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.