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.
'Evolutionary history shows that human populations likely originated in Africa, and the Genographic Project, the most extensive survey of human population genetic data to date, suggests where they went next: Modern humans migrated out of Africa via a southern route through Arabia, rather than a northern route by way of Egypt,' a spokesman for IBM said, who were involved in the research.