Geoscience Reference
In-Depth Information
Given its proximity to Europe and its aura of mystery, the Sahara has long exerted
an attraction for hardy travellers from that continent. Scientific explorers from many
nations have contributed to our knowledge of past and present environments in the
Sahara, but those from France in particular have made an outstanding contribution
to our understanding of past climates in this vast region (
Chapter 5
). This work
ranges from the heroic era of more or less solitary exploration and mapping (Bordet,
1952
; Monod,
1958
; Coque,
1962
;Faure,
1962
; Chavaillon,
1964
; Williams,
1966
;
Rognon,
1967
; Conrad,
1969
; Black and Girod,
1970
;Fabre,
1974
) to the intensive
multidisciplinary teamwork of today (Beuf et al.,
1971
; Fontes et al.,
1983
; Fontes
et al.,
1985
; Pachur et al.,
1990
; Hoelzmann et al.,
2004
; Osborne et al.,
2008
; Williams
et al.,
2010b
; Drake et al.,
2011
).
It is appropriate to consider the Sahara, Afar and Arabian deserts together, because
for much of their geological history, they were part of the same continent and subject
to the same Mesozoic and early Cenozoic northward lithospheric plate movement.
Their history only began to diverge as a result of late Cenozoic uplift, rifting and the
creation of the Red Sea and Gulf of Aden. The Cenozoic (65 Ma to present) includes
the Quaternary (2.6 Ma to present), and the Quaternary includes the Pleistocene
(2.6 Ma to 11.7 ka) and the Holocene (11.7 ka to present).
During the Triassic, Africa was part of the Gondwana supercontinent, together with
South America, Antarctica, Australia and India. The separation of Gonwana into the
two continents of West Gondwana (Africa and South America) and East Gondwana
(Australia, Antarctica and India) was accomplished during the Jurassic, although it
had begun before then. Further break-up of these two huge continents took place
during the Cretaceous (
Chapter 3
,
Figure 3.2
). In Africa, the Cretaceous equator ran
diagonally across the Sahara from southern Nigeria through central Chad, northern
Sudan and Egypt into Arabia. The northward movement of Africa during the late
Mesozoic and Cenozoic resulted in a southward shift of the equatorial rainforest and
led to the long-term desiccation of North Africa as it moved into tropical latitudes
characterised by dry subsiding air. The slight clockwise rotation of the African plate
during the Miocene and Pliocene brought it into contact with the European plate
(Habicht,
1979
; Owen,
1983
; Williams et al.,
2004
). The ensuing crustal deformation
in North Africa led to uplift of the Atlas Mountains in the north-west and was roughly
synchronous with volcanism and uplift of the Hoggar, Tibesti, Aır and Jebel Marra
uplands, creating the major elements of the present-day topography (
Figure 18.3
). The
onset of aridity was not synchronous across the Sahara; it began earlier in Morocco,
Algeria and Tunisia than it did in Egypt and Sudan, as shown by the abundance of
Mesozoic and younger evaporite formations in the north-west Sahara, which by then
had already reached dry tropical latitudes (Coque,
1962
; Conrad,
1969
; Williams
1984a
).
Cenozoic volcanism and tectonism contributed to the desiccation effected by the
northward movement of the African plate, bringing North Africa into dry subtropical
