This chapter is specifically focused on the events that occurred at the boundary between the Mesozoic and Cenozoic eras, commonly referred to as the K–Pg extinction because it marks the boundary between the Cretaceous (K) and Paleogene (Pg) geologic periods. It is the most recent of the five mass extinctions introduced in Chapter 9 and has been a topic of considerable public interest because it also marks the end of the “Age of Dinosaurs” (see Chapter 12). The two main hypotheses for the cause of the extinction center on a giant asteroid impact and massive volcanic eruptions in India.

As the nineteenth century turned to the twentieth, the overwhelming majority of geologists thought that Earth’s great geographic variety was primarily the consequence of bodies of rock moving up and down. Put simply, mountains were places that recently moved up and oceans were places that had recently moved down. Sometimes it was suggested that regions had gone from being high to being low in several episodes. The proposed driving mechanism for these changes were cooling of the Earth and gravitational instabilities. Cooling was used to explain contraction, compression, and formation of mountain belts.

Today, glaciers are found at high latitudes, closer to the pole, or at high altitudes, typically about 4,000 meters above sea level. However, the rock record tells us that during the past two billion years, Earth has experienced several episodes in which the distribution of ice across the globe was distinctly different than the modern arrangement. Earth has experienced about eight major glacial periods throughout its history, including the current stage (Figure 8.1).

NASA defines life as a self-sustaining chemical system capable of Darwinian evolution. Attributes of life typically include an organized internal structure surrounded by a membrane, the ability to regulate the internal environment of the organism (or cell), metabolism that allows growth and reproduction, and the ability to adapt to external change over time. The last point is key to the process of evolution that has driven the development of life on Earth, as introduced in Chapter 6. Even the most primitive prokaryotic bacterial cells are exceedingly complicated and include an external membrane and complex molecules, such as DNA and ribosomes that manufacture proteins.

Rocks are literally underfoot and can tell marvelous stories. The authors of this book have been spellbound by rocks for most of our lives. We have been astounded at the story of how the dinosaurs were killed by an asteroid crashing into Earth from outer space, and we smile at the idea that the rocks now at the top of Mount Everest were formed beneath the surface of an ocean. We have both spent time enjoying the splendor of Grand Canyon and pondering the incredible stories recorded in the rocks that are exposed in the cliffs and along the bottom (Figure 1.1).

The Himalaya is the highest mountain range on Earth, stretching for more than 2,500 km (1,500 miles) with dozens of peaks having elevations over 6,300 m (20,000 feet). At the summit of Mount Everest, the highest point on Earth, at 8850 m (29,029 feet) above sea level, we find limestone, originally deposited below sea level. The great height of the mountains led to our understanding of isostasy via the controversy of topographic compensation debated by Pratt and Airy (see Chapter 3). The first formal geologic study of the region may have been by Swiss geologists Arnold Heim and Augusto Gansser, in 1936, soon after the concept of continental drift was taking hold in Europe (see Chapter 5).

As introduced in Chapter 9, from the beginning of the Paleozoic, life began to leave a much more tangible fossil record as well as experiencing major diversification during the Great Ordovician Biodiversity Event (Figure 9.24). Extinction events at the end of the Ordovician and the end of the Devonian decreased diversity by about 50%, but in each instance recovery to previous levels occurred within about five million years. The first part of this chapter reviews some of the major evolutionary events in the mid to later Paleozoic, with a particular focus on how the evolution of land plants paved the way for colonization by terrestrial animals and affected global climate, triggering a prolonged ice age. Massive volcanism reversed this trend and caused a hothouse at the end of the Paleozoic Era that initiated the most devastating of the five recorded mass extinctions known as the Great Dying and is the focus of the second part of the chapter.

William “Strata” Smith’s “principle of faunal succession,” introduced in Chapters 1 and 2, codified one of the key observations in geology: strata of different ages contain unique, age-diagnostic fossils. Today anyone can repeat these observations, either in northern England, where Smith worked, or with any other fossiliferous sequence of rocks. The rock record is clear, life on Earth has changed over time. As one species becomes extinct it is often succeeded by a new species. This is evolution, a simple and straightforward observation. The ability of life to diversify was made possible by the immensity of geological time and was largely driven by the changes in local or global environments. The only theoretical framework needed to start this work is Steno’s principle of superposition, which we explained in Chapter 1. Applying this idea with observations of fossils in different strata clearly show that life has evolved. Observations from other fields of science, including anatomy and physiology, molecular biology, and genetics, are consistent with the data from paleontology and stratigraphy and show that all life is related. The idea that best explains the observed evolution of life over time, as well as how species are related, is the theory of natural selection and is the focus of this chapter.

The largest canyon on land today is the Grand Canyon, one of the seven natural wonders of the world, 446 kilometers long, 29 kilometers wide, and 1,857 meters deep, that we reviewed in Chapter 1. Of course, these dimensions pale compared to oceanic basins, but despite their far more extensive dimensions, oceans are always filled with seawater. Or are they? As we have seen in the chapter on plate tectonics, oceans do not last forever.

The eastern part of North America contains rocks that tell the detailed story of the closing of the Iapetus Ocean during the assembly of the Pangean supercontinent in the Paleozoic that was the subject of . Since about 200 Ma, eastern North America recorded the rifting of Pangea and evolved from a divergent tectonic boundary during the rift phase into its current passive margin at the margin of the modern Atlantic Ocean (seeand ). The western parts of North America contain a geologic record that reflects a similar history but essentially flipped in time. While major orogenic deformation was impacting eastern North America in the Paleozoic, western North America was a passive margin on which a thick sedimentary sequence was deposited on a Proterozoic and older basement.