Examining the roles of rocks and minerals in the rock cycle reveals their relationships to the Earth system. For a good summary of these relationships, take a look at the Visual Overview at the beginning of the chapter, on pages 30 and 31. Many rocks and minerals can also be placed in a broader conceptual context by examining their relationship to the water cycle (see the "Interactions with the Rock Cycle" section of Chapter 1, page 22).
Although we classify rocks by assigning them to categories, various classes of rocks are interrelated, partly because they are transformed into one another. For example, high-grade metamorphic rocks are hard to distinguish from igneous rocks, and it is debatable whether certain volcanic rocks, such as tuff, are igneous or sedimentary. Tuff is deposited like a sedimentary rock, but it is usually classified as volcanic because it consists of volcanic particles. Also, some aragonite needles in modern seas are produced by direct precipitation from seawater, whereas others are produced by the disintegration of carbonate skeletons, particularly of algae. Thus, the chemical components of calcium carbonate sediment may or may not have passed through an organism. Simple rock transformations have profound implications. It is a striking fact that the weathering of granitic rocks, the most common rocks of the continental crust, automatically produces clay particles and quartz sand, the two most abundant components of siliciclastic sediment.
In short, minerals and rocks should not be seen as discrete entities, but as parts of a vast dynamic system.
