Mount Kenya at 17,058 feet and Mount Kilimanjaro at 19,340
feet, both in East Africa. Th e main reason that Africa has a
relatively small percentage of mountainous landscape has to
do with Africa’s placement on the tectonic plate system. Th e
African plate is moving toward the northeast, pushing into
the Eurasian plate and separating from the Arabian plate.
Since only the northern and eastern edges of the African con-
tinent are near the edges of the much larger African tectonic
plate (which extends far into the Atlantic Ocean) upon which
it sits, there is relatively little volcanic activity throughout the
rest of the continent.
Th e major mountain ranges formed well before 10,000
years ago as a result of the movement patterns of the Afri-
can tectonic plate. Th ese mountainous regions are on the
northern and eastern sides of the continent and include the
mountains that divide eastern and central Africa and the
Atlas Mountains in northwestern Africa. Th e other areas of
Africa are not subjected to tectonic plate pressure or separa-
tion and therefore are not particularly mountainous. Moun-
tains oft en have great eff ects on local weather conditions. In
eastern Africa (Tanzania, Kenya, and Ethiopia) the climate
is much drier than it is directly on the western side of the
central mountains that divide central and eastern Africa, ow-
ing to the rain-shadow eff ect. Moist air comes from the west
and when it hits the mountains, it is forced to rise; as it rises,
it cools and is not able to hold the moisture, which produces
rain. Th us, once this air reaches the other side of the moun-
tains, it is no longer moist, creating a much drier climate in
eastern Africa.
Th e other major land features of Africa are the Sahara
(which dominates the northern region of Africa, running west
to east) and the Kalahari and Namib deserts (which dominate
southern Africa). Th e Sahel is an east-to-west band of semi-
arid savanna that separates the dry Sahara environment from
the lush and moist tropical forests of central Africa.AFRICAN HUMID PERIOD
At the start of the Holocene geologic epoch (10,000 years ago
to the present), Africa had emerged from a long period of dry
and cool temperatures associated with the last major ice age.
Evidence from sediment cores from many of Africa’s lakes,
ice cores from Mount Kilimanjaro in East Africa, and animal
and plant fossil records show that much of Africa (includ-
ing the present-day Sahara region) was more humid during
the fi rst part of the Holocene than it is today. Th is period is
known as the African Humid Period, which lasted from ap-
proximately 11,000 years ago to 4,000 years ago (though there
have been numerous minor swings between humid and dry
conditions during this generally humid period).
Th e African Humid Period can be explained in part by
the earth’s place within the overarching celestial cycles dur-
ing this time period. Specifi cally, the increased moisture and
warmth in Africa are associated with variation in the amount
of solar radiation hitting the earth’s surface during the North-Grass, sand, and volcanic formation in the Sahara, southern Algeria (© Board of Regents of the University of Wisconsin System. Photographer: Jeanne
Tabachnick)
236 climate and geography: Africa