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idge segments. Here seafloor produced at one ridge axis moves in the opposite direction as seafloor is produced at an opposing ridge segment. Thus, between the ridge segments these adjacent slabs of oceanic crust are grinding past each other along a transform fault. Beyond the ridge crests are the inactive zones, which are preserved as linear topographic scars. These fracture zones tend to curve such that small segments roughly parallel the direction of plate motion at the time of their formation. In another role, transform faults provide the means by which the oceanic crust created at ridge crests can be transported to a site of destruction, the deep-ocean trenches. Figure 7.16 illustrates this situation. Notice that the Juan de Fuca plate moves in a southeasterly direction, eventually being subducted under the West Coast of the United States. The southern end of this relatively small plate is bounded by the Mendocino transform fault. This transform fault boundary connects the Juan de Fuca ridge to the Cascade subduction zone (Fig- Juan de Fuca Ridge Pacific plate Transform fault Mendocino Fault Juan de Fuca plate Cascadia subduction zone Chapter 7 Plate Tectonics 211 ure 7.16). Therefore, it facilitates the movement of the crustal material created at the ridge crest to its destination beneath the North American continent (Figure 7.16). Another example of a ridge-trench transform fault is found southeast of the tip of South America. Here transform faults on the north and south margins of the Scotia plate connect the trench to a short spreading axis (see Figure 7.8). Although most transform faults are located within the ocean basins, a few cut through continental crust. Two examples are the earthquake-prone San Andreas Fault of California and the Alpine Fault of New Zealand. Notice in Figure 7.16 that the San Andreas Fault connects a spreading center located in the Gulf of California to the Cascade subduction zone and the Mendocino transform fault located along the northwest coast of the United States. Along the San Andreas Fault, the Pacific plate is moving toward the northwest, past the North American plate. If this movement continues, Oregon California Mendocino Fault Relative motion of Pacific plate PACIFIC PLATE ▲ ▲ ▲ ▲ ▲ Juan de Fuca plate San Francisco Cascadia subduction zone San Andreas Fault Los Angeles Gulf of California NORTH AMERICAN PLATE Figure 7.16 The role of transform faults. The Mendocino transform fault permits seafloor generated at the Juan de Fuca ridge to move southeastward past the Pacific plate and beneath the North American plate. Thus, this transform fault connects a divergent boundary to a subduction zone. Futhermore, the San Andreas Fault, also a transform fault, connects two spreading centers—the Juan de Fuca Ridge and a divergent zone located in the Gulf of California.
212 Unit Three Forces Within that part of California west of the fault zone, including the Baja Peninsula, will become an island off the West Coast of the United States and Canada. It could eventually reach Alaska. However, a more immediate concern is the earthquake activity triggered by movements along this fault system. ?S TUDENTS SOMETIMES ASK... If the continents move, do other features like segments of the oceanic ridge also move? That’s a good observation, and yes, they do! It is interesting to note that very little is really fixed in place on Earth’s surface. When we talk about movement of features on Earth, we must consider the question, “Moving relative to what?” Certainly, the oceanic ridge does move relative to the continents (which sometimes causes segments of the oceanic ridges to be subducted beneath the continents). In addition, the oceanic ridge is moving relative to a fixed location outside Earth. This means that an observer orbiting above Earth would notice, after only a few million years, that all continental and seafloor features— as well as plate boundaries—are indeed moving. The exception to this are hot spots, which seem to be relatively stationary and can be used to determine the motions of other features. Testing the Plate Tectonics Model With the birth of the plate tectonics model, researchers from all of the Earth sciences began testing it. Some of the evidence supporting continental drift and seafloor spreading has already been presented. Some of the evidence that was instrumental in solidifying the support for this new concept follows. Note that some of the evidence was not new; rather, it was a new interpretation of old data that swayed the tide of opinion. Evidence: Paleomagnetism Probably the most persuasive evidence to the geologic community for the acceptance of the plate tectonics theory comes from the study of Earth’s magnetic field. Anyone who has used a compass to find direction knows that the magnetic field has a north pole and a south pole. These magnetic poles align closely, but not exactly, with the geographic poles. (The geographic poles are simply the top and bottom of the spinning sphere we live on, the points through which passes the imaginary axis of rotation.) In many respects the magnetic field is very much like that produced by a simple bar magnet. Invisible lines of force pass through Earth and extend from one pole to the other. A compass needle, itself a small magnet free to move about, becomes aligned with these lines of force and thus points toward the magnetic poles. The technique used to study ancient magnetic fields relies on the fact that certain rocks contain minerals that serve as fossil compasses. These iron-rich minerals, such as magnetite, are abundant in lava flows of basaltic composition. When heated above a certain temperature called the Curie point, these magnetic minerals lose their magnetism. However, when these iron-rich grains cool below their Curie point (about 580°C), they become magnetized in the direction parallel to the existing magnetic field. Once the minerals solidify, the magnetism they possess will remain frozen in this position. In this regard, they behave much like a compass needle inasmuch as they point toward the existing magnetic poles. Then, if the rock is moved or if the magnetic pole changes position, the rock magnetism will, in most instances, retain its original alignment. Rocks formed thousands of millions of years ago thus remember the location of the magnetic poles at the time of their formation and are said to possess fossil magnetism, or paleomagnetism. Polar Wandering. A study of lava flows conducted in Europe in the 1950s led to an amazing discovery. The magnetic alignment in the iron-rich minerals in lava flows of different ages was found to vary widely. A plot of the apparent positions of the magnetic north pole revealed that during the past 500 million years, the location of the pole had gradually wandered from a spot near Hawaii northward through eastern Siberia and finally to its present site (Figure 7.17A). This was clear evidence that either the magnetic poles had migrated through time, an idea known as polar wandering, or that the lava flows had moved—in other words, the continents had drifted. Although the magnetic poles are known to move, studies of the magnetic field indicated that the average positions of the magnetic poles correspond closely to the positions of the geographic poles. This is consistent with our knowledge of Earth’s magnetic field, which is generated in part by the rotation of Earth about its axis. If the geographic poles do not wander appreciably, which we believe is true, neither can the magnetic poles. Therefore, a more acceptable explanation for the apparent polar wandering is provided by the plate tectonics theory. If the magnetic poles remain stationary, their apparent movement was produced by the drifting of the continents. Further evidence for plate tectonics came a few years later when polar wandering curves were constructed for North America and Europe (Figure 7.17A). To nearly everyone’s surprise, the curves for North America and Europe had similar paths, except that they were separated by about 24 degrees of longitude. When these rocks solidified, could there have been two magnetic north poles that migrated parallel to each other? This is very unlikely. The differences in these migration paths, however, can be reconciled if the two presently
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