11487_02_ch02_p032-069.qxd 2/16/06 9:31 AM Page 32 Chapter 2 Plate Tectonics: A Unifying Theory 11487_02_ch02_p032-069.qxd 2/27/06 4:04 PM Page 33 Essential Questions to Ask 2.1 Introduction Ⅲ Why should you know about plate tectonics? 2.2 Continental Drift Ⅲ What were some early ideas about Earth’s past geography? Ⅲ What is the continental drift hypothesis and who proposed it? 2.3 Evidence for Continental Drift Ⅲ What is the evidence for continental drift? 2.4 Paleomagnetism and Polar Wandering Ⅲ What is paleomagnetism? Ⅲ What is the Curie point and why is it important? Ⅲ How can the apparent wandering of the magnetic poles be best explained? 2.5 Magnetic Reversals and Seafloor Spreading Ⅲ What evidence is there that Earth’s magnetic field has reversed in the past? Ⅲ What is the theory of seafloor spreading, and how does it validate continental drift? Ⅲ How was the theory of seafloor spreading confirmed? 2.6 Plate Tectonics: A Unifying Theory Ⅲ What are the main tenets of plate tectonic theory? Ⅲ Why is plate tectonics a unifying theory of geology? Ⅲ What is the supercontinent cycle? 2.7 The Three Types of Plate Boundaries Ⅲ What are the three types of plate boundaries? Ⅲ What are divergent boundaries? Ⅲ What features in the geologic record indicate ancient rifting? Ⅲ What are convergent boundaries? Ⅲ How can ancient subduction zones be recognized in the geologic record? Ⅲ What are transform boundaries? 2.8 Hot Spots: An Intraplate Feature Ⅲ What are hot spots and what do they tell us about plate movement? 2.9 Plate Movement and Motion Ⅲ How can the rate and direction of plate movement be determined? 2.10 The Driving Mechanism of Plate Tectonics Ⅲ What drives plates? Ⅲ How do thermal convection cells move plates? Ⅲ Can plate movement be gravity driven? 2.11 Plate Tectonics and the Distribution of Natural Resources Ⅲ How does plate tectonic theory relate to the origin and distribution of natural resources? Ⅲ What is the relationship between plate boundaries and various metallic mineral deposits? USGS/NASA The Himalayas, Southwestern China Geology Matters Ruby and sapphire veins spread over white like GEOLOGY IN FOCUS: fossils of ferns on rock. Oil, Plate Tectonics, and Politics This satellite image of the Himalayas reveals an ever-changing GEOLOGY IN UNEXPECTED PLACES: tapestry of peaks, ridges, A Man’s Home Is His Castle and rivers woven by Earth GEOLOGY AND CULTURAL CONNECTIONS: millions of years ago when India collided with Asia. The Struggle toward Scientific Progress —A. W. GEOLOGY AT WORK: The Geologic Exploration of Mars This icon, which appears throughout the book, indicates an opportunity to explore interactive tutorials, animations, or practice problems available on the Physical GeologyNow website at http://now.brookscole.com/phygeo6. 33 11487_02_ch02_p032-069.qxd 2/16/06 9:31 AM Page 34 34 Chapter 2 Plate Tectonics: A Unifying Theory have also profoundly influenced the geographic distribu- 2.1 Introduction tion, evolution, and extinction of plants and animals. Fur- thermore, the formation and distribution of many geologic Imagine it is the day after Christmas, December 26, 2004, resources, such as metal ores, are related to plate tectonic and you are vacationing on a beautiful beach in Thailand. processes, so geologists incorporate plate tectonic theory You look up from the book you’re reading to see the sea sud- into their prospecting efforts. denly retreat from the shoreline, exposing a vast expanse of seafloor that had moments before been underwater and Ⅲ Why should you know about plate tectonics? teeming with exotic and colorful fish. It is hard to believe If you’re like most people, you probably have no idea or that within minutes of this unusual event, a powerful only a vague notion of what plate tectonic theory is. Yet tsunami will sweep over your resort and everything in its plate tectonics affects all of us. Volcanic eruptions, earth- path for several kilometers inland. Within hours, the coasts quakes, and tsunami are the result of interactions between of Indonesia, Sri Lanka, India, Thailand, Somalia, Myan- plates. Global weather patterns and oceanic currents are mar, Malaysia, and the Maldives will be inundated by the caused, in part, by the configuration of the continents and deadliest tsunami in history. More than 220,000 people will ocean basins. The formation and distribution of many nat- die, and billions of dollars in damage will be wreaked on the ural resources are related to plate movement and thus have region. an impact on the economic well-being and political deci- One year earlier, on December 26, 2003, violent shaking sions of nations. It is therefore important to understand this from an earthquake awakened hundreds of thousands of unifying theory, not only because it affects us as individuals people in the Bam area of southeastern Iran. When the and as citizens of nation-states but also because it ties to- magnitude-6.6 earthquake was over, an estimated 43,000 gether many aspects of the geology you will be studying. people were dead, at least 30,000 were injured, and approx- imately 75,000 survivors were left homeless. At least 85% of the structures in the Bam area were destroyed or damaged. Section 2.1 Summary Collapsed buildings were everywhere, streets were strewn ● with rubble, and all communications were knocked out. Plate tectonic theory is the unifying theory of geology. 1 It affects all of us because it explains where and why such Now go back another 12 ⁄2 years to June 15, 1991, when Mount Pinatubo in the Philippines erupted violently, dis- natural disasters as earthquakes, volcanic eruptions, and charging huge quantities of ash and gases into the atmos- tsunami occur as well as the formation and distribution of phere. Fortunately, in this case, warnings of an impending many economically valuable natural resources. eruption were broadcast and heeded, resulting in the evacu- ation of 200,000 people from areas around the volcano. Un- fortunately, the eruption still caused at least 364 deaths not only from the eruption but also from ensuing mudflows. 2.2 Continental Drift What do these three recent tragic events have in common? They are part of the dynamic interactions involving Earth’s Ⅲ What were some early ideas about Earth’s past plates. When two plates come together, one plate is pushed geography? or pulled under the other plate, triggering large earthquakes The idea that Earth’s past geography was different from to- such as the one that shook India in 2001, Iran in 2003, and day is not new. The earliest maps showing the east coast of Pakistan in 2005. If conditions are right, earthquakes can pro- South America and the west coast of Africa probably pro- duce a tsunami such as the one in 2004 or the 1998 Papua vided people with the first evidence that continents may New Guinea tsunami that killed more than 2200 people. have once been joined together, then broken apart and As the descending plate moves downward and is assimi- moved to their present locations. lated into Earth’s interior, magma is generated. Being less During the late 19th century, the Austrian geologist dense than the surrounding material, the magma rises to- Edward Suess noted the similarities between the Late ward the surface, where it may erupt as a volcano such as Paleozoic plant fossils of India, Australia, South Africa, Mount Pinatubo did in 1991 and others have since. It there- and South America as well as evidence of glaciation in the fore should not be surprising that the distribution of volca- rock sequences of these southern continents. The plant noes and earthquakes closely follows plate boundaries. fossils make up a unique flora in the coal layers just above As we stated in Chapter 1, plate tectonic theory has had the glacial deposits of these southern continents. This significant and far-reaching consequences in all fields of ge- flora is very different from the contemporaneous coal ology because it provides the basis for relating many seem- swamp flora of the northern continents and is collectively ingly unrelated phenomena. The interactions between mov- known as the Glossopteris flora, after its most conspicuous ing plates determine the locations of continents, ocean genus (ᮣ Figure 2.1). basins, and mountain systems, which in turn affect atmos- In his book, The Face of the Earth, published in 1885, pheric and oceanic circulation patterns that ultimately de- Suess proposed the name Gondwanaland (or Gondwana as termine global climate (see Table 1.3). Plate movements we will use here) for a supercontinent composed of the 11487_02_ch02_p032-069.qxd 2/16/06 9:31 AM Page 35 2.2 Continental Drift 35 ᮣ Figure 2.1 Fossil Glossopteris Leaves Plant fossils, such as continents that eventually broke apart and migrated toward these Glossopteris leaves from the Upper Permian Dunedoo Formation the equator after Earth’s rotation was supposedly slowed in Australia, are found on all five Gondwana continents. The presence by gigantic tidal forces. According to Taylor, these tidal of these fossil plants on continents with widely varying climates today is evidence that the continents were at one time connected. forces were generated when Earth captured the Moon The distribution of the plants at that time was in the same climatic about 100 million years ago. latitudinal belt. Although we now know that Taylor’s mechanism is in- correct, one of his most significant contributions was his suggestion that the Mid-Atlantic Ridge, discovered by the 1872–1876 British HMS Challenger expeditions, might mark the site along which an ancient continent broke apart to form the present-day Atlantic Ocean. Ⅲ What is the continental drift hypothesis and who proposed it? Alfred Wegener, a German meteorologist (ᮣ Figure 2.2), is generally credited with developing the hypothesis of conti- Courtesy of Patricia G.