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CHATPER 4 LESSON 2 THE RELATIVE AGE OF ROCKS breathtaking-rock-formation/weird- rocks-formation-01 /

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1 CHATPER 4 LESSON 2 THE RELATIVE AGE OF ROCKS breathtaking-rock-formation/weird- rocks-formation-01 /


3 LAW OF SUPERPOSITION ( Good animation on DE – law of superposition) According to the law of superposition,undisturbed horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it. Ordering the Grand Canyon’s History The law of superposition can be applied to the layers exposed in the Grand Canyon. Interpreting Illustrations Which layer is the oldest? youngest?

4 Clues from Igneous Rock Extrusion Lava that hardens on the surface and forms igneous rock. Intrusive Magma may push into bodies of rock below the surface. There the magma cools and hardens in to a mass of igneous rock called intrusive. An intrusion is always younger than the rock layers around and beneath it. Fault A break in Earth’s crust. Forces inside the Earth cause movement of the rock on opposite sides of a fault.


6 FIG. 24. Sketch showing intrusions and extrusions in cross section and plan views. A, Volcanic neck, the feeder pipe for a volcano. B, Volcanic neck after erosion has removed the cone. C, Sill, sheet of igneous rock that has spread laterally along the beds. D, A sill exposed by erosion. E, Intrusive plug that forced its way upward through bedded rocks. F, The top of a plug uncovered by erosion with upturned beds exposed on the flank. G, Laccolith, a tackhead-shaped intrusion that spread laterally along the bedding and arched the overlying rocks. H, Eroded laccolith exposed at the surface. The dip of the flanking hogbacks is less than in the plug (E, F) that cuts across the bedding. J, Dike, intrusive rock that fills a fissure that cuts across bedded rocks. This fissure filling cuts across the bedding and thereby differs from a sill (C). K, Lava flow that came from a dike feeder fracture, which is the source of most lava in the Big Bend. ns/state/tx/1968-7/sec1.htm

7 San Andreas fault. Aerial photograph of the San Andreas fault crossing the Carrizo Plain of California, USA. The San Andreas fault is one of the world's great seismic faults, forming the boundary between the Pacific and North American tectonic plates. It is marked by the valley-like depression running from top to bottom of the picture. Ridges and rifts line either side. This fractured landscape is formed by the pressures created by the opposing, lateral movements of the two tectonic plates. The San Andreas fault extends almost the full length of California and is responsible for major earthquakes. media/167148/ enlarge DE – fault animation

8 How do fossils show age? Index Fossil These fossils help geologists match rock layers. A fossil must be widely distributed and represent an organism that existed for a geologically short period of time. Index fossils are useful because they tell the relative ages of the rock layers in which they occur. How did you layer location 2, 3, and 4. What did you infer about the history of location 4? (Click and find out) Layer B must have eroded over time.

9 Unconformity Rock layers erode away, an older rock surface may be exposed. Then deposition begins again, building new rock layers. Unconformity is a gap in the geologic record. It shows where rock layers have been lost due to erosion.

10 Did you label and circle correctly?

11 FOLDING Sometimes, forces inside Earth fold rock layers are turned over completely. 1. What does the photo show? (an unconformity/ folding). 2. What evidence do you see for your answer to Question 1? 3.What can you infer about the history of this area? folding The layers are bent at an angle. After the rock layers formed, forces inside Earth raised and folded them.

12 The Relative Age of Rocks 1.What is the youngest rock layer? Explain. 2.Is the extrusion older or younger than rock layer B? Explain. 3.Is the fault older or younger than rock layer A? Explain. 4.How could a geologist use the fossil in rock layer B to date a rock layer in another location? N 5.___ fault 6.___ extrusion 7.___ unconformity 8.___ relative age 9.___ law of superposition 10.___ intrusion 11.___ absolute age 12.___ index fossils a.the number of years since a rock has formed b.a break in Earth’s crust c.the way to determine relative ages of rocks d.a hardened layer of magma beneath Earth’s surface e.the age of a rock compared with the age of other rocks f.fossils used to help geologists match rock layers g.the surface where new rock layers meet a much older rock surface beneath them h.a hardened layer of lava on Earth’s surface

13 Answer Key 1.Layer C is the youngest because the law of superposition says that a layer is younger than the layers below it. 2.The extrusion is younger because extrusions are always younger than the rock layers below them. 3.The fault is younger than layer a because rock layers are always older than the faults they contain. 4.The fossil might be an index fossil. Geologists can use index fossils to match rock layers at locations that are far apart. 5.b 6.h 7.g 8.e 9.c10.d 11.a12.f

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