Geoscience Reference
In-Depth Information
within the rift, currently monitored rates can be highly variable in time and space,
ranging from as much as 0.1-0.45 cm/year to as little as 0.01 cm/year (Williams et al.,
2004
).
The three dominant directions followed by the Red Sea, the Gulf of Aden and the
Main Ethiopian Rift are, respectively, south-east-north-west, west-south-west-north-
north-east and south-west-north-east. All three are evident in the course pursued by
the Nile River, with its sudden, sharp bends and long, linear reaches, and all three may
be traced back to ancient lineaments within the Precambrian basement and overlying
sedimentary rocks (Adamson and Williams,
1980
; Adamson et al., 1992; Avni et al.,
2012
).
18.5 Cenozoic desiccation of East Africa
The late Cenozoic desiccation of East Africa, including Ethiopia, had several causes.
Continuing tectonic uplift in East Africa during the past 6-8 Ma (Gani et al.,
2007
)
created a major topographic barrier and caused a change in atmospheric circulation,
reducing rainfall both in East Africa and the Chad Basin (Sepulchre et al.,
2006
).
The change in rainfall regime over East Africa resulted in a change from tropical
forest to open grassland and woodland and was associated with the proliferation of
the Pliocene hominids unique to Africa, discussed in
Chapter 17
(Cerling et al.,
1997
;
Williams et al.,
1998
; Sepulchre et al.,
2006
).
Uplift and rifting in East Africa created the Neogene sedimentary basins, with their
unrivalled record of Pliocene and Pleistocene hominid evolution. It is possible that
the emergence in this region of the early Pliocene hominids may be linked to the
Messinian Salinity Crisis (5.96 to 5.33 Ma: Cosentino et al.,
2013
), during which the
Mediterranean Sea dried out, refilled and dried out repeatedly, resulting in the creation
of a salt desert and the genetic isolation of Africa from Eurasia (van Zinderen Bakker,
1978
; Williams et al.,
1998
). Both van Zinderen Bakker (
1978
) and Williams et al.
(
1998
) believed that the Messinian Salinity Crisis was probably closely linked to
the glacial evolution of Antarctica (Mercer,
1978
), namely, expansion of the West
Antarctic ice sheet and the concomitant sea level lowering in the very late Miocene.
This remains a working hypothesis.
The late Miocene Nile responded to this change in base level by cutting a gorge
more than 1,000 km long and up to 2 km deep at its northern end. This prompts us
to ask when the Ethiopian tributaries of the main Nile River (i.e., the Blue Nile and
Tekazze rivers) first originated, since we know that the White Nile and its parent Lake
Victoria are relatively young features of the African landscape and probably no more
than about 0.3 Ma in age (Talbot and Williams,
2009
; Williams and Talbot,
2009
). In
order to answer this question, we need to consider the impact of certain tectonic and
volcanic events on the Ethiopian drainage system, which is discussed in
Section 18.6
.
It is often hard to distinguish the precise causes of environmental change. The
causes and consequences of the late Pliocene build-up of ice over North America
