Chapter 17 Unraveling Geologic History

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396 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY A B C Interpreting Profiles The profiles in Figure 17-8 illustrate how a geologic history can be inferred for each profile. The law of superposition tells us that unless there is evidence of overturning or an igneous intrusion, the youngest layers are on the bottom. Profile A starts with deposition of sediments to form conglomerate, sandstone, and shale, in that order. These layers are faulted, followed by deposition (of limestone) and erosion. Profile B has igneous rock at the bottom, so it starts with crystallization of magma. This must have happened before the deposition of sediments because no metamorphism is shown in the siltstone layer. The law of original horizontality tells you that shale, sandstone, and conglomerate above were deposited and then folded. Later, the top layers were eroded. Water Key to Rock Types Sedimentary Rocks Conglomerate Sandstone Siltstone Shale Limestone Igneous Rocks Granite Basalt Metamorphic Rocks Schist Gneiss Contact Metamorphism Figure 17-8 Geologic profiles contain clues to the sequence of events that created them. The series of events responsible for each of these three profiles is explained in the text.
Intrusion and extrusion at the surface are the final events, unless you wish to point out that erosion at the surface is an ongoing process. Profile C has a metamorphic rock at the bottom. This rock could have two possible origins. Therefore, this history begins either with deposition and sedimentary rock formation followed by metamorphism or by solidification of an igneous rock followed by metamorphism. Gneiss can begin as a sedimentary or an igneous rock. Intrusion of basalt occurred next. Erosion (note the unconformity) is followed by deposition of limestone. The second basalt body must have been an extrusion because there is contact metamorphism below it, but not above. Deposition of shale sediments is probably the last step in this profile because the water above (probably a lake or an ocean) would support renewed deposition. Do you know the square root of 4? Most people think only of 2, but negative 2 is also correct. This question has two equally valid answers. In the same way, geologic profiles are often subject to different interpretations. As long as the sequence of events supports the features shown in the profile, and all the features are explained, a sequence is considered correct. Multiple interpretations are common in science. This is especially true in an environment as complex as the geosciences. HOW DO GEOLOGISTS ESTABLISH ABSOLUTE TIME? HOW DO GEOLOGISTS ESTABLISH ABSOLUTE TIME? 397 In the first part of this chapter, you learned about techniques to determine a sequence of events in relative time. However, scientists want to know more. They want to know how old Earth is, how long the events of mountain building and erosion have been taking place, and how long Earth has been inhabited by living creatures. Recent discoveries about the structure and stability of atoms, along with the technology to count atoms, have finally made it possible to answer these questions.
Page 1 and 2: Chapter 17 UNRAVELING MYSTERIES Unr
Page 3 and 4: have been able to piece together a
Page 5 and 6: Original Horizontality HOW CAN WE D
Page 7 and 8: Figure 17-3 It is clear that this f
Page 9 and 10: Contact Metamorphism Metamorphism i
Page 11 and 12: Figure 17-6 Formation of an unconfo
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Page 17 and 18: Figure 17-9 Carbon atoms always con
Page 19 and 20: HOW DO GEOLOGISTS ESTABLISH ABSOLUT
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Page 23 and 24: Base your answers to questions 2-4
Page 25 and 26: Which sequence of events below is b
Page 27 and 28: 12. The graph below represents data
Page 29 and 30: Use the diagram below for questions

Chapter 17 Unraveling Geologic History

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