The mineral grains that form the solid phase of a soil aggregate are the product of rock weathering. The size of the individual grains varies over a wide range. Many of the physical properties of soil are dictated by the size, shape, and chemical composition of the grains. To better understand these factors, one must be familiar with the basic types of rock that form the earth’s crust, the rock-forming minerals, and the weathering process. On the basis of their mode of origin, rocks can be divided into three basic types: igneous, sedimentary, and metamorphic. Figure 2.1 shows a diagram of the formation cycle of different types of rock and the processes associated with them. This is called the rock cycle. Brief discussions of each element of the rock cycle follow.