Geology 101 Study Guide #4 Our last unit is the study of the earthquakes, the Earth s interior and plate tectonics. We live in a region which has been added on to North America through plate tectonics. The application of this part of the quarter is in our backyards. I hope you will enjoy learning about this information! PART I: Focus On: Earthquakes: What types of information are needed to locate the epicenter of an earthquake? What factors determine the amount of damage from an earthquake? How do deep versus shallow soils influence the amount of shaking during an earthquake? Liquefaction tends to happen more often in what type of soils? Identify the s-p interval and amplitude on a seismogram and their relationship to the distance of a seismograph from the epicenter. How do these waves and internals change at greater distances from the epicenter? What is a tsunami? How does a tsunami develop? What is the Mercalli Intensity Scale? What factors influence the amount of damage due to an earthquake? What is the difference between the motion of an s- wave and a p-wave? Inner Earth: You should know the basic subdivisions of the inner earth (crust, mantle, outer core, and inner core), what type of material is in each zone, and how we know about its existence. How does the density of the Earth change with depth inside the Earth? Which of the zones of the Earth has the greatest volume? What is the relative thickness of the continental crust versus the oceanic crust? What role do the seismic wave velocities play in our interpretation of the Earth s interior? How do we know the earth s magnetic field is not formed by a big magnet inside the earth? What forms the Earth s magnetic field? What is the effect of high temperature (like those inside the Earth) on magnets? What determines the velocity of seismic waves? How are p and s waves different? What creates the p-wave and the s-wave shadow zones? What do these zones tell us about the structure of the Earth s interior? You should be able to describe the principle of isostasy and how it helps shape the Earth s surface. Why are there ocean basins? What is the density of the oceanic crust compared to the continental crust? Why is there a difference? Plate tectonics: What were the types of evidence that Wegner used to support his hypothesis of continental drift? You should be able to describe these techniques and identify how they help support his idea. What are magnetic reversals and how do they help support seafloor spreading? What is the pattern of ages in the ocean basins? What are the three major types of plate boundaries? How do we recognize them? Is Hawai i and the other islands of that chain, a product of a divergent plate boundary? Why/ Why not? You should be able to draw a cross-section (not just a profile) starting from the Juan de Fuca ridge through Washington. In this cross section you should show the location of the Olympic Mountains, the Puget lowlands, and the Cascade Mountains. Why are there volcanoes in the Cascade Mountains (such as Mt. St. Helens)? How would you recognize the location of a divergent plate margin? A convergent plate margin? A transform plate margin? Why are there deep sea trenches at subduction zones? How does magma form in divergent plate boundaries? In convergent plate boundaries? 1
Sample Questions 1. Buildings in which of the areas in the map below will experience the greatest damage during an earthquake? (Assume that the quality of construction is the same throughout the map area.) Describe the specific hazards you would anticipate in that area. What area would be the best foundation for a building in terms of the seismic hazard of the area? 2. a) Below are two seismograms of the same earthquake. Label the p, s, and l waves on the A seismogram. b) Assume that site A was closer to the epicenter of the earthquake. What is incorrect about these two seismograms, if A was closer to the epicenter? 3. How does the Mercalli Scale differ from the Richter Magnitude Scale? Explain their uses. How does the Richter and Moment Magnitude Scales differ? 4. A tsunami has such a long wave length that a ship at sea can have the wave pass by it without knowing it. How can the tsunami create such disaster when it reaches shore? 2
5. In 1921, the epicenter of a 4.9 earthquake was at A in the diagram below. Earthquakes in 1935, 1945, and 1970 were located at points C, B, and D respectively. These three earthquakes were from 5.0-6.2 on the Richter Magnitude Scale. The shaded areas around A, B, C, and D represent the aftershock areas from those earthquakes. If you were responsible for the prediction of the next earthquake in the area illustrated by the map view, which two sites would you select for future intensive research efforts. Explain, in detail, the logic you used to select these two sites. 6. Label the regions of the interior of the earth in the cross-sectional diagram of the earth below: For each of the regions noted: a) describe whether the material is a solid or liquid. b) discuss the composition of the material in the region and the lines of logic which indicate this composition. 3
7. Imagine that you have observed the following geographic map and you suspect that there may be a divergent plate boundary between Land A and Land B. What techniques could you use to determine if a divergent plate boundary is present? (The more techniques you suggest the better- as long as they will work!) Assume that you have sufficient time and money to do this. 7b. Are the earthquakes that happen at divert plate margins shallow or deep hypocenters? Explain your answer. 8. Describe two methods one could use to determine the relative velocity of the earth's lithospheric plates. (Do not use the satellite (GPS) method). 9. Describe two techniques one could use to determine the upper surface of the Juan de Fuca plate as it is subducted under North America. 10. Why are there deep sea trenches at the leading edge of a subduction zone? 11. At the mid-oceanic ridges, geologists believe that the Earth s crust is pulling apart along what we call a divergent plate boundary. If the Earth is under tension in this area (which appears to be true from earthquake studies), why is there a mountain range along this zone? 12. The extinction of the dinosaurs has puzzled earth scientists for a long time. About 15 years ago a hypothesis was proposed that mass extinctions, such as the one in which the dinosaurs became extinct, were created by the collision of meteorites and/or comets and the Earth. In the past 15 years a significant amount of data suggests that meteorite and/or comet impacts have had a major effect on life on Earth. There are about 150 known impact craters on the Earth. Most of these are "terrestrial," (on land). Since a majority of the Earth's surface is covered by water, it is likely that a significant number of meteorites and/or comets have landed in the oceans. (A crater has been found off of the coast of Mexico which dates to about 65 million years and is now assumed to be the impact site for the event at the end of the Cretaceous which ended the reign of the dinosaurs.) It is possible (likely) that there have been more impacts of meteors/comets in the ocean basins than the number of craters under the oceans. Explain two reasons that meteorites may have hit in the oceans, but today we cannot find a sufficient number of craters in the ocean basins (the water does not significantly "cushion" the impact of the object!). 4
13. If the distance from the oceanic trench to the line of andesitic volcanoes is 200 miles in one location and 300 miles in another location, what reasons can you suggest for this difference in distance? Explain your ideas as "far" as you can. 14. Describe the relative motion of the lithospheric plates of the western United States. Discuss the types of plate boundaries found between the various plates. Using arrows illustrate the relative motion of the various plates of the Pacific Northwest and California. 5
15. In the map below, a "new" hot spot has just formed under the Pacific plate. The Pacific plate is now moving in a northwesterly direction. Let's say for this problem that in 10 million years the Pacific plate changes its direction to a straight northerly direction. Draw what you think the series of seamounts would look like from this hot spot at 20 million years in the future. 16. Use the information on page 45 to explain the cross section of the Pacific Northwest below: W Mtn Range A ocean E Mtn Range B What range of mountains is Mtn Range A? Mtn Range B? Can you find the Juan de Fuca Ridge? The Juan de Fuca plate? The Pacific plate? Where would you find oceanic crust? Continental crust? Benioff zone? 6