Chapter 17 Unraveling Geologic History

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392 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY 1 Deposition of Layers of sediment Key: Sandstone Shale Contact metamorphism 2 Intrusion and Extrusion Limestone Basalt Extrusion Subsequent deposition Intrusion Contact metamorphism (transition zone) 3 Subsequent Deposition Figure 17-5 Contact metamorphism—the first diagram shows sedimentary rocks that will be intruded by magma in diagram 2. The portion of the magma that comes to the surface is known as an extrusion. In diagram 3, new deposition has left a sandstone layer on top of the basalt. By looking at diagram 3 alone, you can tell that the basalt magma was extruded and that the top sandstone layer is younger than the extrusion because there is no contact metamorphism on top of the basalt. erosion surface, the buried erosion surface is known as an unconformity. Figure 17-6 shows the formation of an unconformity. Deposition, which most often occurs in water, creates sediments that become sedimentary rock. The region is uplifted and the layers are exposed to weathering and erosion. After erosion has occurred over a period of time, subsidence (sinking of the land) and flooding leads to the deposition of a new layer on top of the erosion surface. Unconformities are common throughout New York State. New York’s sedimentary rocks show a continuous record of the early development of fish and land animals. For a long time, a shallow sea covered New York. Then, about 350 million years ago, the land was pushed up above sea level. This was probably the result of a collision between ancient North America and Africa. The disappearance of the sea not only stopped the deposition, but it also led to the erosion of some layers of previously deposited sediments. Wherever new layers were deposited on top of the erosion surface, a buried erosion surface, or unconformity, was created. Figure 17-7 on page 394 shows an unconformity in Utah.
Figure 17-6 Formation of an unconformity—layers of sedimentary rock are created by deposition and compression. Uplift exposes the layers to weathering and erosion, which wear down the land to a new level. Deposition on top of the erosion surface creates an unconformity: a gap in the geologic record. HOW CAN WE DETERMINE THE SEQUENCE OF EVENTS? 393 1. Deposition 2. Uplift 3. Erosion 4. New Deposition Key to Rock Types Shale Water Erosion surface Water Sandstone Limestone Unconformity Conglomerate
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: Contact Metamorphism Metamorphism i
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Page 15 and 16: Intrusion and extrusion at the surf
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
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Chapter 17 Unraveling Geologic History

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