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.
Homo sapiens are thought to have originated from Africa and then moved and populated different parts of the world, especially Asia (out of Africa model). Another contrasting theory is the thought that these hominins existed independently in many regions of the world (multiregional hypothesis).