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Unit 7 Lesson 1 The Theory of Plate Tectonics Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics Drifting Apart What is continental drift? • Continental drift is the hypothesis, proposed by Alfred Wegener, that all of today’s continents were once part of a single landmass. • Wegener called this single landmass Pangea and proposed that broke into pieces that “drifted” to the continents’ present locations. • Alexander du Toit suggested that Pangea first separated into Laurasia and Gondwana. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports continental drift? • Wegener developed his hypothesis by looking at the shapes of the continents, climatic evidence, rocks, fossils, and land features. • As mapmaking advanced, maps began to show more accurate shapes of the continents. • The matching coastlines support continental drift because they suggest that the continents were once connected. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports continental drift? • Fossils of the same plant and animal species are found on continents separated by vast oceans. • Scientists inferred the continents were joined when these plants and animals were alive. • Different continents also have matching rock layers and land features. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports continental drift? • Locations of coal deposits and past glacial activity provide climatic evidence for continental drift. • Coal deposits found in cooler climates suggest that these continents were once closer to the equator. • The past movement of glaciers across South America, India, Africa, and Australia only made sense if the continents were connected when the glaciers existed. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What is plate tectonics? • At first, continental drift was not widely accepted by the scientific community, but the hypothesis evolved into the theory of plate tectonics. • Plate tectonics states that Earth’s surface is made up of giant, moving slabs called tectonic plates. • A theory requires a great deal of evidence and is a system of ideas that explains many related observations about the natural world. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports plate tectonics? • The theory of plate tectonics explains how moving plates shape land features and how the motion relates to Earth processes. • During sea-floor spreading, new sea floor forms and pushes existing rock in opposite directions. • The far edges of the spreading sea floor are pushed into deep-ocean trenches through subduction, the process by which one tectonic plate is pulled beneath another plate. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports plate tectonics? • The north and south poles have repeatedly switched throughout Earth’s history, resulting in a a magnetic reversal. • As new sea floor forms, magnetic minerals align with the current arrangement of Earth’s poles and become stuck in that position as the rock hardens. • This alignment switches each time Earth’s poles shift, resulting in a magnetic reversal pattern. This provides evidence for sea-floor spreading and plate tectonics. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics What evidence supports plate tectonics? • The locations of earthquakes and volcanoes also provide evidence for plate tectonics. • Most active volcanoes are found near plate boundaries, and most earthquakes take place near or on these boundaries as well. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics Plates Make It Possible! How does research related to plate tectonics affect society? • Knowledge about plate tectonics allows scientists to identify areas at risk for earthquakes and volcanic eruptions. • Scientists are also able to locate valuable resources, such as minerals and fossil fuels. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 1 The Theory of Plate Tectonics How does research related to plate tectonics affect science? • The theory of plate tectonics has had a great impact on scientific thought because it has unified many theories of Earth science. • Plate tectonics help scientists understand fossil distribution, earthquake patterns, volcanic eruptions, and climatic changes • Scientists also use plate tectonics to observe how Earth’s surface has changed and continues to change. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Plate Tectonics and Landforms Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms Stressed Out What does the theory of plate tectonics explain? • Earth’s outermost layer is called the lithosphere, and the layer directly below is called the asthenosphere. Both are solid layers of rock. • The lithosphere is divided into large, moving pieces called tectonic plates. • The theory of plate tectonics explains how lithospheric plates move around on the slowflowing rock of the asthenosphere. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What are some properties of tectonic plates? • The lithosphere is broken into 15 major tectonic plates that differ in size, shape, density, thickness, and composition. • Continental lithosphere is thicker and older than oceanic lithosphere. • Tectonic plates move slowly around Earth’s surface and interact with one another. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What types of stress are related to the movement of tectonic plates? • As tectonic plates move and interact, stress is put on rock. • This stress causes deformation, which is the bending, tilting, and breaking of rock. • Deformation changes the size and shape of features on or below Earth’s surface. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What types of stress are related to the movement of tectonic plates? • Compression is a stress that squeezes or shortens material. • This type of stress can cause folds as rock is squeezed and shortened. • Compression can also cause breaks in rock, or faults, where Earth’s lithosphere is more rigid. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What types of stress are related to the movement of tectonic plates? • Tension is a stress that lengthens a material or pulls a material apart. • Two plates moving away from one another cause tension. • When tension is greater than the strength of the rock, the rock break and faults form. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What types of stress are related to the movement of tectonic plates? • Shear stress causes material to twist or become distorted. • This type of stress can occur when two tectonic plates move past one another and grind against each other. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What are the different types of plate boundaries? • Most deformation occurs at boundaries where tectonic plates meet. • These plate boundaries may be on the ocean floor, around the edges of continents, or even within continents. • Certain landforms are formed at different types of boundaries. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What are the different types of plate boundaries? • A convergent boundary forms when tectonic plates collide. • In oceanic-continental convergence, the denser oceanic plate sinks beneath the continental plate. • In continental-continental convergence, the plates push against each other and buckle. • In oceanic-oceanic convergence, the denser plate sinks beneath the less dense plate. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What are the different types of plate boundaries? • A divergent boundary forms when two tectonic plates move away from one another. • Magma rises up through cracks that form when two plates move away from each other along a divergent boundary. • Most divergent boundaries are under the ocean. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What are the different types of plate boundaries? • A transform boundary forms when two tectonic plates move past one another in opposite directions. • The plates slowly scrape against one another before shifting suddenly, which shear stress breaks or distorts rock. • The San Andreas Fault is a transform boundary, but most transform boundaries are on the ocean floor. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms Push Up What features are associated with convergent boundaries? • Compression along convergent boundaries can cause rock to be folded and move upwards, forming mountain ranges • The Appalachian Mountains formed from faulting and folding when the North American plates collided with the Eurasian and African plates. • The Himalayas formed when the Indian plate collided with the Eurasian plate. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What features are associated with convergent boundaries? • When oceanic and continental plates collide, magma can rise to Earth’s surface and eventually form a continental volcanic arc. • Volcanoes can also form when two oceanic plates converge and one sinks beneath the other. This type of collision results in a volcanic island arc. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What features are associated with convergent boundaries? • Ocean trenches are the deepest landforms found on the ocean floor. • They can form when a oceanic plate collides with and slides beneath an oceanic or continental plate. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What features are associated with divergent boundaries? • Mountains may also form at divergent boundaries. • Fault-block mountains form when block of lithosphere drop down or are lifted up along faults. • Tension at diverging plates also causes rifting, where Earth’s lithosphere pulls apart to form long, narrow, faulted rift valleys. • Rifting also forms rift zones, which can form volcanic mountains and tall, flat-topped plateaus. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What features are associated with transform boundaries? • Some chains of volcanic islands, known as hot spots, form far from plate boundaries. • At a hot spot, magma rises from deep within the Earth. Repeated lava flows form an undersea volcano that eventually reaches above the ocean’s surface to form a volcanic island. • As the oceanic plate shifts, a new island may form over the hot spot. Copyright © Houghton Mifflin Harcourt Publishing Company Unit 7 Lesson 2 Tectonic Plates and Landforms What volcanic landforms occur way from plate boundaries? • Major and minor earthquakes are associated with transform boundaries • Transform boundaries can also cause mid-ocean ridges to be offset. • Offset streams and fences can be a sign of a transform boundary on land. Copyright © Houghton Mifflin Harcourt Publishing Company