236 climate and geography: Africa Grass, sand, and volcanic formation in the Sahara, southern Algeria (© Board of Regents of the University of Wisconsin System Photographer: Jeanne Tabachnick) Mount Kenya at 17,058 feet and Mount Kilimanjaro at 19,340 feet, both in East Africa The main reason that Africa has a relatively small percentage of mountainous landscape has to with Africa’s placement on the tectonic plate system The 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 continent 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 The major mountain ranges formed well before 10,000 years ago as a result of the movement patterns of the African tectonic plate These 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 The other areas of Africa are not subjected to tectonic plate pressure or separation and therefore are not particularly mountainous Mountains often have great effects 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, owing to the rain-shadow effect 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 Thus, once this air reaches the other side of the moun- tains, it is no longer moist, creating a much drier climate in eastern Africa The 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) The Sahel is an east-to-west band of semiarid 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 (including the present-day Sahara region) was more humid during the first part of the Holocene than it is today Th is period is known as the African Humid Period, which lasted from approximately 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) The African Humid Period can be explained in part by the earth’s place within the overarching celestial cycles during this time period Specifically, 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-