Plate Tectonics Purpose The purpose of this lab is to have you analyze Earth forces, faults, and plate boundaries to determine if Earth’s volume (size) is decreasing, increasing, or staying the same. You will also measure and calculate rates of plate tectonic processes. Earth Forces and the Faults They Produce When Wegener proposed his Continental Drift Hypothesis involving lateral movements of Earth’s crust and creation of new ocean basins, his critics were preoccupied with geologic models favoring vertical motions of Earth’s crust. They reasoned that if Earth were cooling from a semi-molten state, then it should be contracting and shrinking in size (volume). This would force the continents to compress together and close ocean basins. Continental drift would be impossible. The possibility that Earth’s size might change actually brings up three fundamental questions about Earth’s global rocky sphere. Is Earth shrinking in size? Is Earth expanding in size? Or is Earth staying about the same size? These questions can be evaluated by studying Earth’s natural forces and faults in relation to those that you predict for by models of a shrinking, expanding, or constant Earth volume. Three kinds of directed force (stress) can be applied to a solid mass of rock (Figure 1). Compression compacts a block of rock and squeezes it into less space. This can cause reverse faulting, in which the headwall block is forced up the footwall block in opposition (in reverse of) to the pull of gravity (Figure 2). Tension pulls a block of rock apart and increases its length. This can cause normal faulting, in which gravity pull the headwall block down and forces it to slide down (in the normal direction of gravity) off of the footwall block (Figure 2). Shear smears a block of rock and may eventually tear it apart into two blocks of rock that slide past one another along a strike-slip fault (Figure 2). Causes shortening Causes lengthening Causes tearing and smearing Figure 1: Three kinds of stress (applied force, as indicated by arrows) and the strain they cause in an undeformed block of rock. Figure 2: Chart for comparing fault types (columns A and D) to stress (column C) and strain (column B) in Question 1 and to plate boundary types (column E) in Question 4. Column D Column A Column B Column C Column E Block Diagram Has the crust: Was the stress: Fault Is the plate boundary Shortened? Shear? Type type: Lengthened? Neither? Compression? Tension? Transform? Divergent? Convergent? Normal fault Reverse fault Strike-slip fault Questions 1. Analyze the three block diagrams in Figure 1 and the three kinds of faults shown in Figure 2. Complete columns B and C in Figure 2 to infer what kind of stress and strain is associated with each kind of faulting. 2. Refer to your partially completed chart in Figure 1. Think of Earth’s lithosphere (crust and rigid mantle) as the shell of an egg. What one kind of faulting would you expect to occur in Earth’s thin eggshell-like lithosphere if: a. Earth is expanding in size? b. Earth is shrinking in size? c. Earth’s outer shell fractures but there is no expansion or shrinking of its size? 3. Refer to Figure 3, a map showing the distribution of Earth’s lithospheric plates and their rates of motion in centimeters per year. Study this figure to determine the nature and distribution of Earth’s three kinds of plate boundaries: divergent (red), convergent (hachured), and transform (dashed). a. What stresses, plate motions, and other geologic processes occur along divergent plate boundaries (also called mid-ocean ridges, rifts, rises, or spreading ridges)? b. What stresses, plate motions, and other geologic processes occur along convergent plate boundaries? c. What stresses, plate motions, and other geologic processes occur along transform plate boundaries (also called transform faults)? 4. Based on your characterization of plate boundaries in question 3, complete column E in Figure 2. 5. Re-examine Figure 3 and notice the locations and lengths of Earth’s three kinds of plate boundaries. Then visually estimate the following data. a. What percentage (based on length) of Earth’s plate boundaries are transform plate boundaries? b. What percentage (based on length) of Earth’s plate boundaries are divergent plate boundaries? c. What percentage (based on length) of Earth’s plate boundaries are convergent plate boundaries? 6. Do you think Earth’s size is increasing (expanding), decreasing (shrinking), or staying about the same? Justify your answer by citing evidence from your answers to questions 1-5. Measuring and Evaluating Plate Tectonics Hot spots are centers of volcanic activity that persist in a stationary location for tens of millions of years. Geologists think that they are either: A) the result of long-lived narrow plumes of hot rock rising rapidly from Earth's mantle and forming magma by decompression melting the) a stream of heated lava rising in a lava lamp), or B) the slow decompression melting of a large mass of hot mantle rock in the upper mantle that persists for a long interval of geologic time. A volcano develops directly above the hot spot. When the plate slides on, the volcano that was over the hot spot becomes dormant, and over time it migrates many kilometers from the hot spot. Meanwhile, a new volcano arises as new lithosphere passes over the hot spot. The result is a string of volcanoes, with one end of the line located over the hot spot and quite active, and the other end distant and inactive. In between is a succession of volcanoes that are progressively older with distance from the hotspot. The Hawaiian Islands and Emperor Seamount chain are thought to represent such a line of volcanoes that formed over the Hawaiian hotspot. 7. Figure 4 shows the distribution of the Hawaiian island chain and Emperor Seamount chain. The numbers indicate the age of each island in millions of years (m.y.), obtained from the basaltic igneous rock of which each island is composed. a. What was the rate and direction of plate motion in the Hawaiian region from 4.7 to 1.6 million years ago? b. What was the rate and direction of plate motion from 1.6 million years ago to the present time? c. How does the rate and direction of Pacific plate movement during the past 1.6 million years differ from the older rate and direction of plate motion? d. Locate the Hawaiian island chain and the Emperor Seamount chain (submerged volcanic islands) in the top part of figure 4. How are the two island chains related? e. Based on the distribution of the Hawaiian Islands and Emperor Seamount chains, suggest how the direction of Pacific plate movement has generally changed over the past 60 million years. 8. Another hot spot is beneath Yellowstone National Park in the United States. There are no erupting volcanoes on the Yellowstone hotspot today, but there are Hot Springs and geysers. The high heat flow also causes buckling and faulting of Earth's crust. Geologist Mark Anders has observed that as the North American plate slides over this hot spot, it causes development of a U-shaped set of faults (with the closed end of the U pointing northeast). Also, as layers of volcanic ash and lava flows accumulate on deforming crust, they tilt. By dating the layers of tilted rocks, and mapping the U-shaped fault systems that moved beneath them, Anders has been able to compose a map of circular regions that were once centered over the hot spot at specific times. This line of deformation circles and their ages are shown in Figure 5. 9. Examine the part of Figure 5 showing the distribution of circular areas that were centers of crustal faulting and buckling when they were located over the Yellowstone hot spot. The numbers indicate the ages of deformation, as determined by Mark Anders. a. What direction is the North American plate moving, according to Anders' data? Explain your reasoning. b. What was the average rate in centimeters per year (cm/year) that the North American plate has moved over the past 11 million years? c. Beside the Yellowstone hot spot in Figure 5, place an arrow and rate of motion to indicate the velocity of the North American plate. You will find the direction and rate of motion from items a and b above. 10. Notice the ages of seafloor volcanic rocks in Figure 5. The modern seafloor rocks of this region (located at 0 million years old) are forming along a divergent plate boundary called the Juan de Fuca Ridge. The farther one moves away from the plate boundary; the older are the seafloor rocks. a. What has been average rate of seafloor spreading along the line B-D, east of the Juan de Fuca Ridge, over the past 8 million years? b. What has been average rate of seafloor spreading along line A-B, west of the Juan de Fuca Ridge, over the past 8 million years? c. Notice that seafloor rocks older than 8 million years are present west of the Juan de Fuca Ridge, but not east of the ridge. What could be happening to the seafloor rocks along the line C-D that would explain their absence from the map? d. Based on your reasoning in item 9C, what kind of plate boundary is represented by the red line running through location C on figure 5? e. Notice the line of volcanoes in the Cascade Range located at the center of the map. How could magma form beneath these volcanoes (be as specific as you can).