How did West African gold end up traded for Saharan salt?

Gold and salt are both ordinary things, but in medieval West Africa they were exactly what each side lacked. West Africa held gold. The Sahara held salt, and the northern lands that wanted gold sat across the desert. Northern economies were short of gold but at times controlled salt mines such as Taghaza; West African states like Wangara had plenty of gold but needed salt. That gap built an empire.

That empire was Ghana, also called Wagadu, in what is now southeast Mauritania and western Mali. Its name was really a title: Ghana meant warrior or war chief, and its kings were also called Kaya Maghan, king of gold. It is not the modern country, which borrowed the name at independence in 1957.

What made the crossing possible was the camel. Introduced to the western Sahara in the 3rd century, it was a major catalyst for the empire's formation, and by the 7th century camels had turned irregular desert tracks into a network running between North Africa and the Niger River. Centuries later, the traveller Ibn Battuta wrote that an average caravan had 1,000 camels, and some had as many as 12,000.

Ghana sat in the middle and taxed the salt that passed through: al-Bakri reports one gold dinar on imported salt and two on exported salt. The king kept all the gold nuggets for himself and let everyone else have only gold dust. Demand was high because Muslim states used gold coins, the dinar, as money. By the 970s Ibn Hawqal was calling the king of Ghana the wealthiest king on the face of the earth.