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Transcript
Earth Science Chapter 17: Plate Tectonics Chapter Overview Section 1: Drifting Continents During the average human lifespan, Earth’s surface appears to be unchanging. On the geologic time scale, however, Earth’s surface is changing at rates almost too great to imagine. For example, South America is moving away from Africa at 2 to 3 centimeters per year and Mount Everest is slowly rising. Just beyond the perception of our physical senses the Earth is constantly changing. 1. Early Observations Some of the first people to consider the idea of moving landmasses were early mapmakers. In the late 1500s, a Dutch mapmaker noticed the apparent fit of continents on either side of the Atlantic Ocean. He proposed that the Americas had been separated from Europe and Africa by floods and earthquakes. In the late 1800s an Austrian geologist hypothesized that the present southern continents had once been joined as a single landmass he named Gondwanaland. In 1912, Alfred Wegener presented the first serious, scientific hypothesis about continental movement. 2. Continental Drift Wegener called his hypothesis continental drift, which proposed that Earth’s continents had once been joined as a single landmass. He called this supercontinent Pangaea, a Greek word that means “all the earth”. Wegener proposed that Pangaea began to break apart around 200 million years ago. Wegener collected and organized rock, fossil, and climatic data to support his hypothesis • Evidence from Rock Formation When Pangaea began to break apart large geologic features, like mountain ranges, would have fractured as the continents separated. Wegener found that the Appalachians in the United States and mountains in Greenland and Europe shared similar features • Evidence from Fossils Similar fossils of several different animals and plants that once lived on land had been found on widely separated continents. This implies that at some point in time these continents were joined together. Also, the fossilized remains of an aquatic reptile that lived in fresh water were found on different continents. Wegener reasoned that these reptiles could swim across the oceans to separate continents. • Ancient Climatic Evidence Coal deposits found in Antarctica indicate that at one time the climate on that continent was warm enough to support large swampy areas and extensive vegetation. Evidence of glacial deposits on Africa, India, Australia, and South America suggests that these landmasses were once closer to the South Pole. 3. A Rejected Hypothesis Wegener’s hypothesis had two major flaws that prevented it form being accepted. First, he could not explain what was causing the continents to move; and second, he could not how they were moving. Section 2: Seafloor Spreading Until the mid-1900s, most people thought that the ocean floor was essentially flat. Advances in technology in the 1940s and 1950s proved this idea, and others about the nature of the see floor, wrong. 1. Mapping the Ocean Floor • One advance that allowed scientists to study the ocean floor was the development of echosounding methods, including sonar. Sonar uses sound waves to measure water depth by timing how long it takes for a sound to travel to the ocean floor and back to the surface. • Another technological advance that was used to study the ocean floor was the magnetometer. A magnetometer is a device that can detect small changes in magnetic fields. 2. Ocean Floor Topography The maps made from the data collected by sonar and magnetometers revealed a sea floor topography that included mountain ranges that are the longest on the planet, and trenches, some almost six times as deep as the Grand Canyon. Scientists were unable to explain why these features existed. 3. Ocean Rocks and Sediments Scientists also collected samples of deep-sea rocks and sediments. Analysis of these sediments produced two important discoveries. First, the ages of the rocks that make up the seafloor vary in different places, and these variations change in a predictable way. Rock samples taken near seafloor ridges were younger than samples taken from areas near deep-sea trenches. In fact, the farther from the ridge, the older the rock. Scientists also discovered that the oldest rock on the seafloor is only 180 million years old, compared to rocks 3.8 billion years old on the continents. The second discovery was that the thickness of the sediment deposited on the ocean floor was less than expected, only a few hundred meters thick. On the continents, however, sediment layers could be from a few kilometers to 20 kilometers thick. 4. Magnetism Scientists know that rocks containing iron-bearing minerals provide a record of Earth’s magnetic field. The study of this magnetic record is called paleomagnetism. • The Geomagnetic Time scale Studies of continental basalt flows in the early 1960s revealed a pattern of magnetic reversals over geologic time. A magnetic reversal is a change in Earth’s magnetic field. Comparing the magnetic data of the seafloor with that of the continent, scientists found an interesting. In some places, the magnetic field strength was greater than normal, while in other places it was lower than normal. The higher than normal was explained by the combining of the magnetic fields of the seafloor and Earth’s current magnetic field while the lower than normal was the result of a magnetic reversal of the field on the seafloor • Magnetic Symmetry The positive and negative areas form a series of stripes that were parallel to ocean ridges, and the data matched the pattern of magnetic reversals found in basalt flows on the land. By matching the patterns on the seafloor with the known pattern of reversals on land, scientists were able to determine the age of the ocean floor. They were able to construct isochron maps. Isochrons are lines on a map that connect points that have the same age. These maps revealed that the youngest seafloor was closest to the ocean ridges while the oldest seafloor rock were close to ocean trenches 5. Seafloor Spreading An American scientist proposed a theory that could explain the topographic, age, and magnetic data from the seafloor. This theory, called seafloor spreading, states that new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches. • The theory of sea floor spreading explained how landmasses could move long distances over time and supported Wegener’s hypothesis of continental drift Section 3: Theory of Plate Tectonics The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous slabs called plates. There are a dozen or so major plates and several smaller ones. These tectonic plates move in different directions at different rates over Earth’s surface 1. Plate Boundaries Tectonic plates interact at places called plate boundaries. At some boundaries the plates come together, or converge. At others, plates move away from one another, or diverge. At the third type of plate boundary the plates move horizontally past one another. Each type of boundary has certain geologic features and processes associated with it • Divergent Boundaries Places where two tectonic plates are moving apart are called divergent boundaries. Most divergent boundaries are found on the seafloor where they form ocean ridges. The formation of new ocean crust at most divergent boundaries accounts for the high heat flow, volcanism, and earthquakes associated with these boundaries. Occasionally divergent boundaries form • • on continents. When the continental crust begins to separate, the stretched crust forms a long, narrow depression called a rift valley Convergent Boundaries Places where two tectonic plates are moving toward each other are convergent boundaries. There are three types of convergent boundaries based on the type of crust involved. The types are: oceanic crust converging with oceanic crust, oceanic crust converging with continental crust, and continental crust converging with continental crust. In an oceanicoceanic boundary, one of the plates descends beneath the other in a process called subduction. This process creates a deep oceanic trench and the formation of island arcs that parallel the trench. Transform Boundaries A place where two plates slide horizontally past each other is a transform boundary. At transform boundaries the crust is deformed and fractured. Transform boundaries are characterized by long faults, sometimes hundreds of kilometers in length. Movement along these faults is responsible for many of the earthquakes Section 4: Causes of Plate Motion 1. Convection The transfer of heated material is called convection. The heating of matter causes it to expand and to decrease in density. The warm matter then rises because of buoyancy. As the matter rises it cools, contracts, becomes more dense, and then sinks because of gravity. This pattern of up and down motion is called convection. Convection currents in the mantle are thought to be the driving mechanism of tectonic plate motion. 2. Push and Pull During the formation of an ocean ridge forces in the mantle cause the asthenosphere to rise. The weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone in a process called ridge push. In addition to ridge push, the horizontal flow at the top of a convection current could create drag on the lithosphere and thereby contribute to plate motion. A sinking region of a mantle convection current could suck an oceanic plate downward onto a subduction zone. The weight of a subducting plate helps pull the trailing lithospheric plate into the subduction zone in a process called slab pull