What is a lingua franca, and what led to the lingua francas used in East Africa? (lingua franca)

This language is both the native tongue of a specific people - "the Swahili" - and a lingua franca spoken by more than 50 million people throughout the region. Swahili is one the few lingua francas among the more than 1,000 languages spoken on the African continent, and therefore one of the most widely used.

Explanation:

The language dates from the contacts of Arabian traders with the inhabitants of the east coast of Africa over many centuries. Under Arab influence, Swahili originated as a lingua franca used by several closely related Bantu-speaking tribal groups.

heat from the core of the earth causes convection currents to occur in the mantle. these currents slowly move the crust around.