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17.4 THEORY OF PLATE TECTONICS • The theory of plate tectonics states that the lithosphere is divided into 12 large sections (plates) and about 20 smaller ones. • These plates ‘float’ on the more dense, fluid-like asthenosphere VOCABULARY theory of plate tectonics divergent boundary rift valley convergent boundary subduction zone oceanic–oceanic convergent boundary oceanic–continental convergent boundary continental–continental convergent boundary transform boundary strike–slip fault 17.4 THEORY OF PLATE TECTONICS • Three types of boundaries exist where tectonic plates meet. These create Earth’s geological features and events. • Ridges, rifts, volcanoes and earthquakes are created at divergent boundaries, where plates are moving away from each other. • At convergent boundaries (where plates move toward each other), we find mountains, trenches, subduction zones, volcanoes, and earthquakes, depending on the types of plates involved. • Earthquakes and strike-slip faults are created along transform boundaries, where plates move past each other in opposite directions. VOCABULARY theory of plate tectonics divergent boundary rift valley convergent boundary subduction zone oceanic–oceanic convergent boundary oceanic–continental convergent boundary continental–continental convergent boundary transform boundary strike–slip fault 17.4 Theory of Plate Tectonics THREE TYPES OF PLATE BOUNDARIES 17.4 Divergent Boundaries Continental Rifts • When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift. East African Rift Valley 17.4 review Convergent Boundaries A subduction zone occurs when one oceanic plate is forced down into the mantle beneath a second plate. Oceanic-Continental • Denser oceanic slab sinks into the asthenosphere. • Pockets of magma develop and rise. • Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent. • Examples include the Andes, Cascades (Juan de Fuca), and the Sierra Nevadas. Cascadia Subduction Zone West of Vancouver Island, and extending from the north tip of the Island to northern California, the oceanic Juan de Fuca plate is moving towards North America at about 2-5 cm/year. This region is called the Cascadia subduction zone. Here, the much smaller Juan de Fuca plate is sliding (subducting) beneath the continent (it is about 45 km beneath Victoria, and about 70 km beneath Vancouver). The ocean plate is not always moving though. There is good evidence that the Juan de Fuca and North America plates are currently locked together, causing strain to build up in the earth's crust. It is this squeezing of the crust that causes the 300 or so small earthquakes that are located in southwestern British Columbia each year, and the less-frequent (once per decade, on average, damaging crustal earthquakes (e.g., a magnitude 7.3 earthquake on central Vancouver Island in 1946). At some time in the future, these plates will snap loose, generating a huge offshore "subduction" earthquake - one similar to the 1964 M=9.2 Alaska earthquake, or the 1960 M=9.5 Chile earthquake. Current crustal deformation measurements in this area provide evidence for this model. Geological evidence also indicates that huge subduction earthquakes have struck this coast every 300-800 years. Convergent Boundaries OCEANIC-CONTINENTAL CONVERGENT BOUNDARY 17.4 17.4 Convergent Boundaries Oceanic-Oceanic • Two oceanic slabs converge and one descends beneath the other. • This kind of boundary often forms volcanoes on the ocean floor. • Volcanic island arcs form as volcanoes emerge from the sea. • Examples include the Aleutian, Mariana, and Tonga islands. 17.4 Convergent Boundaries OCEANIC-OCEANIC CONVERGENT BOUNDARY 17.4 Convergent Boundaries Continental-Continental • When subducting plates contain continental material, two continents collide. • This kind of boundary can produce new mountain ranges, such as the Himalayas. 17.4 CONVERGENT BOUNDARIES Continental-Continental Convergent Boundary 17.4 CONVERGENT BOUNDARIES ‘Collision of India and Asia’ 17.4 Transform Boundaries At a transform fault boundary, plates grind past each other without ‘destroying’ the lithosphere. Transform faults • When the faults ‘overcome’ the friction holding them in place, the ‘slip’ causes earthquakes 17.4 Transform Boundaries TRANSFORM FAULT BOUNDARIES 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT • Scientists now believe that three forces work together to move the tectonic plates: 1. mantle convection 2. ridge push 3. slab pull VOCABULARY mantle convection ridge push slab pull •Forces at plate boundaries produce landscape features such as mountains, trenches, and island arches. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT • Uplifted mountains, volcanic belts, and island arcs are found parallel to convergent boundaries. • Ocean trenches lie along convergent boundaries. • Mid-ocean ridges and rift valleys mark divergent boundaries. VOCABULARY mantle convection ridge push slab pull 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Scientists believe tectonic plates are moved by three forces: o Mantle convection is the idea that there is a treadmill-like current in the asthenosphere that moves the crust that sits above it. (hot magma rises in one area and cool magma sinks in another) • Convective flow is the motion of matter resulting from changes in temperature. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Convection causes Ridge Push and Slab Pull... Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement. Ridge Push Slab Pull Ridge Push Slab Pull Ridge Push 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Scientists believe tectonic plates are moved by three forces: o Ridge Push is the idea that the hot magma pushing up the mid-ocean ridges is also pushing the ocean plates apart. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Scientists believe tectonic plates are moved by three forces: o Slab Pull is when one plate slides underneath the other. The subduction zone causes the bottom plate to be pushed back into the mantle and this pulls the plate away from the midocean ridge. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Mountains form at convergent boundaries. o Ocean – continent boundary causes mountains to form along the coast. These often push land that was once underwater up high causing ocean fossils to be found in the mountains. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Mountains form at convergent boundaries. o Continent – continent boundary causes mountains to form from the uplifting of the two continent edges. This can also cause former coast lines and marine fossils to be found in the mountains. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT • Rifts form at divergent boundaries. • If this happens on land a rift valley forms. • Often volcanoes also form along these rifts since the crust is thinner. CAUSES AND EFFECTS OF PLATE 18.1 MOVEMENT Trenches can form at a convergent boundary. o They occur where one plate is subducted under the other. If there are mountain chains along a coast then you know that a few hundred kilometers out into the ocean that there must be a trench. 18.1 CAUSES AND EFFECTS OF PLATE MOVEMENT Trenches can form at a convergent boundary. o When trenches form at oceanic-oceanic convergent boundaries, a deep ocean trench is also formed o The Mariana trench is Earth’s deepest trench, still sinking, and is ~ 11 km below the surface of the ocean! QUESTIONS PG. 508 # 1 – 9 PG. 521 # 2