Chapter 11: A Biography of Earth
Study Plan
A geologist in the field, here on the shore of Kangaroo Island, Australia. Such field observations have allowed geologists to work out the history of Earth.
Credit: Stephen Marshak
Guide to Reading
The last chapter dealt with how Earth's history has been worked out; this one deals with the story itself. It begins by pointing out that the history of Earth should include information about orogenies, changing shapes of continents, changing positions of continents, past environments, past climates, changes in sea level, and the evolution of life. The chapter organizes all of this information in terms of the major divisions of geologic time: the Precambrian (from Earth's beginning 4.57 billion years ago up to 545 million years ago) and its subdivisions (Hadean Eon, Archean Eon, and Proterozoic Eon) and the rest of all time, the Phanerozoic Eon, including its subdivisions (the Paleozoic Era, including the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods; the Mesozoic Era, including the Triassic, Jurassic, and Cretaceous Periods; and the Cenozoic Era, with special attention paid to the Pleistocene Ice Age of the Quaternary Period). That's a lot of names, but do learn them. They're basic to any discussion of Earth history.
The rest of Chapter 11 is the story itself; highlights of 4.57 billion years of change. This presents a problem. Even the most concise summary of 4.57 billion of anything is going to be long. What is essential to learn? The author has chosen major milestones in Earth's development that any well-informed general geologist should know. Still, basic literacy means different things to different students, to different teachers, and in different situations. For example, the particular facts required of you could depend on the location of your school. If you're in the Denver area, no doubt you'll have to know about the Ancestral Rockies, the Laramide Revolution, and the Western Interior Seaway. If you're located in Boston or Seattle, other particulars may seem more important to you or to your teacher. This chapter presents a wonderful opportunity to bring basic concepts to life by indulging your curiosity about the history of your present (or hometown) surroundings.
Whatever your goal, it's easy to get mired down in all the details and names. To avoid this, as you study, try to picture our planet as it was in the past-a truly alien world. It's not dull stuff; it's been the subject matter for popular comics and blockbuster movies for decades. Who hasn't heard of the Flintstones or Jurassic Park? Asteroid encounters have been exciting from Superman days up to Deep Impact. People everywhere wonder why the dinosaurs became extinct; maybe after reading this chapter you'll also wonder why the trilobites became extinct. As you read, remind yourself that this is not a fanciful movie plot; it's all based on scientific data and reasoning. Truth really is stranger than fiction.
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