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9.4 Testing Plate Tectonics Section 9.4 HSES_1eTE_C09.qxd 5/16/04 10:16 AM Page 265 9.4 Testing Plate Tectonics Section 9.4 1 FOCUS Section Objectives Key Concepts Vocabulary Reading Strategy 9.11 Explain how paleomagnetism What evidence supports ◆ paleomagnetism Predicting Copy the table. Write a and magnetic reversals provide the theory of plate ◆ normal polarity prediction of where earthquakes will occur. evidence that supports the tectonics? ◆ reverse polarity After you read, if your prediction was incorrect or incomplete, write where earthquakes theory of plate tectonics. How does paleomagnetism ◆ hot spot support the theory of plate actually occur. 9.12 Evaluate how earthquakes, tectonics? Probable Locations Actual Locations ocean drilling, and hot spots a. ? b. ? provide evidence that supports the theory of plate tectonics. Reading Focus Evidence for Plate Tectonics L2 With the birth of the plate tectonics model, researchers from all of the Build Vocabulary Earth sciences began testing it. You have already seen some of the evi- Word Parts Have students break the dence supporting continental drift and seafloor spreading. Additional vocabulary term paleomagnetism into evidence for plate tectonics came as new technologies developed. roots, prefixes, or suffixes. Students may need to use a dictionary to find the Paleomagnetism If you have ever used a compass to find direc- meanings of some parts. (Paleo- is a tion, you know that the magnetic field has a north pole and a south combination form of the Greek word pole. These magnetic poles align closely, but not exactly, with the geo- Figure 16 Paleomagnetism palaios meaning “ancient.” The word graphic poles. Preserved in Lava Flows As the magnetism comes from the Greek root lava cools, it becomes magnetized In many ways, Earth’s magnetic field is much like that produced parallel to the magnetic field words Magnes (lithos), literally meaning by a simple bar magnet. Invisible lines of force pass through Earth and present at that time. When the a stone of Magnesia, an ancient city in extend from one pole to the other. A compass needle is a small magnet polarity randomly reverses, a Asia Minor.) record of the paleomagnetism is that is free to move about. The needle aligns with these invisible lines preserved in the sequence of L2 of force and points toward the magnetic poles. lava flows. Reading Strategy Certain rocks contain iron-rich minerals, such as magnetite. When a. at convergent plate boundaries heated above a certain temperature, these magnetic minerals lose their Normal b. at all plate boundaries magnetism. However, when these iron-rich mineral grains cool down, magnetic 0.4 m.y. field they become magnetized in the direction parallel to the existing ago 2 INSTRUCT magnetic field. Once the minerals solidify, the magnetism they 0.8 m.y. (normal) ago possess stays frozen in this position. So magnetized rocks (reversed) behave much like a compass needle because they point 1.2 m.y. Evidence for Plate ago toward the existing magnetic poles. If the rock is moved (normal) Tectonics or if the magnetic pole changes position, the rock’s mag- Integrate Biology L2 netism retains its original alignment. Rocks formed Birds and Magnetism Tell students millions of years ago thus show the location of the mag- that birds use Earth’s magnetic field to netic poles at the time of their formation, as shown in locate places to stop and eat along their Figure 16. These rocks possess paleomagnetism. migration route. In addition, the birds use Earth’s magnetic field to navigate. Plate Tectonics 265 They read the angle at which magnetic fields enter the ground and thus determine their latitude relative to the magnetic poles. Ask: Why is it so important for birds to locate food sources? (The location of these places is critical because birds must have large quantities of food to provide energy during their long migrations.) Verbal, Logical Plate Tectonics 265 HSES_1eTE_C09.qxd 5/16/04 10:16 AM Page 266 Section 9.4 (continued) Geophysicists learned that Polarity of Ocean Crust Earth’s magnetic field periodically reverses polarity. The north mag- A netic pole becomes the south magnetic pole, and vice versa. A rock Testing Minerals solidifying during one of the periods for Magnetism L1 Magma of reverse polarity will be magnet- Purpose Students test various minerals Period of normal magnetism ized with the polarity opposite that with a magnet to determine whether of rocks being formed today. they have magnetic properties. B When rocks show the same Materials magnet, minerals (include magnetism as the present magnetic at least one sample of a mineral that field, they are described as having normal polarity. Rocks that show contains iron or cobalt), compass Magma the opposite magnetism are said to Procedure Have students test the Period of reverse magnetism have reverse polarity. A relation- mineral samples with the magnet to see ship was discovered between the if they are attracted by it. Have students C place the compass near each mineral magnetic reversals and the seafloor- sample to see if the needle moves. spreading hypothesis. Ships towed If it does, the material is magnetic. instruments called magnetometers Magma across segments of the ocean floor. Expected Outcomes Minerals that Period of normal magnetism This research revealed alternating contain iron or cobalt, such as strips of high- and low-intensity lodestone, have magnetic properties. Figure 17 A As new material is magnetism that ran parallel to the ridges. The strips of high-inten- Meteorites also have magnetic added to the ocean floor at the sity magnetism are regions where the paleomagnetism of the ocean properties. oceanic ridges, it is magnetized crust is of the normal type. These positively magnetized rocks Kinesthetic, Visual according to Earth’s existing magnetic field. B This process enhance the existing magnetic field. The low-intensity strips repre- records each reversal of Earth’s magnetic field. C Because new sent regions where the ocean crust is polarized in the reverse direction Use Visuals L1 rock is added in approximately and, therefore, weaken the existing magnetic field. As new basalt is equal amounts to the trailing added to the ocean floor at the oceanic ridges, it becomes magnetized Figure 17 Have students study the edges of both plates, strips of equal size and polarity parallel according to the existing magnetic field, as shown in Figure 17. figure. Ask: Could the rocks in a strip both sides of the ocean ridges. The discovery of strips of alternating polarity, which lie as possessing reverse polarity ever possess Applying Concepts Why are mirror images across the ocean ridges, is among the strongest evi- normal polarity? (No, once the rocks the magnetized strips about equal width on either side of dence of seafloor spreading. solidify, their polarity is permanently set.) the ridge? How do you think the width of a strip Earthquake Patterns Scientists found a close link relates to the seafloor spreading rate? between deep-focus earthquakes and ocean trenches. Also, the (The faster the spreading rate is, the wider absence of deep-focus earthquakes along the oceanic ridge system the strip will be.) was shown to be consistent with the new theory. Visual, Logical Compare the distribution of earthquakes shown in Chapter 8 on page 226 with the map of plate boundaries on pages 256–257. The close link between plate boundaries and earthquakes is obvious. When the depths of earthquake foci and their locations within the trench sys- tems are plotted, a pattern emerges. 266 Chapter 9 Customize for English Language Learners Explain to students that there are many uses of polar molecules have partial charges. Polar also the term polar, both in science and in everyday means diametrically opposite. Have students usage. For example, in magnetism, polarity look up the various meanings of the term polar refers to the magnetic poles. In chemistry, and use each meaning in a sentence. 266 Chapter 9 HSES_1eTE_C09.qxd 5/16/04 10:17 AM Page 267 Build Science Skills L2 Volcanic island arc Trench Interpreting Diagrams Have students Marginal sea study Figure 18. Ask: • From the map, identify the direction Oceanic crust in which the sinking slab of oceanic China lithosphere is moving. (from right to left) e lithosph re • Locate Korea on the map. Why do ic 100 km ean oc you think Korea has relatively few Partial melting ing ct Japan enioff zonedu ub earthquakes compared to Japan? S 200 km adati-B W (Korea is located far from ocean Japan Asthenosphere trenches; Japan is close to a trench.) 300 km • What pattern does the map show? trench Key (Deeper earthquakes occur farther from Shallow the trench.) Be sure students can Figure 18 Distribution of Earthquake Foci Note Intermediate distinguish the blue dots from the Deep that intermediate- and deep-focus earthquakes occur only within the sinking slab of oceanic lithosphere. green dots. • What can geologists learn from this pattern? (They can use the plotted foci Look at Figure 18. It shows the distribution of earthquakes near to track the plate’s descent into the the Japan trench. Here, most shallow-focus earthquakes occur within mantle.) or adjacent to the trench. Intermediate- and deep-focus earthquakes Visual, Logical occur toward the mainland. In the plate tectonics model, deep-ocean trenches are produced where cool, dense slabs of oceanic lithosphere plunge into the mantle. Shallow-focus earthquakes are produced as the descending plate inter- acts with the lithosphere above it. As the slab descends farther into the mantle, deeper-focus earthquakes are produced. No earthquakes have been recorded below 700 kilometers. At this depth, the slab has been heated enough to soften. Ocean Drilling Some of the most convincing evidence confirm- ing the plate tectonics theory has come from drilling directly into ocean-floor sediment.
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