Define weathering and distinguish between the two main categories of weathering.
Weathering involves the physical breakdown (disintegration) and chemical alteration (decomposition) of rock at or near Earth’s surface. Weathering goes on all around us, but it seems like such a slow and subtle process that it is easy to underestimate its importance. Yet weathering is a basic part of the rock cycle and thus a key process in the Earth system.
Weathering is also important to humans—even to those of us who do not study geology. For example, many of the life-sustaining minerals and elements found in soil, and ultimately in the food we eat, were freed from solid rock by weathering processes. As the chapter-opening photo, Figure, and many other images throughout this book illustrate, weathering also contributes to the formation of some of Earth’s most spectacular scenery.
Of course, these same processes are also responsible for causing the deterioration of many of the structures we build.
There are two basic categories of weathering. Mechanical weathering is accomplished by physical forces that break rock into smaller and smaller pieces without changing the rock’s mineral composition. Chemical weathering involves a chemical transformation of rock into one or more new compounds.
These two concepts can be illustrated by a large log. The log disintegrates when it is split into smaller and smaller pieces, whereas decomposition occurs when the log is set afire and burned. Figure provides other examples of weathering.
Why does rock weather? Simply, weathering is the response of Earth materials to a changing environment.
For instance, after millions of years of uplift and erosion (the removal and transport of weathered rock material by water, wind, or ice), the rocks overlying a large, intrusive igneous body may be removed, exposing it at the surface.
This mass of crystalline rock—formed deep below ground, where temperatures and pressures are high—is now subjected to a very different and comparatively hostile surface environment. In response, this rock mass will gradually change. This transformation of rock is what we call weathering.
In the following sections we will examine the various types of mechanical and chemical weathering. Although we will consider these two categories separately, keep in mind that mechanical and chemical weathering processes usually work simultaneously in nature and reinforce each other.