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14. Describe the probable source of all the earthquakes that occur in 3. Discuss evidence that would explain why plate tectonics occurs
southern California. on Earth but not on other planets.
15. The northwestern coast of the United States has a string of 4. Analyze why you would expect most earthquakes to be localized,
volcanoes running along the coast. According to plate tectonics, shallow occurrences near a plate boundary.
what does this mean about this part of the North American
Plate? What geologic feature would you expect to find on the
seafloor off the northwestern coast? Explain. INVITATION TO INQUIRY
16. Explain how the crust of Earth is involved in a dynamic, ongoing
recycling process. Measuring Plate Motion
Tectonic plate motion can be measured with several relatively new tech-
FOR FURTHER ANALYSIS nologies, including satellite laser ranging (SLR) and use of the Global
Positioning System (GPS). Start your inquiry by visiting the Tectonic
Plate Motion website at http://cddis.nasa.gov/926/slrtecto.html. Study
1. Why are there no active volcanoes in the eastern United States or
Canada? Explain why you would or would not expect volcanoes the regional plate motion of the North American Plate, for example.
there in the future. Note the scale of 50 mm/yr, then measure to find the rate of movement
at the different stations shown. Are these recent data on plate motion
2. Describe cycles that occur on Earth’s surface and cycles that
occur between the surface and the interior. Explain why these consistent with other available information on plate motion? How can
cycles do not exist on other planets of the solar system. you account for the different rates of motion at adjacent stations?

PARALLEL EXERCISES

The exercises in groups A and B cover the same concepts. Solutions to group A exercises are located in appendix E.

Group A Group B
1. The rate at which the temperature increases with depth is called 1. The rate at which the temperature increases with depth is called
the geothermal gradient. What is the geothermal gradient in a the geothermal gradient. What is the geothermal gradient in a
tectonically stable region where the temperature is 100°C at a tectonically stable region where the temperature is 140°C at a
depth of 6 km? (Assume a surface rock temperature of 10°C.) depth of 5 km? (Assume a surface rock temperature of 10°C.)
2. Based on a geothermal gradient of 15°C/km, what is the 2. Based on a geothermal gradient of 15°C/km, what is the
temperature at the bottom of the deepest oil well mentioned in temperature at the bottom of the deepest mine mentioned in the
the Overview? (Assume the temperature at the surface is 20°C.) Overview? (Assume the temperature at the surface is 20°C.)
3. Based on a geothermal gradient of 15°C/km, what is the 3. Based on a geothermal gradient of 25°C/km, what is the
temperature at the bottom of the continental crust? (Assume the temperature at the bottom of the continental crust? (Assume the
temperature at the surface is 20°C and a 35 km thick crust.) temperature at the surface is 20°C and a 35 km thick crust.)
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4. What is the pressure, in N/m , at a depth of 20 km in the 4. What is the pressure, in N/m , at the depth of the deepest mine?
continental crust? 5. What is the pressure, in N/m , at a depth of 75 km in the
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5. What is the pressure, in N/m , within a mountain range at a continental crust?
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depth of 35 km? 6. What is the pressure, in N/m , at the base of the mantle? (Ignore
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6. What is the pressure, in N/m , at a depth of 100 km in the the crust and assume ρ mantle = 3.3 g/cm .)
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mantle? (Ignore the crust and assume ρ mantle = 3.3 g/cm .) 7. Wood floating in water can demonstrate how the position at
7. Ice floating in water demonstrates how the position at which which crust floats in the mantle can be estimated from the
crust floats in the mantle can be estimated from the thickness of thickness of the crust and the relative densities of crust and
the crust and the relative densities of the crust and mantle. If a mantle. If a 9.0 cm thick block of wood is placed in a pan of
5 cm thick block of ice is placed in a pan of water, how much of it water, how much of it will be exposed above water?
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will be exposed above water? (ρ ice = 0.92 g/cm .) (ρ wood = 0.72 g/cm .)
8. A teacher would like to demonstrate the behavior of continental 8. A teacher would like to demonstrate the behavior of continental
and oceanic crust floating in the mantle by using wood blocks and oceanic crust floating in the mantle by using wood blocks
and a pan of salad oil. What density of wood is required to model and a pan of water. What density of wood is required to model
how the continental crust floats in the mantle? What density of how the continental crust floats in the mantle? What density of
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wood is required to model the oceanic crust? (ρ oil = 0.91 g/cm .) wood is required to model the oceanic crust?
18-21 CHAPTER 18 Plate Tectonics 475