Perhaps no scientific question has been more widely considered across the span of human history as the age of Earth. The question, “How long has this planet been here?” dovetails quite reasonably into, “How long have we been here?” and so the problem of geochronology is essential to much more than just rocks. Whether we consider this problem from Earth or from outer space (Figure 4.1), the issue remains essential.

In Chapter 5 we introduced the evidence that at the end of the Paleozoic Era most of the Earth’s continents were joined together, forming the supercontinent Pangea, which in Latin translates as “whole mother-Earth land.” The formation of the Atlantic and Indian oceans primarily records the subsequent breakup of Pangea and led to the hypothesis of continental drift. Although continental drift seemed like a viable mechanism to create new oceans, until we had a full understanding of the Wadati–Benioff subduction zones, where oceans were being destroyed (see Chapter 5), it was reasonable to consider alternate hypotheses.

It’s hard these days to avoid dinosaurs. Kids’ bedrooms are commonly full of toy dinosaurs, tourist spots like Niagara Falls feature dinosaur mini-golf (Figure 12.1), and the Jurassic Park movie franchise continues to delight many movie goers. Most readers of this book probably had both toys and books about dinosaurs growing up. These depictions, as well as more scientific documentaries such as the BBC series Walking with Dinosaurs, consistently show them as quick, smart, agile, and above all, dangerous, but this view of dinosaurs is actually quite recent. This chapter focuses on the evolution and lifestyles of dinosaurs that dominated the Mesozoic Era.

It was during the time of the Greek philosopher Aristotle (384–322 BC) that someone may have first argued that rocks can be viewed as recorders of ancient environments, although just how ancient was still not clearly understood in his time. By walking along the seashore and observing the types of sediments present and the various organisms living there, philosophers as early as Aristotle were able to deduce that the rocks which made up the landscape shared many characteristics of the modern ocean. From this, Aristotle would argue that “where there is sea, there is at another time land.” This was perhaps the first application of the now-foundational dictum of geology: “the present is the key to the past” – the idea that we can apply our understanding of the natural world today to gain an understanding of the past. This chapter reviews some of the historical milestones and key people in the development of geological reasoning and concludes with a discussion of how modern geological science works as an academic and scientific discipline.

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.

In Charles Darwin’s 1871 book, The Descent of Man, a follow-up to his landmark book On the Origin of Species, he made it clear that humans were likely an offshoot of “old world simians,” which include the great apes – chimps, gorillas, and orangutans. The “Old World,” refers to Africa, Asia, and Europe, versus the “New World,” which refers to the Americas and Oceania. Although Gregor Mendel published his first paper on genetics in 1865, Darwin was not aware of this, so he began his discussion of the evolutionary origin of humans by detailing the physical similarities of humans with apes and mammals.