Humans have demonstrated the capacity to damage land and extinguish species from very early times (figure 13.1). Our impacts have grown immensely as populations and technological prowess increased (figure 13.2), and by the twentieth century, they had begun changing the landscape in major ways. To generations born since 1945, the pervasive human sculpting of natural landforms may even seem part of the natural scene. The effects certainly have reached a scale comparable to natural geological forces. Humans directly displace approximately 35 billion tons of soil and rock per year worldwide, exceeding the work of rivers and streams and greatly surpassing natural erosion from glaciers or wind. In the United States, road building, mining, construction, urban expansion, recreation, and military training and bomb testing move approximately 28 tons of earth per person each year—far outranking the world average of about six tons per person per year. Unintentional agricultural displacements are even greater—about 1,500 billion tons per year. Natural processes obey the physical laws of motion and energy, which never take a break. In this book, we have tried to explain how human changes add to nature’s effects (figure 13.3), in many cases multiplying the impacts of natural processes and causing severe environmental damage. Most people simply do not understand how the Earth works—but if nothing else, Hurricane Katrina’s 2005 devastation of New Orleans made it clear that ignoring or underestimating the power of natural forces can severely imperil our present and future well-being. Natural forces constantly are acting on and inside the Earth, building up land and tearing it down at the same time. Humans experience the Earth’s internal heat engine through tectonic effects, which have segmented the surface along boundaries that define mobile tectonic plates. Plate interactions at the boundaries generate faults and earthquakes, raising hills and mountains or depressing basins and troughs. Plate boundaries also contain most of the world’s volcanoes, generated by the internal melting that creates igneous rocks.
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