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Slavery was a huge effect on Africa, since the Europeans were fond of using slavery for centuries to gain cheap labor. I also think they made use of the resources of Africa by mining to improve the economy. They also modernized the areas they conquered, which had a more beneficial effect if anything, since it meant people now had higher standards of living.
There were few positive effects that I know of for the African people themselves. One of these was the fact that a very limited number of Africans were offered a western education, but the negatives for the people far outweighed this. [ (The imperialism process was definitely good for the European powers. Raw materials from colonies boosted the European economies and added to their power and influence.) There were, however, several negative effects. The literacy rates were low and infant mortality rates were very high in Africa and other colonized countries. Also, industrialization and civilization in general were almost nonexistent in Africa at the time. In general, there was injustice for the African people. Africa was controlled by European powers who never took quality of life for African citizens into consideration; their only thought was for personal gain, to increase their influence. Even after African territories gained independence, the governments were anything but stable, and war broke out all over the continent. Imperialism basically created injustice in Africa. The most long-lasting effect was the arbitrary political boundaries which European monarchies established, and which ignored the previous tribal system. Most of these boundaries are still here today, and have been one of the major causes for African civil wars and conflicts in general. (See the Wikipedia article "Scramble for Africa". It's pretty accurate. Also see the question "What were the negative and positive effects of imperialism in Africa on the colonised?")