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Theory of Plate Tectonics Earth’s Plates • Are large slabs of rock composed of crust and solid part of upper mantle collectively called the lithosphere • Lithosphere floats on liquid like molten rock of upper mantle called asthenosphere • Lithosphere is broken up into different sections called plates • 12 major plates and many smaller ones exist on Earth’s surface • All these plates fit together like pieces of jigsaw puzzle Earth’s Tectonic Plates Theory of Plate Tectonics • Explains many of the features and events that take place at and below Earth’s surface • It explains the following geological processes: a) how and why the continents move b) how and why sea floor spreading occurs c) how and why and where earthquakes, volcanoes, and mountain formation occur Type of Tectonic Plates • Some are composed solely of oceanic crust • Other plastes are composed of only continental crust • Most plates are mixed oceanic and continental crust Movement of Tectonic Plates • Earth’s Tectonic Plates float and move slowly over the asthenosphere because plates are less dense than underlying asthenosphere • Convection currents in asthenosphere can cause fractures of Earth’s plates and are also responsible for their movement Divergent Plate Boundaries • Are regions where 2 plates separate and create new oceanic crust • This process occurs where the sea floor spreads along a mid-ocean ridge • Can also occur in middle of continent called “continental rifting” Divergent Plate Boundaries cont… Convergent Plate Boundaries • Are regions where 2 plate boundaries collide • Typically the heavier and more dense oceanic plate slides under the less dense and lighter continental plate • This process is called subduction and region where this occurs is called the “subduction zone” Convergent Plate Boundaries cont…. • Subduction causes deep sea trenches to form • Volcanoes and volcanic mountains such as Garibaldi and Mount Baker, are some examples of volcanic mountains formed at these regions • Earthquakes are also common at these regions Convergent Plate Boundaries cont…. Transform Plate Boundaries • Are regions where 2 or more plates slide past each other • Typically occurs when both plates are of the same type and therefore same density and weight • Identified by Canadian Geologist Tuzo Wilson • Earthquakes are very common at these boundaries Transform Plates cont….. Movement of Earth’s Tectonic Plates • Earth’s tectonic plates are constantly moving • Using sensors embedded in Earth’s plates and using satellites we know that they move at a rate of about 1-15 cm per year depending on the plate • The following map shows in which direction the different plates are moving Direction of Movement of Earth’s Plates Movement of Earth’s Plates •Now we will see the different processes responsible for making the Earth’s plates move Mantle Convection • Part of the Earth’s mantle is made of molten (i.e. liquid) material • Energy from radioactive decay of some elements as well as heat from the Earth’s core provides enough heat to melt some of the mantle resulting in aesthenosphere Earth’s Aesthenosphere Convection Currents in Aesthenosphere • Warmer, less dense material rises and cooler, denser material sinks with any liquid • The asthenosphere is no exception • Warmer, less dense asthenosphere (closest to the Earth’s core) rises and cooler, more dense asthenosphere (closest to the lithosphere) sinks downward resulting in convection currents in the asthenosphere • These currents drag the tectonic plates with them through friction Aesthenosphere Convection Currents Two Other Processes That Aid In Plate Movement • There are two other processes (other than convection currents in the aesthenosphere) that also aid in movement of the Earth’s Plates. These processes are referred to as: a) Slab Pull b) Ridge Push Ridge Push • Because mid-ocean ridges lie at a higher elevation than the rest of the ocean floor, gravity causes the ridge to push on the lithosphere that lies farther from the ridge. • Weight of elevated ridge pushes the oceanic plate towards the subduction zone Ridge Push Slab Pull • As the leading edge of the subducting plate sinks, it pulls the rest of the plate with it at convergent plate boundaries • Gravity and convection assist in this movement Slab Pull In Class/Homework • Complete questions 1, 2, 3, 5, 6, 7 on page 295 in your textbooks • Complete the Interactions of Earth’s Tectonic Plates Worksheet