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LESSON 2 EARTH’S MOVING CONTINENTS Chapter 5 Changes Over Time OBJECTIVES Discuss evidence for continental drift and plate tectonics. Explain seafloor spreading. MAIN IDEA Earth’s crust is made of moving plates that slowly but constantly change its surface. VOCABULARY continental drift – the slow movement of the continents over eons plate tectonics – the theory that Earth’s surface is broken into piece’s, or plates, that slide over the magma in the mantle magma – hot, fluid rock below Earth’s surface seafloor spreading – the moving apart of plates on the ocean floor that is caused by magma flowing up between the plates and then hardening ARE THE CONTINENTS MOVING? • Alfred Wegener published a book proposing that the continents had been connected. • He called this supercontinent Pangea. • The separation of the continents became known as continental drift. • He showed that the continents fit together like a puzzle. • Fossil remains, rocks, glaciers, and mountain ranges matched. • Also fossils of tropical species were found on Antarctica. PLATE TECTONICS • In Wegener's time, people wondered how the continents could move through solid rock. • New evidence in the 1950’s supported Wegener's proposal. • Scientists mapping the floor of the Atlantic Ocean found an underwater mountain chain in the middle of the Atlantic. • On both sides of these mountain chains, the ocean floor was moving. • Scientists developed a model called plate tectonics to explain how the continents and the ocean floor could move. • According to this model, Earth's surface is broken into pieces, or plates. • The plates move over the hot, fluid rock, or magma, in the mantle. • Uneven heating in the mantle produces slowmoving currents of plastic-like, fluid rock. • The cooler, rigid rock of the lithosphere rests on top of this fluid rock. • The slow movements in the fluid part of the mantle drag the lithosphere and its plates sideways. • As the lithosphere moves, so do the ocean floor and the continental plates. QUICK CHECK Draw Conclusion What conclusions did Alfred Wegener draw about Pangaea? He concluded that it split apart and that the continents “drifted” to their present positions. Critical Thinking How does the movement of the ocean floor support Wegener’s idea of continental drift? It provides evidence for the theory of plate tectonics by demonstrating how plates can move. HOW DO OCEANS CHANGE SIZE? • The processes that move the continents also help form new crust on the ocean floor. • As some crustal plates move apart, magma enters the cracks and flows outward. • The magma cools, hardens and builds up into parallel ridges, or raised structures, on the ocean floor. • The new rock exerts a sideways force called compression. • Magma continues to flow between the plates, forcing them farther apart. • This process is called seafloor spreading. • Seafloor spreading explains how plates move apart and new crust forms. • At the Mid-Atlantic Ridge in the Atlantic Ocean, new seafloor crust is formed at the rate of about 3 centimeters (1 inch) per year. EVIDENCE OF SEA FLOOR SPREADING • Several pieces of evidence support seafloor spreading. • For example, the youngest rock on the ocean floor is found at mid-ocean ridges. • The older rock is found further away from the ridge. • Rock that makes up the continents is generally older than the seafloor rock. • The type of rock found on the ocean floor is also significant. • Most of the seafloor rock is volcanic in origin. • Magma from the mantle cooled and hardened. • The magnetism of seafloor rock provides further evidence for seafloor spreading. • Earth has a north-south magnetic field; occasionally it reverses completely, resulting in a south-north orientation. • The iron particles in the magma line up according to the direction of Earth's magnetic field. • As the magma cools and solidifies, the iron particles "freeze" in the direction of the magnetic field at the time. • When scientists studied rock along the seafloor, they found that the magnetism in the rock altered from one direction to the other. • This pattern matched on both sides of the mid-ocean ridges. QUICK CHECK Draw Conclusions Why would continental rock generally be older than seafloor rock? Rock is formed at mid-ocean ridges. To become continental rock, the material is pushed away from the ridge over millions of years. Critical Thinking Summarize the evidence that supports seafloor spreading. The evidence includes the age of seafloor rocks (the youngest are at mid-ocean ridges); the type of rock involved (volcanic); and the magnetism of the rock (which indicates that rock formed when poles were aligned in different directions). WHAT HAPPENS AT PLATE BOUNDARIES? • Plates can move in three ways. • They can move apart from each other, • they can collide, • or they can slide past each other. • Locations where plates move apart are called divergent boundaries. • Seafloor spreading occurs at divergent boundaries. • Divergent boundaries also occur on land. • Iceland is located on a divergent boundary at the northern end of the Mid-Atlantic Ridge. • The Great Rift Valley in Africa is also located on a divergent boundary. • There, the continent of Africa is splitting. • The split may one day form a new ocean. • Some divergent boundaries are less visible, occurring within a continental plate. • Locations where plates collide are convergent boundaries. • If both colliding plates include continents, the pressure lifts and crumples the plates, forming mountains. • Earthquakes and volcanic activity can occur at convergent boundaries. • When one plate carries part of an ocean floor and the other a continent. • The oceanic plate slides under the continental plate into the mantle and melts, this is subduction. • Some of the magma beneath rises through the cracks between the plates. • At the surface, a volcano forms. • When plates move toward each other on the ocean floor, one plate sinks under the other. • This movement forms an ocean trench. SLIDING PLATES • Some plates simply slide past each other. • The boundary between these plates is a transform boundary. • Earthquakes can occur along transform boundaries as strain on the rock builds up and then is quickly released. • Rock along these boundaries shatters and breaks. • Eventually, this rock may pile up and form narrow ridges and valleys. QUICK CHECK Draw Conclusions If a continental valley begins to widen, what conclusion might you draw about the region’s plate movement? The valley may be on a divergent boundary and the plates may be moving apart. Critical Thinking Where would it be easiest to drill through Earth’s crust and reach the mantle? The ocean floor is where Earth’s crust is thinnest and would therefore be the easiest location in which to drill. WHERE ARE THE PLATES? • Earth's crust has seven major tectonic plates. • • Some of the plates are diverging or moving apart. • Others are converging, or pushing together. • Mid-ocean ridges occur along diverging plates. • • The Mid-Atlantic Ridge is located where the South American Plate is moving away from the African Plate. • Convergent boundaries can be found at many large mountain chains. • The Himalayas are located where the Indian Plate is colliding with the Eurasian Plate. • The Alps are located where the African Plate is colliding with the Eurasian Plate. • Most transform boundaries are located on the ocean floor. • On land the most noticeable transform boundary is the San Andreas Fault between the North American Plate and the Pacific Plate. • This fault is the site of many powerful earthquakes. QUICK CHECK Draw Conclusion India continues to move into the Asian continent. What will happen to the Himalayas? They will continue to grow. Critical Thinking How might the movement of Earth’s tectonic plates affect you? Plate movement can cause earthquakes, which can affect people’s lives.