Further Reading

Turcotte, Donald L., and Gerald schubert. Geodynam-ics. 2nd ed. Cambridge: Cambridge university Press, 2002.

Vanicek, Petr, and Nikolaos T. Christou. Geoid and Its Geophysical Interpretations. New York: CRC Press, 1994.

geological hazards Geological hazards take many shapes and forms, from earthquakes and volcanic eruptions to the slow, downhill creep of material on a hillside and the expansion of clay minerals in wet seasons. Natural geologic processes constantly operate on the planet. They are considered hazardous when they go to extremes and interfere with the normal activities of society. For instance, the surface of the Earth constantly moves through plate tectonics yet is not noticeable until sections of the surface move suddenly, causing an earthquake.

The Earth is a naturally dynamic, hazardous world, with volcanic eruptions spewing lava and ash, earthquakes pushing up mountains and shaking Earth's surface, and tsunamis that sweep across ocean basins at hundred of miles per hour (500 km/hr), rising in huge waves on distant shores. Mountains may suddenly collapse, burying entire villages under massive landslides. other mountain slopes are gradually creeping downhill, slowly tilting and moving everything built on them farther downslope. storms sweep coastlines and remove millions of tons of sand from one place and deposit it in another in a single day. Large parts of the globe are turning into desert. Glaciers that once advanced are rapidly retreating, and sea level is beginning to rise faster than previously imagined. All of these natural phenomena are expected consequences of the way the planet works, and as scientists better understand these geological processes, they are better able to predict when and where natural geologic hazards could become disasters and take preventative measures.

The slow but steady movement of tectonic plates on the surface of the Earth is the cause of many geologic hazards, either directly or indirectly. Plate tectonics controls the distribution of earthquakes and the location of volcanoes, and causes mountains to be uplifted. other hazards are related to Earth's surface processes, including floods of rivers, coastal erosion, and changing climate zones. Many of Earth's surface processes are parts of natural cycles but are considered hazardous to humans because people did not adequately understand the cycles before building on exposed coastlines and in areas prone to shifting climate zones. A third group of geologic hazards is related to materials such as clay minerals that dramatically expand when wetted, and sinkholes that develop in limestones. still other hazards are extraterrestrial in origin, such as the occasional impact of meteorites and asteroids with Earth. The exponentially growing human population on Earth worsens the effect of most of these hazards. species on the planet are now experiencing a mass extinction event, the severity of which has not been seen since the extinction event 66 million years ago that killed the dinosaurs and many other species at that time.

Many geologic hazards are the direct consequence of plate tectonics, associated with the motion of individual blocks of the rigid outer shell of the Earth. With so much energy loss accommodated by plate tectonics, it is clear why plate tectonics is one of the major energy sources for natural disasters and hazards. Most of the earthquakes on the planet are directly associated with plate boundaries, and these sometimes devastating earthquakes account for much of the motion between the plates. single earthquakes have killed tens and even hundreds of thousands of people, such as the 1976 Tangshan earthquake in China that killed a quarter million people, and the 2008 sichuan earthquake that killed about 100,000 people in southern China. Earthquakes also cause enormous financial and insurance losses; for instance, the 1994 Northridge, California, earthquake caused more than $14 billion in losses. Most of the world's volcanoes are also associated with plate boundaries. Thousands of volcanic vents are located along the midocean ridge system, and

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The Basic Survival Guide

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