Chemical weathering is the process by which rocks are broken down by chemical reactions. There are different types of chemical weathering. Hydrolysis is the chemical breakdown of a substance when combined with water. The most common example of hydrolysis is feldspar in granite rocks changing to clay.
Racially Africans originated in Africa, Europeans in Europe and so on. (before that the human species originated in Africa, but that's ancient history) African Americans are an ethnic group, or a subrace of the African race.
The African Diaspora, which can be found all over the world, comes from three major waves of migration: historic, related to de-colonization process, and the current diaspora motivated by socio-economic and political situation of African countries[i].