by ejecta blankets (Theilen-Willige, 2021 ). An outer rim also occurs 18-20 km from the centre of the structure. Also, in the centre of the Bosumtwi impact crater exists a central uplift with a diameter of 1.9 km (Reimold and Koeberl, 2014 ). The lake in the impact crater has no outlet and is mainly controlled by rainfall and the rate of evaporation from the surface (Turner et al., 1996 ).
As far back as 1899, people wondered about the origins of the Bosumtwi Impact Crater after locals complained about a two-year cycle of sudden bubbling on the lake accompanied by a strong sulphurous smell (Jones, 1985 ). This phenomenon and the crater depression indicated volcanic origins to scientists at that time. Due to the lack of evidence of volcanic activity in the observed rocks, this theory was not generally accepted. The first suggestions of a meteorite impact origin were first promulgated in 1931 by Maclaren . He categorically ruled out volcanic origins since no evidence could be found to support such a claim. He then proposed meteorite impact as the cause of the crater. Maclaren(1931 ) also arguably conducted the first geophysical survey using a prismatic compass and dip compass to make magnetic observations. He observed fluctuations in inclination and magnetic intensity, implying the existence of a magnetic body. But volcanic origin theory remained an alternative theory until the second half of the 20thcentury. In the 1960s, a series of expeditions to the Bosumtwi crater resulted in samples being sent to the United States for laboratory investigations (Jones, 1985 ).
In addition, a swarm of tektites were discovered in Cote D’Ivoire and associated with the Bosumtwi impact crater. The laboratory investigations were mainly based on comparing the ages and compositions of the tektites from Cote D’Ivoire to glass samples in tuffaceous rocks and other rock samples from Bosumtwi. Using techniques such as the K/Ar technique, fission track dating, Rb/Sr technique, and XRF analyses, the content of radioactive elements (U, Th, K), O18/O16 ratio analysis large body of evidence was accumulated showing similarities between the Cote D’Ivoire tektites and the rocks samples from Bosumtwi. These data reinforced the impact crater theory based on the tektites being derived from ejecta originating from the Bosumtwi crater. With the meteorite impact crater theory now firmly established, subsequent research from the 1970s onwards (Jones, 1985; JONES et al., 1981; Palme et al., 1978)focused on identifying other characteristics reminiscent of impact craters.