In simple terms, a rift can be considered as a fissure in the earth's surface that broadens over time; or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. The East African Rift runs from the Afar Triple Junction in Ethiopia, southward through eastern Africa.
The
rift is a narrow zone in which the African Plate is in the process of splitting
into two new tectonic plates called the Somalian Plate and the Nubian Plate,
which are sub plates or protoplates. These two plates are moving away form each other and
also away from the Arabian plate to the north. The point where these three
plates meet in the Afar region of Ethiopia forms what is called a
triple-junction.
The oldest and best defined rift
occurs in the Afar region of Ethiopia and this rift is usually referred to as
the Ethiopian Rift. Further to the South, a series of rifts occur which include
a Western branch, the "Lake Albert Rift" or "Albertine
Rift", which contains the East African Great Lakes, and an Eastern branch
that roughly bisects Kenya north-to-south, on a line slightly west of Nairobi
(Figure 2). These two branches together have been termed the East African Rift
(EAR), while parts of the Eastern branch have been variously termed the Kenya
Rift or the Gregory Rift (after the geologist who first mapped it in the early
1900's). The two EAR branches are often grouped with the Ethiopian Rift to form
the East Africa Rift System (EARS). The complete rift system therefore extends
1000's of kilometers in Africa alone; and several 1000’s more if we include the
Red Sea and Gulf of Aden as extensions.
The stretching process associated
with rift formation is often preceded by massive volcanic eruptions, which flow
over vast areas and are usually preserved or exposed on the edges of the rift. Some
geologists consider these eruptions to be "flood basalts", that is,
the lava is erupted along fractures, rather than at individual volcanoes; and
runs over the land in sheets like water does during a flood.
The East African Rift Zone comprises
a number of both active and dormant volcanoes, including the following: Mount
Kilimanjaro, Mount Kenya, Mount Longonot, Menengai Crater, Mount Karisimbi,
Mount Nyairagongo, Mount Meru and Mount Elgon, including Crater Highlands in
Tanzania.
The rifting of East Africa is
complicated by the fact that two branches have emerged, one to the west which
hosts the African Great Lakes, where the rift filled with water; and another
nearly parallel rift about 600 kilometers to the east, which nearly bisects
Kenya north-to-south before entering Tanzania where it seems to cease. Lake
Victoria lies between these two branches. It is thought that these rifts are
generally following old sutures between ancient continental masses that
collided billions of years ago, to form the African craton; and that the split
around the Lake Victoria region occurred due to the presence of a small core of
ancient metamorphic rock, the Tanzania craton, that was too hard for the rift
to tear through. Considering that the rift could not penetrate this area, it
instead diverged around it, leading to the two branches that can be seen today.
The East African Rift System is an excellent field laboratory to study a
modern, actively developing rift system.
This region is also imperative for
grasping the roots of human evolution. The bones of several hominid ancestors
of modern humans have been discovered there, including Lucy, a partial yet
eye-opening australopithecine skeleton dating back to 3 million years. Some
other hominid material recently discovered was found to be 10 million years
old. The structure and evolution of the rift may have rendered East Africa more
sensitive to climate changes, which led to many alternations between wet and
arid periods.
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