layers, or strata, is called stratigraphy. In methods of relative dating, geologists apply principles of stratigraphy such as these:
Rock layers below are generally older than rock layers above.
All sedimentary rock layers are originally formed in a horizontal position.
When a different rock is cutting through layers of rock, the cross-cutting rock is younger than the layers it cuts through.
These principles and a few others that I describe in Chapter 16 guide geologists called stratigraphers in interpreting the order of rock layers so that they can form a relative order of events in Earth’s history.
However, sometimes simply knowing that something is older than — or younger than — something else is not enough to answer the question being asked. Absolute dating methods use radioactive atoms called isotopes to determine the age in numerical years of some rocks and rock layers. Absolute dating methods may determine, for example, that certain rocks are 2.6 million years old. These methods are based on the knowledge, learned from laboratory experiments, that some atoms transform into different atoms at a set rate over time. By measuring these rates of change in a lab, scientists can then measure the amount of the different atoms in a rock and provide a fairly accurate age for its formation.
If the process of obtaining absolute dates from isotopes seems very complex, don’t worry: In Chapter 16, I explain in much more detail how absolute dates are calculated and how they are combined with relative dates to construct the geologic timescale: a sequence of Earth’s geological history separated into different spans of time (such as periods, epochs, and eons).
Witnessing evolution in the fossil record
The most fascinating story told in the rock layers is the story of Earth’s evolution. To evolve simply means to change over time. And indeed, the earth has evolved in the 4.5 billion years since it formed.
Both the earth itself and the organisms that live on Earth have changed through time. In Chapter 17, I briefly explain the biological understanding of evolution. Much of modern understanding about how species have changed through time is built on evidence from fossilized or preserved life forms in the rock layers. Fossilization occurs through different geologic and chemical processes, but all fossils can be described as one of two forms:
Body fossils: Remains of an organism itself, or an imprint, cast, or impression of the organism’s body.
Trace fossils: Remains of an organism’s activity, such as movement (a footprint) or lifestyle (a burrow) but without any indication of the organism’s actual body.
Earth did not always support life. In Chapter 18, I describe the very early Earth as a lifeless, hot, atmosphere-free planet in the early years of the solar system’s formation. It took billions of years before simple, single-celled organisms appeared, and their origins are still a scientific mystery.
Simple, single-celled life ruled Earth for many millions of years before more complex organisms evolved. Even then, millions of years passed with soft-bodied life forms that are difficult to find in the fossil record. It wasn’t until 520 million years ago that the Cambrian explosion occurred. Chapter 19 describes this sudden appearance of shell-building, complex life as well as the millions of years that followed when life was lived almost entirely in the oceans until amphibians emerged on the land.
Chapter 20 delves into the Age of Reptiles, when dinosaurs ruled the earth and reptiles filled the skies and seas. During this period, all the earth’s continents were connected as Pangaea, Earth’s most recent supercontinent. But before the Age of Reptiles ended, Pangaea broke apart into the separate continents you recognize today. Evidence for Pangaea is still visible in the coastal outlines of South America and Africa — indicating where they used to be attached as part of the supercontinent.
In relatively recent time, geologically speaking, mammals took over from reptiles to rule the earth. The Cenozoic era (beginning 65.5 million years ago), which we are still experiencing, is the most recent and therefore most detailed portion of Earth’s history that can be studied in the geologic record (the rocks). Many of the most dramatic geologic features of the modern Earth, such as the Grand Canyon and the Himalayan Mountains, were formed in this most recent era. In Chapter 21, I describe the evolution of mammal species (including humans) and the geologic changes that occurred to bring us to today.
At various times in the history of Earth, many different species have disappeared in what scientists call mass extinction events. In Chapter 22, I describe the five most dramatic extinction events in Earth’s history. I also explain a few of the common hypotheses for mass extinctions, including climate change and asteroid impacts. Finally, I explain how the earth may be experiencing a modern-day mass extinction due to human activity.
Chapter 2
Observing Earth through a Scientific Lens
IN THIS CHAPTER
Geology is one of many sciences that study the natural world. Before moving on to the details of geologic science, I want to spend a little time sorting out what exactly science is and does. In this chapter, I describe the elements of science and the scientific method, and I explain how you do science every day perhaps without even realizing it!
Realizing That Science Is Not Just for Scientists
Science is not a secret society for people who like to wear lab coats and spend hours looking into microscopes. Science is simply the asking and answering of questions. Any time you make a decision by considering what you know, collecting new information, forming an educated guess, and figuring out whether your guess is right, you participate in acts of science.
Take a very simple example: choosing a shampoo. You’ve probably tried different types or brands of shampoo, observed how each one leaves your hair looking and feeling, and then decided which shampoo you wanted to purchase the next time. This process of observation, testing, and decision-making is all part of the scientific approach to problem-solving. You follow this process every day in multiple situations as you make decisions about what to buy, what
