The earth is a complex, dynamic system. Although it seems stable and permanent to us, the crust is in constant motion. Huge blocks called tectonic plates slide over the surface of the ductile mantle.

They crash into each other in ponderous slow motion, crumpling their margins into mountain ranges and causing earthquakes. Sometimes one plate will slide under another, carrying rock layers down into the mantle where they melt and flow back toward the surface to be formed into new rocks. Rocks are classified according to composition, structure, and origin.

The three basic types of rock are igneous, metamorphic, and sedimentary. These rock types can be transformed from one to another by way of the rock cycle, a continuous process of weathering, transport, burying in sediments, metamorphism, melting, and recrystallization. During the cooling and crystallization process that forms rock from magma, minerals and metals can become concentrated enough to become economically important reserves if they are close enough to the surface to be reached by mining. Having a reliable supply of strategically important minerals and metals is vital in industrialized societies. A few places in the world are especially rich in mineral deposits.

South Africa and the former Soviet Union contain most of the world's supply of several strategic minerals. Less-developed countries, most of which are in the tropics or the Southern Hemisphere, are often the largest producers of ores and raw mineral resources for the strategic materials on which the industrialized world depends. The major consumers of these resources are the industrialized countries.

Worldwide, only a small percentage of metals are recycled, although it is not a difficult process technically. Recycling saves energy and reduces environmental damage caused by mining and smelting. It reduces waste production and makes our metal supplies last much longer. Substitution of materials usually occurs when mineral supplies become so scarce that prices are driven up. Many of the strategic metals that we now stockpile may become obsolete when newer, more useful substitutes are found.

Both mining and processing of metals and mineral resources can have negative environmental effects. Mine drainage has polluted thousands of kilometers of streams and rivers. Fumes from smelters kill forests and spread pollution over large areas. Surface mining results in removal of natural ecosystems, soil disruption, creation of trenches and open pits, and accumulation of tailings. It is now required that strip-mined areas be recontoured, but revegetation is often difficult and limited in species composition. Smelting and chemical extraction processes also create pollution problems.