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.
Scientists confirm early humans were from Africa but their route out was via Arabia not Egypt. Our human ancestors did come from Africa but left the continent to spread across the world via a different route than first thought, scientists have revealed. A six-year study mapping genetic patterns found that people who ended up in Europe, Asia and Oceania got there by crossing the sea to Arabia around 70,000 years ago. Scientists had thought that humankind left for other continents in a northern direction through Egypt's Sinai region but now it seems they wandered further south, probably via Yemen.