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.
List of African-American firsts. African Americans (also known as Black Americans and Afro-Americans) are an ethnic group in the United States. The first achievements by African Americans in various fields historically marked footholds, often leading to more widespread cultural change. The shorthand phrase for this is breaking the color barrier.