Key Points
- Soils come from the breaking down of rocks and are usually described as gravels, sands, silts, or clays.
- Physical weathering breaks rocks into smaller pieces without changing what they are made of.
- Chemical weathering not only makes rocks smaller but also changes their chemical makeup.
- Gravels and sands are called coarse-grained soils, while silts and clays are called fine-grained soils.
- Coarse-grained soils are mostly made of quartz, a very hard mineral.
- Clays are mainly made of three types of minerals: kaolinite, illite, and montmorillonite.
- Clay minerals are built from tiny layers of silica and alumina sheets. How these layers stick together is very important. In montmorillonite, the layers are held together weakly, so water can easily get between them and make the clay swell.
- A thin layer of water, called adsorbed water, sticks to the surfaces of soil particles and greatly affects how fine soils behave.
