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146 Part Two Oceanography Eurasian plate North American plate JUM de Fuca plate Carbed piata Arabian plate Frie Date Pacifoto Cocos plate Alican plate Na plale Austral plate South American plato Antinctio plate Divergent plate boundary boundary Transforma boundary 2000 2000 5000 3000 Miles 2000 4000 5000 5000 10,000 Kilometers Figure 10.2 The most of rigid plates that comes Carth's uterShell called lithospheric plates, or simply plates (Figure Plate Boundaries 10.2). Plates are continually moving and where they move apart along the mid-ocean ridges, upwelling of Earth scientists recognize the distinct types of plate hot mantle rock creates new seafloor. The axis (nline boundaries, each distinguished by the relative move down the center) of some ridge segments contains ment of the plates on the opposite sides of the bound- deep down-faulted structures called rift valleys Ary. Divergent plate boundaries are located along the Along other plate margins plates slide pare cach other crests of oceanic ridges and are constructieplater or collide to form mountains. In additiot one plate is where new oceanic lithosphere is generated. Al- may descend beneath an overriding plate to form a though new lithosphere is constantly being produced deep-ocean trench at the oceanic ridges, our planet is not growing langer: The discoveries of the global mid-ocean ridge sy its total surface are remains the same. To balance the tent and deep-ocean trenches were of major impor addition of newly created lithosphere, older portions tance in the development of plate tectonic theory: of oceanic lithosphere descend into the mantle along convergent plate boundaries. Because lithosphere is 1. On Figure 9.6 in Exercise (page 137), outline the destroyed at convergent boundaries, these bound- Mid-Atlantic Ridge and Inbel the rift valley aries are also called ifestatieplate margines. Although (Note: The ridge is at least 2000 kilometers wide all convergent zones have the same basic characteris in most of the regions shown) ties, they are highly variable, each controlled by the 2. Using an atlas or world map for reference, type of crustal material and tectonic setting. Conver draw the axis of the global mid-ocean ridges gent boundaries can form between two oceanic plates between one ocenic plate and one continental plate tem on the map in Figure 103 or between two continental plates. Along transform 3. Label the following deep-ocean trenches on fault boundaries plates slide horizontally past one Figure 103. Draw a line to represent each french another, without the production or destruction of lith 1. Puerto Rico 6. Japan osphere. Transform foutis were first identified where 2. Cayman 7. Mariana they join offset segments of an ocean ridge. Although most are located within the ocean basin a few cut 3. Peru-Chile & Tonga through continental crust. An example of such 4. Aleutian 9. Kemiadec boundary is California's earthquake prone San Andres 5. Kuri 10. Java Sunda) Fault UUUU
To The Dynamic Ocean Floor 147 Similar and XVI 290000000000 1202 149 Figure 10.3 World Use Figure 104, which illustrates the plate bound aries, to complete the following 4. Does Figure 104A representa (convergente di vergenti plate boundary? This is a plate boundary 2. Are the plates along this boundary moving apart or moving together? The plates are b. Does this plate boundary occarat (deep ocean trenches or mid-ocean ridges? The plate boundary occurs at Does this plate boundary result in the con struction er destruction of thosphere This plate boundary results is of lithosphere 5. Does gore 10:40 representa convergent, diver gent, or transform plate boundary The figure represents plate boundary 2. Are the plates along this type of boundary moving aparter moving together! The plates are h. Damine Figure 105, which shows the three types of convergent plate boundaries. Label each of the three diagrams with the appropri ale mocni-continental convergence scenic-ock convergence, or continental- continental convergence. Find one example of each type of convergent boundary on the plate map in Figure 102 Label och boundary with the terms from question 5 6. Does Figure 104C represent a convergent, di vergent, or transform) plate boundary? The figure is plate boundary a. Isithosphetic material (being created, being destroyed, er remaining unchanged) along this type of boundary? Lithospheric material is along this type of boundary b. What is the name given to faults that facilitate plate movement along this type of boundary? Lauts Which type of fault (dip-slip or strike) is alus trated in gure 10:41 fault
148 Part Two /Oceanography PR Pine Figure 10.4 tons of the three types of plate bonde Figure 10.5 of types of contate boundaries Evidence for Plate Tectonics Evidence: Fit of the Continents Examine the east coast of South America and the west coast of Africa on the world map in Figure 103. Then complete the following 7. Does the shape of the east coast of South Ameri CA conform to the shape of the foot or west) side of the mid-ocean ridge in the Atlantic Ocean? It conforms to the side 8. Does the shape of the west coast of Africa con form to the shape of the least or west) side of the mid-ocean ridge in the Atlantic Ocean! It conforms to the side 9. If South America md Africa were moved to the mid-ocean ridge would their shapes fit together long the ridge! (Note: The fit of the continents is more exact on a globe because flat maps have distortions) The margins fit together
Esercise Ten/ The Dynamic Ocean Floor 149 Present Figure 10.3 shows the location of an outcrop of an cient rocks in northeastern South America that are sim- ilar in rock type and age of rocks to western Africa, 10. Does your answer to question support the idea that these continents were once joined? 11. In Figure 10.1A, label the present-day continents that comprised Pangaea. 1 million years ago mal 2 mya D 3 mya 4 туа Evidence: Earthquakes The distribution of earthquakes provide some of the most convincing evidence for plate tectonics. Compare the plate map shown in Figure 10.2 to the earthquake distribution map shown in Figure 8.9 in Exercise 8 (page 125). 12. Are most deep-focus earthquakes associated with (mid-ocean ridges or deep-ocean trenches)? Most deep-focus earthquakes are associated with Figure 10.6 Chronology of magnetic polarity during the past 4 million years. Periods of commal polarity, when a compass die would have been oriented as it is today, are shown in color (Data from Allan Cox and G. Dalrymple) 13. Are the earthquakes that occur along mid-ocean ridges (shallow focus or deep-focus) earthquakes? These are earthquakes 14. What occurs along ocean trenches (but not along mid-ocean ridges) that accounts for the occur rence of deep-focus earthquakes? Scientists have reconstructed Earth's magnetic po tarity reversals over the past several million years, as shown in Figure 10.6. The periods of normal polarity (when a compass needle would have pointed north, as it does today) are shown in various colors and labeled A- Use Figure 10,6 to answer the following questions 15. How many intervals (3, 5, 7) ofrebersed polarity (shown in tan color) have occurred during the past & million years? intervals of reversed polarity 16. Approximately how many years ago did the cur- rent period of normal polarity labeled A) begin? years ago 17. Did the Earth experience normal or reverse po- Larity 1.5 million years ago Farth experienced polarity 15 million years ago 16. Did the period of normal polarity, begin (1, 2 or 3 million years ago? Nomial polarity began million years ago 19. During the past 4 million years, has each interval of reverse polarity lasted (more or less than 1 million years? Reverse polarity lasted than 1 million years Evidence: Paleomagnetism The iron-rich mineral magnetite, common in small amounts in lava flows, becomes slightly magnetized and aligned with Earth's magnetic field when molten rock cools and crystallizes. These mineral grains are similar to tiny compass needles because they point toward the magnetic poles. The ancient magnetism called paleomagnetism, present in rocks on the ocean floor can be used to determine the rate at which plates are forming and moving away from the ridge cresti Scientists have also discovered that the polarity of Earthy's magnetic field has periodically reversed, such that the North Magnetic Pole becomes the South Mag netic Pole and vice versa As plates move apart along the mid-ocean ridge magma from the mandler ses to the surface and creates new ocean floor. As the magna cools, the minerals align themselves with the prevailing magnetic field. In the event that Earth's magnetic field reverses polarity new material forming at the ridge will be magnetted In an opposite for more direction >>>>
150 Part Two Oceanography 20. Based on the pattern of magnetic reversals exhib- ited in Figure 106, does it appear as though an- other polarity reversal will occur in the near future? Paleomagnetism and Seafloor Spreading The patter of magnetic polarity reversals shown in Figure 10,6 has been found in rocks of all the major ocean basins, Figure 107 illustrates this pattern of e- versals for sections of the mid-ocean ridges in the North Atlantic, South Atlantic, and Pacific. As new ocean crust forms along the mid-ocean ridge, it spreads out equally on both sides of the ridge. Therefore, the pattern of polarity reversals that occurs on one side of the ridge ads can be matched with the polarity of the rocks on the other side of the ridge (Figure 10.7) Use Figures 10.6 and 10.7 to complete the following 21. On Figure 10.7, identify and mark the periods of normal polarity with the letters A-F (as in Figure 10.6). Begin at the ridge crest and label along both sides of each ridge (Note: The left side of the South Atlantic has been completed as a guide) 22. Using the South Atlantic as an example, label the beginning of the normal polarity period that began 2 million years ago on the left sides of the Pacific and North Atlantic diagrams. Ridge A North Atlantic Ocean Ocean foorust B. South Atlante Ocean FE Magnetic polarity record mony C. Pacib Ocean EN 10 20 49 50 00 30 sice Fine 10.7 G che mi pomoc North Am. So, and Face Prostornind town 10.6
Determining the Ages of Ocean Basins The following procedure will allow you to calculate the age of a portion of the North Atlantic and South Atlantic basins 23. Using the distance scale at the bottom of Figure 10.7, measure the distance from the ridge axis to the beginning of the normal polarity period for each ridge. Place the distances on Figure 10.7 How many kilometers has the left side of each of these ocean basins spread during the past 2 mil- lion years? South Atlantic basin: km 26. Using Figure 10.3, measure the distance from Point A located off the Carolina coast to Point B off the African coast. Determine the distance in kilometers and then convert that distance into centimeters Distance: kom North Atlantic basin: km Pacific basin km cm The distances you obtained in question 23 are for only one side of the ridge. Assuming that a ridge spreads equally on both sides, the actual distance ench ocean basin has opened would be twice this amount 24. How many kilometers has each ocean basin opened in the past 2 million years? South Atlantic basin km North Atlantic basin kom Pacific basis km When the distance that an ocean basin has opened and the true it took for it to open are known, the rite of seafloor spreading can be calculated. To determine the rate of spreading in centimeters per year for each ocean basin, convert the distance the basinhas opened from kilometers to centimeters and then die vide this distance by the time-2 million years in this example Distance: 27. Divide the distance in centimeters separating the continents obtained in question 26 by the rate of scafloor spreading for the North Atlantic basin that you calculated in question 256. Your answer is the approximate age, in years, of the North Atlantic basin at that location. Distance/Rate of senfloor spreading → Age of the North Atlantic basin: years Repeat the procedure above to determine the age of the South Atlantic basin from Point Calong the east- er edge of Brazil to Point Doff the const of Africa. 25. How many years ago did South America and Africa begin to separate at that location? Age of the South Atlantic basin: years 29. Based on your answers to questions 27 and 28, which part of the Atlantic basin appears to have opened first? The -part of the basin opened first 25. Determine the rate of seafloor spreading for the Pacific and North Atlantic Ocean basins As an example, the South Atlantic has been completed. a. South Atlantic distance 72 km x 100,000 cm/km 7,200,000 chi 7,200,000 cm Rate of spreading 36 / ye 2,000,000 yr b. North Atlantic distance lam X 100,000 cm/km cm Rate of spreading - cm/y c. Pacific distance km X 100,000 cm/km ch Rate of spreading cm/y Hot Spots and Plate Velocities Researchers have determined that the volcanism on the island of Hawaii, known as a hot spot, arises from A plume of noten material moving upward from the mantle (Figure 10,8). It is believed that the position of this hot spot has remained constant over the past few million years in the past, as the Pacific plate moved over the hot spot the successive volcanic islands of the Hawaiian chain were built. The island of Hawali con tinues to grow islava from this mantle plume flows to the sea Radiometric dating of the volcanoes in the Hawallan chain has revealed that the islands increase in age with increasing distance from the island of Hawaii "Het evidence lindicates that the mantieplumes gradually ing no sapeviously thought
152 Part Two Oceanography 0 50 100 150 . 200 Kilometern Kausi 3.3.56 MA Apes von in ons el Yol MIO 1,3-1,5 M Mau Was than 10 M Oh 2.233 Mi Hawa Doel plate mosion H20,7 Mato prevent spot Obolo Cust Figure 10.8 Monent of the Pacific plate over a hot spot and radiometric ages of the wall Islands, in ons of years Companion Website The companion website provides numerous opportu nities to explore and reinforce the toples of this lab ex- ercise. To access this useful tool follow these steps: million years (Figure 108). Based on the age of an island and its dis tance from the hotspot, the velocity of the plate can be calculated Use Figure 10.8 to answer the following questions. 30. What are the minimum and maximum ages of the island of Kaal? Minimum age million years Maximum age 31. What is the approximate distance in kilometers) from the hot spot to the center of Kausi Convert your answer to centimeters Kilometers centimeters 32. Using the data in questions 30 and 31, calculate the approximate maximus and minimum veloce ities of the Pacific plate as it moved over the Hawaiian hot spot in centimeters per year (on/y Maximum velocity cm/y Minima velocity cm/ye 1. Go to www.mygeoscienceplace.com 2. Click on "Books Available" at the top of the page. 3. Click on the cover of Apytications wind testigo Hions Et Science 7e. 4. Select the chapter you want to access. Options are listed in the left column (Introduction. We besef Activities, Retal Websites and Field Trips
Course/Section me Due Date ate 1. The distribution of earthquakes defines the boundaries of which major Earth feature? 2. Deep-focus earthquakes are associated with what prominent ocean-floor feature? Lithosphere 3. Shallow-focus earthquakes occur along what prominent ocean-floor features? Asthenosphere A. Transform fault boundary 4. Describe how paleomagnetism is used to calcu- late the rate of seafloor spreading. Lithosphere Asthenesphere B. Convergent boundary 3. Based on your calculations for question 25, what are the rates of seafloor spreading for the follow- Ing ocean basins? North Atlantic basin: cm/yr Pacific basin: cm/yr 6. Based on your calculations in question 32, what is the maximum velocity for the Pacific plate? cm/yr 7. Based on your calculations in questions 27 and 28, what are the ages for the North and South Atlantic basins at the locations examined? North Atlantic basin: million years old South Atlantic basin million years old 8. Complete the block diagrams in Figure 109 to illustrate the types of plate boundaries listed below each diagram. Include arrows to indicate relative plate motion 9. Use Figure 10.10, which illustrates a generalized cross section of the plate boundary along the Lithosphere Asthenosphere Divergent boundary Figure 10.9 Block diagrams to accompany question 8 153 M
54 Part Two Oceanography 10. List and describe two lines of evidence from this exercise that support the theory of plate tectonics western edge of South America, to answer the following questions a. Label each of the following features on the block diagram Asthenosphere Continental crust Deep-ocean trench Oceanic crust Continental lithosphere Oceanic lithosphere b. What type of plate boundary is illustrated in the figure? Be as specific as possible. Figure 10.10 Block diagram of the plate boundary long the western edge of South America
- Earthquake Distribution Map (below) - use this for questions 12-14 on page 149 « Previous