Chapter Summary

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. Often, one plate slides under another, carrying rock layers down into the mantle, where they melt and flow back toward the surface to form 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, burial 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. Southern Africa and Russia contain some of the world’s richest supplies of strategic minerals. Less-developed countries, most of which are in the tropics or the Southern Hemisphere, are often the largest producers of iron, aluminum, copper, gold, and other metals on which the industrialized world depends. The major consumers of these resources are the industrialized countries.

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 or open pits, and tailings accumulations. 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.

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.

Earthquakes, volcanoes, floods, and erosion are natural geological processes. When they affect human lives and property, however, they become some of the worst “natural disasters” known. Some of these natural disasters, especially floods and erosion, are made more serious by human activities and land-use decisions.