Ch 17 Interpreting Earth History

Ch 17 Interpreting Earth History - 2009 Allan Ludman and...

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1 CHAPTER 17 INTERPRETING GEOLOGIC HISTORY: WHAT HAPPENED AND WHEN DID IT HAPPEN? PURPOSE • To learn how to determine the relative ages of rocks and geologic processes and use these methods to interpret complex geological histories. • To learn how numeric (absolute) ages of rocks are calculated and apply them to dating geologic materials and events. • To see how geologists piece together Earth history from widely separated areas. MATERIALS NEEDED Pen, pencils, and a calculator 17.1 INTRODUCTION You’ve learned to identify minerals, use mineralogy and texture to interpret the origin of rocks, deduce which agents of erosion have affected a given area, and recognize evidence for tectonic events. With these skills you can construct a three-dimensional picture of Earth, using topography and surface map pattern to infer underground relationships. This chapter adds the fourth dimension – time: the ages of rocks and processes. Geologists ask two different questions about age: “Is a rock or process older or younger than another?” (their relative ages) and “Exactly how many years old are they?” (their numeric or absolute ages). We look first how relative ages are determined, then at methods for calculating numeric age, and finally combine them to decipher geologic histories of varying complexity. 17.2 PHYSICAL CRITERIA FOR DETERMINING RELATIVE AGE Common sense is the most important resource for determining relative ages. Most reasoning used in relative age dating is intuitive and the basic principles were used for hundreds of years before we could measure numeric ages. Geologists use two types of information to determine relative age: physical methods based on features in rocks and relationships between them, and biological methods that use fossils. We will focus first on the physical methods and return to fossils later. © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company
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