Have you ever wondered why change occurs and why some processes are rapid whereas others are slow? Why does a match burn when struck, or why does a tombstone weather? Both processes involve reaction with oxygen in the air, but the match burns rapidly, whereas the tombstone weathers slowly. Why should the match or the tombstone react with oxygen, and why do the processes have such different rates? In this chapter we examine the fundamental principles governing all change. Thermodynamics is the subject that provides the answers to the questions about the direction of natural processes, whereas kinetics deals with the rates of processes. Thermodynamics is the study of energy and its transformations. It allows us to determine the direction of natural process and the equilibrium states toward which they strive. The rates of natural processes vary considerably, from the rapid explosion of volcanic gas to the slow motion of tectonic plates. Despite this great variation, these processes obey similar laws. We will also introduce some of the fundamental thermodynamic principles that allow us to determine what to expect if equilibrium were achieved. We then outline some of the important factors that control the rate at which equilibrium is approached. Finally, we look at one of the most remarkable rate processes, that of radioactive decay, and how it is used to determine absolute ages.
The elements that constitute the Earth can combine in a large number of ways to form many different minerals, which, in turn, can combine in different ways to form many different rocks. In addition, one set of elements might form one mineral under one set of conditions and another mineral under another set; e.g., graphite and diamond are both forms of carbon (polymorphs, see Sec. 5.8), but graphite is the stable form at low pressure and diamond the stable form at high pressure. Or a group of minerals that form a rock under one set of conditions might change into a different group of minerals, or even melt, under another set of conditions (Chapter 15). And under still other conditions these minerals may weather and be transported and deposited as sediment (Sec. 11.17).
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