LAST NAME: FIRST NAME: LAS POSITAS COLLEGE OCEANOGRAPHY LAB LAB 6 PLATE TECTONICS

Transcription

1 LAST NAME: FIRST NAME: LAS POSITAS COLLEGE OCEANOGRAPHY LAB LAB 6 PLATE TECTONICS (Modified from Exercise 4, Sea Floor Spreading and Plate Tectonics, Laboratory Exercises in Oceanography, Bernard W. Pipkin et al, 2ed, W. H. Freeman and Co. 1987) 1. It has long been recognized that such features as mountains, earthquakes, and volcanoes are not randomly distributed upon the surface of the earth. In 1912 the German meteorologist Alfred Wegener at tempted to explain this distribution by arguing that if continents could move vertically, then horizontal motion, or continental drift, was also possible. The theory of sea-floor spreading a process whereby new sea floor is created as adjacent crust is moved apart to make room was first proposed in 1960 as a viable alternative to Wegener s theory. 2. In essence, it has been proposed that the earth s outer shell (called the lithosphere, a zone about 100 kilometers thick) is composed of a number of rigid plates. Each of these crustal plates moves in a different direction, and thus plate boundaries are sites of tectonic activity where earthquakes, volcanism, and mountain building occur. Six large plates and a number of smaller ones have been identified (figure 1). 3. The source of energy for driving plate motions is heat produced by the decay of radioactive atoms in the earth s interior. Although these atoms exist in very small concentrations, the combined total heat production is large enough to keep the earth s interior quite hot and provide a constant flow of heat to the earth s surface. 4. Because of this heat production, thermal gradients develop in the earth s interior, causing hot, low density material to rise toward the surface while cold, higher-density material sinks. The convective motion thus generated in the earth s interior moves the lithospheric plates around on the surface of the earth. In a few areas, strong plumes may rise through the earth s mantle and penetrate the lithosphere to produce a hot spot with magmatic activity, which may or may not lie on a plate boundary. Examples of hot spots include Hawaii, Iceland, Easter Island, the Galapagos Islands, and Yellowstone (figure 2). 5. There are three types of plate boundaries: DIVERGENT, CONVERGENT, AND TRANSFORM (figure 3). Where plates are separating, magma rises and cools to form new ocean crust. This boundary also called a spreading center and is expressed

2 topographically as a mid-ocean ridge. Examples of ridges include the Mid- Atlantic Ridge and East Pacific Rise. Of course, if new sea floor is continuously created at spreading centers, old sea floor must be destroyed. This occurs at destructive boundaries, also known as subduction zones, which are expressed topographically by the presence of trenches. As old oceanic crust is subducted, it will be heated, causing partial melting. Magma produced in this fashion will rise, causing volcanism on the overriding plate near the trench. Examples of subduction zones include the Aleutian Trench and Aleutian Islands, and the Peru-Chile Trench and Andes Mountains. The third type of boundary lies along numerous offsets on mid-ocean ridges. These form conservative plate boundaries, where plates slide past one another with no crustal creation or destruction. These are also known as transform faults and may be expressed topographically as fracture zones. Examples of these are the Romanche, Oceanographer, and Mendocino fracture zones. Figure 1. A mosaic of plates forms the earth s lithosphere, or outer shell. According to the theory of plate tectonics, the plates are not only rigid but are in constant motion.

3 6. The boundaries between plates are seldom smooth, and thus plates catch on each other, deforming elastically on the edges until rock failure occurs to produce the abrupt motion we know as earthquakes. The largest and most devastating of these events often occur in subduction zones, but transform faults such as the San Andreas (in California) may also produce very large earthquakes. Occasionally two continental plates will collide in a subduction zone, but because continental crust has too low a density to sink into the earth s interior, the result is the pushing up of large mountain ranges. Collisions such as these created the Himalayas and the Alps. 7. The critical evidence for sea-floor spreading is based on earth magnetism (the force exerted by the earth s magnetic field). In the past (approximately every half million years), the earth s magnetic field has reversed its polarity, so that the north magnetic pole becomes the south magnetic pole (or the presentday normal polarity becomes reverse polarity). Each major reversal is termed a polarity epoch. The sequence of reversals occurring in the course of the past several million years has been dated with the use of radiometric techniques (figure 4). Shorter reversals, termed events, have also been recorded within the longer epochs. 8. When basalt is intruded and cools in cracks at mid- ocean ridges, the polarity of the earth s magnetic field at the time of cooling is preserved. As the crust moves away from the spreading center, each successive unit of the cooled magma gradually moves outward, revealing zones of fossil magnetism (figure 5), in which the orientation of the earth s magnetic field at the time of formation is preserved. This fortunate preservation allowing us to detect fossil magnetism gives us a method for measuring the rate at which new sea floor is formed. 9. Where the rocks have the same magnetic polarity as the present-day normal magnetic field we find stronger than average magnetisms and we have a positive anomaly; where the rocks preserve reverse polarity we measure weaker than average magnetic field and we have a negative anomaly. We can determine the rate at which new sea floor is formed by measuring the distance from the ridge crest to a magnetic anomaly of known age. To calculate the spreading rate, simply divide the distance traveled by the age of the oldest reliably dated anomaly. With the aid of this technique, plate velocities of from 1 to 10 centimeters per year have been calculated (figure 6). 10. Although mid-ocean ridges are areas in which relatively large temperature increases should be observed in sediments beneath the sea floor, measurement of these gradients has shown that they are only slightly larger than those observed at locations far away from ridge crests. At first, these observations were difficult to reconcile with the predictions of the plate tectonic model. Then, suggestions were made that perhaps seawater enters fractures

4 Figure 2. Location of hot spots. They are found on all major plates: on continental land masses as well as oceanic plates. Their distribution is random. Figure 3. New sea floor is created at the spreading center or oceanic ridge as magma rises from below. The magma solidifies as lava and preserves the prevailing magnetic polarity. Transform faults are perpendicular to the ridges. As they slide past each other, they produce earthquakes. Shallow earthquakes occur at the ridges, while deep earthquakes occur at subduction zones.

5 Figure 5. Evidence for sea floor spreading has been obtained by determining the polarity of fossil magnetism in rocks lying on both sides of oceanic ridges. In the illustration rocks of normal polarity are represented by the lighter gray stripes. Two blocks represents a fracture zone. The symmetry suggests that the rocks welled up on a molten or semimolten state and gradually moved outward. Figure 4. Illustration of how magnetic polarities of lave flows are used to construct the time scales of magnetic reversals over the pas 5 million years. In no one place is the entire sequence found; the sequence is worked out by patching together the ages and polarities from lava beds all over the world. Each event is named after a geologist.

6 Figure 6. These are magnetic reversals recorded from the ocean floor. There are two values to note. Distance, in terms of kilometers below the magnetic anomaly and time, in terms of millions of years, above the magnetic anomalies. Do note that spreading rates are calculated in centimeters per year. in the oceanic crust and flows toward hot, recently formed rock. Where it encounters hot rock, water would be heated, causing it to rise and exit near ridge crests. This suggestion seemed reasonable because hot springs and geysers are frequently observed on continents near magma sources. This hydrothermal flow would provide a mechanism for cooling the newly formed crust and thus reducing the temperature gradients. 11. In 1977, an expedition went to the Galapagos spreading center to search for vents of hot hydrothermal waters. Using the submersible Alvin, they explored the sea floor for evidence of hot springs. Their search was successful, and resulted in some remark able additional discoveries. Plumes of hot water were found exiting from fissures in the crust. These plumes carry sulfide in solution, which bacteria can combine with oxygen from seawater in order to derive energy for growth. The bacteria can then be consumed by higher organisms, including large clams and tube worms, allowing a unique ecosystem to develop, fueled by chemical energy. Similar vents have since been found at a number of other locations on ridge crests, some of which may vent water nearly 300 C in temperature. 12. Although each vent may have only a modest flow and be active for only a few thousands of years, there may be large numbers of them. It has been estimated that the entire volume of the ocean is circulated through ridge crests in million years. Examination of the composition of these hot waters has answered some longstanding questions regarding ocean

7 chemistry, because reactions between seawater and hot rocks help regulate the abundance of some elements in seawater. Metals are leached from hot rocks and carried upward by hydrothermal flow. As the water rises and cools, some of these metals may precipitate in the fractures or fissures that permit flow, forming ore deposits. Other metals are carried into bottom water and provide a significant fraction of the dissolved metal flux to the sea. EXERCISES 1a. Examine figure 3 and state where you would expect to find the highest temperatures on the seafloor. b. Where would deep-focus earthquakes occur? (The focus is the location inside the earth where an earthquake starts). c. Why would you not expect to find deep-focus earthquakes under spreading centers? 2. What types of plate boundaries occur between the following? Refer to the box in figure 1. a. The North American and Pacific plates b. The Nazca and South American plates c. The South American and African plates 3. What stresses (tension = pulling, compression = pushing, or shearing = tearing apart) characterize the following types of junctions? a. Spreading centers b. Subduction zones c. Transform faults

8 4a. A transform fault (single line) is shown offsetting mid-ocean ridges (double lines) in the sketch below. Use an X to show the possible locations of earthquake epicenters (the point on the earth s surface directly above an earthquake s focus) along its strike. Use arrows to show the directions of plate motions. NOTE: no more than 6 x s are allowed. b. Explain the distribution of earthquake epicenters on the sea floor. 5. Refer to figure 6 and calculate the spreading rates in centimeters per year for the three areas shown in the figure. a. South Atlantic b. North Pacific c. Pacific Antarctic d. Which of the ocean basins exhibits the slowest spreading rate? e. Which one is spreading the fastest? f. About how long did it take for the Atlantic Ocean to open to its present width at the equator? g. During what geologic time period did these events take place? Use geologic time scale! 6. The Hawaiian Islands have been created by the passage of the Pacific Plate over a hot spot. On which Hawaiian Island would you expect to find active volcanism and the youngest rocks? (Indicate east or west, or identify the islands on a good map of the sea floor.)

9 7. On a good map of the Pacific sea floor, find the following hot-spot ridges in the Pacific: the Emperor Seamount Chain hot-spot ridge, the Tuamotu Archipelago Line Island hot-spot ridge, and the Austral Gilbert Marshall Island hot- spot ridge. If these oceanic island chains are produced by hot spots, explain the change in the lineation of each of them (mainly westerly for the Hawaiian, Tuamotu, and Austral parts; and mainly northerly for the Emperor, Line, and Gilbert-Marshall parts). 8. If the Pacific Plate is moving at 10 centimeters per year, and this speed has not changed, estimate the time (years before present) when the change described in Question 7 took place. 9. On a good map showing detailed bottom topography, locate the Walvis Ridge and Rio Grande Rise in the South Atlantic. It is believed that these features were produced as volcanic ridges from a hot spot currently at about 40 S on the Mid-Atlantic Ridge. If this is true, why do the lineations of the hot-spot ridges and the lineation of the fracture zones (both produced by plate tectonics) show a different sense of movement (lineations are in different directions)? G12L Form 6 Rev: 13 Aug 2013