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# Faults: The Earth's Crustal Breaking Point (page 2)

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Author: Janice VanCleave

Shape three or more 1 × 1 × 4-inch (2.5 × 2.5 × 10-cm) alternating layers of colored dough on a cookie sheet. Cut two diagonal fault planes for each model. The bottom of the first planes should slant inward with the fault block moved down to represent a graben (see Figure 18.2A). In the second model, the bottom of the planes should slant outward with the fault block moved up to represent a horst (see Figure 18.2B). With adult permission, bake the dough at 275°F (135°C) for 2 hours or until the dough is firm. For information about fault blocks, see David Lambert and the Diagram Group, The Field Guide to Geology (New York: Facts on File, 1988), p. 9l.

1. A fault block displaced downward and bounded by parallel normal faults is called a graben or rift. The steep-walled rift valley (long, narrow breaks in the Earth's crust) that runs down the center of the mid-Atlantic Ridge is a graben. (For more information about rift valleys in this and other mid-ocean ridges, see Chapter 19, "Plate Tectonics.") An up thrust block bounded by parallel faults is called a horst. Use salt dough to make models showing these fault blocks. For each color of dough, mix 2 cups (500 ml) of flour and 1/2 cup (125 ml) of table salt in a bowl. Add 20 drops of food coloring to 3/4 cup (188 ml) of water. Add the colored water to the salt and flour mixture. Knead the dough about 3 minutes or until it is soft and pliable. Note: Add a little more flour if the dough feels sticky or a little more water if it feels dry.
2.

1. How does tensional stress affect the width of the crust in a fault zone (area of Earth's crust that includes the fault blocks on both sides of the fault plane)? On a 4 × 6-inch (10 × 15-cm) piece of cardboard, draw and label the shapes shown in Figure 18.3, then cut out the shapes. Lay the pieces together on a table so that all their edges are even. Measure and record the total width of the assembled pieces. Demonstrate normal faulting caused by tensional stress by moving the two hanging walls down about 1 inch (2.5 cm). Measure and record the total width of the assembled pieces in this normal faulting position.
2. How does compressional stress affect the width of the crust in a fault zone? Assemble the pieces from the previous experiment to form a 4 × 6-inch (10 × 15-cm) rectangle. Demonstrate reverse faulting caused by compressional stress by moving the two foot walls down about 1 inch (2.5 cm), and push the pieces together. Measure and record the total width of the assembled pieces in this reverse faulting position.
3. How would the dip of the fault plane affect the results of the two previous experiments? Repeat each experiment twice. First, use cardboard pieces with a smaller fault plane dip. Then use cardboard pieces with a larger fault plane dip.

### Get the Facts

1. The largest recorded abrupt vertical displacement occurred in 1899 at Yakutat Bay, Alaska. Part of the Alaskan shore was lifted as much as 50 feet (15 m) above sea level. Is movement along fault planes always abrupt? How does the depth of the San Andreas fault affect its movement? For information about movement along fault planes, see Brian J. Skinner and Stephen C. Porter, The Dynamic Earth (New York: Wiley, 1995), pp. 414–415.
2. The Grand Tetons of Wyoming are fault block mountains. Find out more about the formation of fault block mountains. What is a thrust fault? See Steve and Jane Parker, Mountains and Valleys (San Diego: Thunder Bay Press, 1996), pp. 22–23.
3. Faults that are temporarily locked together are called lock faults. How can these and other faults produce earthquakes? For information, see Janice VanCleave's Earthquakes (New York: Wiley, 1993), pp. 12–15 and 24–27.
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