1. Biological Processes, which consist of any and all activities performed by living organisms (eating, breathing, "burning" food, and eliminating waste. 2. Geological Processes, which include volcanic eruptions, the formation and breakdown of rock, and major movements of matter within and below the surface of the earth. 3. Chemical and Physical Processes, which include the formation of clouds and precipitation, the flow of running water, and the action of lightening. 4. Human Activity, which includes the mining and burning of fossil fuels, the clearing of land for building and farming, the burning of forests, and the manufacture and use of fertilizers.