PSc 201 Chapter 3 Homework Critical Thinking Questions 1. (adapted from text) Seawater is denser than fresh water. A ship moving from the Atlantic Ocean into the Great Lakes goes from seawater to fresh water. Will the ship sink farther into the fresh water, stay at the same level, or rise slightly higher? 2. a) Draw a picture of a transverse wave on a slinky. b) Draw a picture of a longitudinal wave on a slinky. c) Is a P-wave a transverse wave or a longitudinal wave? 3. On a warm calm afternoon with no wind the air near the ground will be hot and the temperature of the air will become cooler and cooler as height above the ground increases. The speed of sound is faster in hotter air than in cooler air. I have provided a side view of the ground and the air above it and I have drawn a sound wave crest (straight line) from a speaker which is beginning to travel in a direction parallel to the ground, as shown by the arrow. Draw more positions of this sound wave crest as it moves across the ground and decide whether this sound wave will continue to travel parallel to the ground, or will angle upward away from the ground or downward toward the ground. This is an example of wave refraction.
4. a) Mark the approximate position of the P-wave shadow zone on the surface of the Earth on this cross section diagram. The x shows the epicenter of an earthquake. b) Mark the approximate position of the S-wave shadow zone on the surface of the Earth on this cross section diagram. The x shows the epicenter of an earthquake. 5. What specific information does the S-wave shadow zone give us about the interior of the Earth? 6. a) When the Earth is divided into the regions called the crust, the mantle, and the core, how are these regions different from each other? b) When the Earth is divided into the regions called the lithosphere, the asthenosphere, the lower mantle, the outer core, and the inner core, how are these regions different from each other? 7. What are the two necessary conditions for a planet or a moon to generate its own magnetic field?
8. The surface of the oceanic lithosphere lies lower than the surface of the continental lithosphere for two reasons, difference in thickness and difference in density. a) Which is thicker? b) Which is denser? 9. Why is lithosphere in the Hudson s Bay region of Canada rising several centimeters higher each year? 10. In the late 1800 s Lord Kelvin made a calculation of the time it would take a planet the size of the Earth to cool from a molten state to a state with the surface temperature of the Earth now. There were no serious errors in his calculation, and his result was about 80 million years. Because of his calculation, he thought that the Earth was probably only about 80 million years old. What did he leave out of his calculations (because no one knew about this yet)? 11. Geologists in the 1800 s had a hypothesis to explain how mountains were formed on the Earth as it cooled. What was this hypothesis? 12. a) In 1788 James Hutton described his hypothesis that erosion of the Earth s surface was a very slow process which had operated in the same way for a very long time in order to wear down entire ranges of mountains, so he decided that the Earth must be very old. What did he name his hypothesis? b) Before James Hutton most geologists believed that the Earth s surface had been created abruptly during the biblical flood. What was the name of this hypothesis?
13. Leonardo da Vinci left records of his interest in many things. He noticed something which has been an important clue to understanding plate tectonics, and he was apparently the first person in history to wonder about this. What did he notice? 14. Alfred Wegener presented new evidence that South America and Africa which are now widely separated by the Atlantic Ocean had once been part of a single supercontinent, and were located in different positions on the Earth than they are now. Briefly explain two of the four types of evidence. a) b) c) d) 15. Where is the true edge of a continent, which is what we have to consider when judging how well two continents would fit together? 16. Wegener s hypothesis of continental drift proposed that the continents had drifted across the ocean floor. There was some missing evidence which caused most other scientists not to believe his hypothesis. What evidence was missing? 17. Between 1912 when Wegener proposed his hypothesis of continental drift and 1960, four additional types of strong evidence were found as more sophisticated scientific methods became available. Explain all four types of modern evidence. a)
b) c) d) 18. In 1965 Matthews and Vine at Cambridge, England published geologic evidence that new seafloor was formed along the Mid-Atlantic Ridge. This convinced geologists that the hypothesis which later came to be called plate tectonics was correct. What type of geologic evidence did Matthews and Vine publish? 19. What is the difference between Wegener s continental drift hypothesis and plate tectonics? 20. The movement of tectonic plates was at first thought to be caused by the convection currents in the asthenosphere dragging along the bottom of the lithosphere plates. Now geophysicists feel that this dragging helps the plates move, but think that there are two more important causes of plate motion. a) Explain slab push. A diagram may be useful. b) Explain slab pull. A diagram may be useful.
21. Explain where the oldest ocean floor is located, and why even the oldest ocean floor is only about 200 million years old. 22. What evidence shows the location of the edges of the plate boundaries most clearly? 23. What is the name of the plate which is subducting under northern California, Oregon, Washington, and British Columbia and creating the Cascade Mountains and their volcanoes? 24. Name the three basic types of plate boundaries. a) b) c) 25. Which type of lithosphere is both created and destroyed? 26. Which type of lithosphere is created, but is not destroyed? 27. List in order the processes which create a new ocean basin.
28. List one location where rifting is occurring on land. 29. Name the ridge where new Pacific Ocean basin is forming. 30. About how long ago did the supercontinent Pangaea begin to break apart? 31. List a geographic location which is a good example of each of the following types of plate boundaries. a) a divergent oceanic-oceanic boundary b) a divergent continental-continental boundary c) a convergent oceanic-continental boundary d) a convergent oceanic-oceanic boundary e) a convergent continental-continental boundary f) a transform boundary 32. Where are deep sea trenches found?
33. Name the two geologic locations which cause volcanoes to form. a) b) 34. How far down into the mantle can subducting oceanic lithosphere slabs survive as they descend? 35. Where within the Earth is the Earth s magnetic field generated? 36. The Earth s magnetic field is similar to the magnetic field of a bar magnet. a) What is the magnetic pole nearest the north geographic pole called? b) If we pretend that there is a giant bar magnet buried inside the Earth, which pole of this bar magnet is nearest the north geographic pole of the Earth? 37. A compass needle points toward magnetic north. If a compass needle points to one end of a bar magnet, which magnetic pole of the bar magnet is this? 38. If a cylindrical core drilled from some volcanic rock is magnetized horizontally, where on the Earth s surface was this volcanic rock when it cooled? 39. If a cylindrical core drilled from some volcanic rock is magnetized vertically upward (meaning that the magnetic field made by this sample points vertically upward), where on the Earth s surface was this volcanic rock when it cooled?
40. If a cylindrical core drilled from some volcanic rock is magnetized at about a 45 degree angle downward, in what general region on the Earth s surface was this volcanic rock when it cooled? 41. It is possible that reversals of the Earth s magnetic field have an effect on the evolution of living things. Explain how this could happen. 42. Explain how volcanoes on land can record magnetic reversals. 43. Explain how the seafloor rock can record magnetic reversals. 44. When a magnetometer is towed behind a ship across a section of the seafloor which was magnetized during a reversed magnetic era, will the magnetometer register a total magnetic field which is higher than average, or lower than average? 45. Explain how paleomagnetism can track the wandering of a continent.
46. Hot spots produce volcanic activity at Earth s surface. a) Give the location of a major hot spot under an ocean. b) Give the location of a major hot spot under a continent. 47. Which Hawaiian Island is oldest? Kauai is at the northwest end of the islands. Hawaii is at the southeast end of the islands. 48. What does the difference in direction of the Hawaiian Island Chain and the Emperor Seamount Chain tell us? 49. Is a location on the seafloor where the sediment layer is thick close to or far from the divergent boundary where new seafloor is formed? 50. The part of California where we live, and most of the western half of California, is made of bits and pieces of continental material and sediments scraped off an oceanic plate as it subducted under the North American Plate. What are these materials which were pasted onto the North American Plate called?
Analytical Thinking Questions 51. I am going to ask you if a particular rock sample is a piece of continental lithosphere or a piece of oceanic lithosphere. Assume that I provided you with a digital balance and you used this balance to find that the mass of the rock is 488 g. Also assume that I provided you with a plastic cylinder with a volume scale on the side which was filled with water to the 100 cm 3 mark, and when you dropped the rock into the water the level rose from 100 to 268 cm 3. Calculate the density of this rock sample and tell me which it is. 52. The present distance between corresponding locations on the east coast of South America and the west coast of Africa is about 5,800 km at present. The Atlantic Ocean basin began to form about 210 million years ago. a) If the Atlantic Ocean basin grew at a steady rate during this period, at what speed in cm/year has the Atlantic Ocean basin increased in width? b) The average period of the Earth s magnetic polarity during the past 5 million years has been about 200,000 years. About how many kilometers wide would a magnetic stripe on the ocean floor be if it corresponded to 200,000 years?