The tectonic forces at work deep within Earth are enormous and outside the control of humans. The power of large earthquakes to devastate cities is evident from the 1906 San Francisco and 1923 Tokyo events. The 2004 mega-disaster arising from the earthquake offshore Sumatra generated tsunamis that caused destruction in areas far from where any shaking was felt. For many people, the dangers from large volcanic eruptions are even more frightening, evinced by the AD 79 eruption of Mount Vesuvius in Italy, that destroyed the cities of Pompeii and Herculaneum.

Devastating earthquakes and volcanic eruptions, infrequent on the scale of human lifetimes, are indeed commonplace tectonic phenomena over timescales of millions of years, the standard geologic time unit required to understand the evolution of Earth. But the key observation is that these kinds of events are restricted in occurrence even over multimillion-year geologic timescales (Figure 2.1). Most earthquake and volcanic activity is localized to the margins of the Pacific Ocean basin, the “Ring of Fire.” Another region where large earthquakes are frequent but volcanism occurs only in scattered clusters is along the great, roughly east-west trending mountainous belt extending from the European Alps to the Himalayas. In the context of plate tectonic theory, we now understand the cause of this activity. The circum-Pacific Ring of Fire is the product of plate convergence (subduction) and plates sliding past one another (strike-slip transform faulting). The greatest mountain belt on Earth is the product of ongoing continental collision of Eurasia with Africa and India.