Although their importance is often overlooked, 

 rocks are essential to life. The solidification of 

 igneous rock involves the release of oxygen and water 

 into the atmosphere. The physical properties of 

 rocks, especially those properties associated with 

 surface temperature, have a bearing on local climatic 

 conditions. Further, climatic associations relate to the 

 topographic features that are really rock accumula- 

 tions. Rocks and climate are important contributors 

 to soil formation. Soil, partly rock, is extremely 

 important in providing a substrate for plants and 

 animals. Because rocks are composed of minerals, 

 they supply certain nutritional needs of living 

 creatures. For example, various salts and water are 

 essential foods. Water, the most essential food, is in 

 its pure form a mineral; however, in nature, because 

 of extent and impurities, it often is a rock! 



This chapter will examine the kinds of rocks in the 

 physical environment and the effects of rocks upon 

 local land forms. The types of rocks — their struc- 

 ture as affected by constructive and destructive 

 geological processes — impart the features of a land- 

 scape. 



MINERALS AND ROCKS 



There are certain criteria for distinguishing be- 

 tween minerals and rocks. Minerals are pure or 



homogeneous substances of definite chemical com- 

 position and are shaped by natural inorganic proc- 

 esses. For example, a molecule of pure table salt is a 

 combination of one atom of sodium and one of 

 chlorine; so is the mineral, halite. Halite is a mineral 

 because it is always pure sodium chloride, a com- 

 pound of definite chemical composition united by 

 processes not involving living creatures. Therefore, 

 the criteria for a mineral would exclude any com- 

 pound that is chemically variable, is a plant or animal 

 product, or is man made. For this reason, man-made 

 synthetic gems and manufactured products, even 

 though they are exact replicas of minerals, are not 

 minerals. There are about 1500 kinds of minerals in 

 nature. 



Rocks are mostly combinations of two or more 

 minerals; however, certain pure substances can be 

 rocks. A true mineral is also considered a rock if 

 it forms a large feature of the landscape, as, for ex- 

 ample, a body of pure water or a hill of serpentine, 

 hematite, or gypsum. Any homogeneous material 

 with variable chemical composition is a rock rather 

 than a mineral (e.g., obsidian whose composition var- 

 ies irregularly from place to place). Organic prod- 

 ucts, even pure substances, are rocks (e.g., pe- 

 troleum, coal, and organic limestone). In this 

 respect, it is of interest to note that pure but or- 

 ganically derived calcium carbonate is limestone. 



68 



