Earth s Changing Surface Tectonics What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. Continents do not move. 2. Earth s mantle is liquid. Key Concepts What is the theory of plate tectonics? What evidence do scientists use to support the theory of plate tectonics? How do the forces created by plate motion change Earth s surface? Motion Even though you usually cannot feel it, Earth s surface is always moving. This motion can cause earthquakes and volcanic eruptions. It also can form mountains. The theory of plate tectonics states that Earth s is broken into rigid plates that move slowly over Earth s surface. The rigid plates are called tectonic plates. You live on the North American, shown in the figure below. Tectonic plates slowly move over Earth s surface. The movement of one plate is described as either moving away from or toward another plate, or sliding past another plate. s move at speeds of only a few centimeters per year. At this rate, it takes moving plates millions of years to make new continents, new mountain ranges, or other landforms. Evidence of Motion The theory of plate tectonics has helped geologists explain many observations about Earth and predict geologic events. Evidence gathered by scientists studying Earth for nearly 100 years supports the theory of plate tectonics. The theory replaced a hypothesis called continental drift. Juan de Fuca Pacific Tectonic s North American Caribbean Make an Outline Summarize the information in the lesson by making an outline. Use the main headings in the lesson as the main headings in your outline. Use your outline to review the lesson. 1. State What is the theory of plate tectonics? 2. Identify What plates does the North American interact with to the west? Reading Essentials Earth s Changing Surface 243
Make a horizontal six-door book to describe the changes in Earth s surface as the result of plate tectonics and forces. Compression Divergent Tension Transform Shear 3. Summarize What evidence do scientists use to support the theory of plate tectonics? Reading Check 4. Explain Why did it take more than 50 years for scientists to accept the hypothesis of continental drift? Drift Long before geologists proposed the theory of plate tectonics, they discovered evidence of continental movement. One piece of evidence is the shape of Earth s continents. Look at the outlines of South America and Africa on a map. If you could push these two continents together, they would fit together like two pieces of a puzzle. In 1912, Alfred Wegener developed the hypothesis that continents move, called continental drift. Fossil Evidence Different plants and animals live on different continents. For example, lions live in Africa but not in South America. Many fossils of animals and plants show the same thing some ancient organisms lived in certain areas but not in others. However, geologists have discovered the same types of fossils on continents that are now separated by vast oceans. For example, fossils of the freshwater reptile Mesosaurus have been found in South America and in Africa. These two landmasses are separated by the Atlantic Ocean. So how did a freshwater reptile cross a saltwater ocean? Scientists hypothesize that when the two continents were together, Mesosaurus probably traveled in freshwater rivers from one area to the other. Geological Evidence Rocks that are made of similar substances and mountains that formed at similar times are present on continents that are now far apart. Scientists can trace these rocks and mountains, as well as the locations of ancient glaciers, deserts, and coal swamps, from one continent to the next. How s Move The hypothesis of continental drift was not accepted for more than 50 years after it was proposed. The main reason was that the hypothesis did not explain how continents could move. Geologists knew that the mantle, the part of Earth underneath continents, was solid. How could a continent push its way through solid rock? An Explanation New discoveries during the 1960s led scientists to propose the theory of plate tectonics. Recall that Earth s is broken into separate tectonic plates. These plates include the beneath the ocean and the continents. Scientists proposed that continents did not just float around the ocean. Instead, Earth s continents are part of tectonic plates. The plates move toward, away from, or past each other, carrying continents with them. 244 Earth s Changing Surface Reading Essentials
The Role of Convection The forces that move plates come from deep within Earth. Earth s mantle is so hot that rocks can deform and move without breaking, much like putty. Convection affects the mantle underneath tectonic plates. Hotter mantle rises toward Earth s surface and cooler mantle sinks deeper into the mantle, as shown in the figure at right. As the mantle moves, it pushes and pulls tectonic plates over Earth s surface. Tectonic The edges of tectonic plates are called plate boundaries. A convergent boundary is where two plates move toward each other. A divergent boundary is where two plates move apart from each other. A transform boundary is where plates slide horizontally past each other. Recall that oceanic is denser than continental. This difference is important in areas where plates meet. When two plates come together, the denser oceanic plate usually is forced down into the mantle. The less dense continental plate remains on Earth s surface, as shown on the left in the figure below. The area where one plate slides under another is called a subduction zone. However, when two continents collide at a plate boundary, both continents remain on the surface. As two continents push together, the rises up and mountains form, as shown on the right in the figure below. Oceanic Subduction zone Deep ocean trench Volcanoes Earth s Mantle and Tectonic s Continent Crust Interactions Continent Mantle Convection currents Inner core Mountains Outer core 5. Identify Trace the movement of the mantle in the figure. Reading Check 6. Describe How does Earth s mantle move tectonic plates? 7. Point Out Highlight the subduction zone in the left part of the figure. Reading Essentials Earth s Changing Surface 245
8. Describe the movement of the plates at a divergent boundary. 9. Describe the movement of the plates at a transform boundary. 10. Summarize How do the forces created by plate motion change Earth s surface? Divergent When plates move apart at divergent boundaries, a rift forms between the two plates, as shown in the figure on the right. A rift valley can form within continents when continental moves in opposite directions. A rift valley also can form at divergent boundaries on the ocean floor. As plates separate, molten rock can erupt from the rift. As the molten rock cools, it forms new. Transform Tectonic plates slide past each other at transform boundaries, as shown in the figure on the right. Each side of the boundary moves in an opposite direction. This movement can deform or break features such as fences, railways, or roads that cross the boundary. Changing Earth s Surface Oceanic Divergent Boundary Mid-ocean Rift valley ridge Transform Boundary Transform fault Lithosphere within Earth cause plates to move. Different types of forces occur at the three types of plate boundaries. The squeezing force at a convergent boundary is called compression. The pulling force at a divergent boundary is called tension. The side-by-side dragging force at transform boundaries is called shear. These forces result in distinct landforms at plate boundaries. Even though plates move slowly, the forces at plate boundaries are strong enough to form huge mountains and powerful earthquakes. Tensional forces pull the land apart and form rift valleys and mid-ocean ridges. Compressional forces also form mountains such as the Himalayas in India. 246 Earth s Changing Surface Reading Essentials
Mini Glossary compression: the squeezing force at a convergent boundary continental drift: the hypothesis that continents move convergent boundary: where two plates move toward each other divergent boundary: where two plates move apart from each other shear: the side-by-side dragging force at transform boundaries subduction zone: the area where one plate slides under another tension: the pulling force at a divergent boundary transform boundary: where plates slide horizontally past each other plate tectonics: theory stating that Earth s is broken into rigid plates that move slowly over Earth s surface 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that shows you understand the theory of plate tectonics. 2. Write shear, tension, or compression in each box below to identify the type of force associated with each type of plate boundary. boundary Divergent boundary Transform boundary 3. Select a term that appears in a main heading of the outline you created when you reviewed the lesson. In the space below, define that word. What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Earth s Changing Surface 247