 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Environmental Geology
BE ALERT FOR: Cycling
Recycling is growing in many parts of our country, but it has not fully insinuated itself into our way of life. This is not true of nature, as you first learned back in Chapter 3 when you studied the biogeochemical cycles of ecosystems. The rock cycle, an important concept in this chapter, is another example of the centrality of the recycling theme in natural processes.
Once you understand how weathering, sedimentation, subduction, and heat and pressure move rock materials around in the lithosphere, you'll see what I mean. Time is the biggest difference between the rock cycle and other cycles with which we are more familiar. The cycle of creation, destruction, transport, and re-creation of rocks can literally take billions of years. It is possible that some of earth's oldest rocks have not yet moved through the complete cycle even once. The rock cycle can also take as few as several million years, and numerous molecules have undoubtedly been spun through the cycle several times.
BE ALERT FOR: Metals
Technological advances and world demand can combine to make it economically profitable to mine relatively low-grade ores containing much less than 1 percent metal. This means that huge piles of waste rock are created per unit of recovered metal.
Many of the ores are sulfur compounds. Exposed to rainwater, dangerously acidified runoff is created, which dissolves metals out of the waste materials.
You may be surprised at the variety of ways resource extraction impacts the environment. Physical disturbance of surface features is an obvious and expected impact of digging holes in the ground. Unfortunately, there is often considerably more to the impacts than creating piles of rock. Many of the heavy metals are particularly toxic to organisms. Chemically locked up in solid rock, these metals can cause no harm. Once dissolved, however, they become biologically active.
Processing the ore to remove the metal is the next stage that is fraught with risks to the environment. Smelting and the new heap-leach extraction method both can produce considerable damage to the environment, as you will learn in this section of the chapter.
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2003 McGraw-Hill Higher Education