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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2 17.2.1 Principles of Original Horizontality and Superposition T he Principle of Original Horizontality states that most sedimentary rocks are deposited in horizontal beds (there are exceptions, such as inclined sedimentation in alluvial fans, dunes, and deltas). Deposition as horizontal beds makes it easy to determine relative ages in a sequence of sedimentary beds if the rocks are still in their original horizontal position . a. Figure 17.1 shows sedimentary rocks in the Painted Desert, Arizona. Assuming original horizontality, label the oldest. The youngest. Where will the oldest rocks be in any sequence of undeformed, horizontal sedimentary rocks? Explain. Figure 17.1 Horizontal strata in Painted Desert National Park, Arizona This bit of common sense was first applied to sedimentary rocks by Nils Stensen (Nicolaus Steno in Latin) almost 400 years ago and is called the Principle of Superposition . It applies equally well to horizontal lava flows and volcanic ash deposits, but cannot be used if rocks have been tilted or folded. 17.2.2
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This note was uploaded on 03/03/2009 for the course GEOL 101 taught by Professor Jackel during the Spring '09 term at CUNY Queens.

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

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