The main motivation of the European colonial powers to colonize Africa was the economic interest, or to put it more specifically, exploiting the natural resources, cheap and abundant labor force, and making wealth out of it.
Africa is a continent that is very rich in natural resources. Lot of those natural resources were very expansive, in high demand, and highly appreciated outside of Africa. The Europeans saw a great opportunity to make themselves even more wealthy, as they had far superior military and weapons, so conquering would not be a problem, and the natives were not using those natural resources nor knew that they are valuable in other places of the world.
The Europeans used the suitable circumstances, and numerous colonial European countries managed to get a piece of the African land, making lot of wealth out if it until the end of the colonial period.
The graphic scale it's a line marked with distance to the scale on the map.
