Soil loss or erosion is a natural process that can profoundly affect human activities. Soil erosion can deplete the topsoil, resulting in desertification of entire regions. It can destabilize buildings and gradually undermine foundations. In your own garden, soil loss can eat away at your landscaping and wash away nourishing compost and topsoil, requiring more work to maintain your plants.
Mass movement soil erosion is driven by the force of gravity. Some forms of mass movement are gradual, while others are sudden and cataclysmic. In landslides, a whole section of land suddenly collapses and flows quickly downhill. This is often caused by over-saturation from too much rainfall, which increases weight in the top layer of soil and decreases friction, causing the top layer to collapse. Similarly, in rock avalanches, a rock structure collapses downhill due to weaknesses in the rock, erosion at the base, or a combination of the two.
Soil creep, by contrast, is more subtle. Particles of dirt gradually roll downhill, slowly eroding the soil. Sometimes, livestock can aid the downhill flow of soil by treading on it and knocking particles loose, a phenomena known as terracetting.
Water erosion is a multistage process. It begins with a phenomena called splash erosion. When raindrops hit the ground, they knock little particles of soil loose and compact the ground under the impact, creating a more densely packed crust. The water flows over this crust, creating small rills or channels. These channels can become deeper gullies, carrying away more soil as the walls of the channels collapse under the fast-flowing water. Alternately, the water can flow over the ground in a continuous sheet, carrying away fine soil particles, a phenomena called sheet erosion.
We have seen that too much water can cause erosion, but too little can do much the same thing. Wind erosion takes place on dry, sandy soil. Because this type of soil is made up of small separate particles that don't cling together well, the wind can lift them off, slowly blowing layers of soil away. The famous Dust Bowl of the 1930s is named after a period of particularly bad soil erosion. Drought and poor farming practices had dried and loosened the soil in the great plains, allowing strong winds to carry it off in great, black clouds.