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Chapter 18 Plate tectonics History of plate tectonics • The earth’s surface is divided into several major and minor plates and the interaction between these plates is known as plate tectonics. There are 8 major and about 30 minor plates. –N. American –S. American –African –Eurasian –Indian –Pacific –Australian –Antarctic • Intense geologic activity occurs at the plate boundaries where plates; –Collide with one another –Move away from one another –Slide past one another • The concept of plate tectonics was developed in 1960s by combining two theories; –Continental Drift –Sea floor spreading 1. Continental Drift: proposed by Alfred Wagener in 1912. • He found similarities in S. America, Africa, India, Antarctica and Australia. • On this basis he proposed; –All the continental landmasses were once joined together in one supercontinent—Pangea. All the oceans formed one super ocean—Tetheys. –The Pangea then split into a northern Laurasia and a southern Gondwanaland. • Revival of continental drift: Wegener’s mechanism of continental drift was not very convincing and his theory remained discarded till 1960s. • It was revived with the advent of paleomagnetism, which confirmed that the continents have moved relative to one another. • Additional evidence for cont. drift: –The continents fit like a jigsaw puzzle –Rock similarity –Fossils and age similarity 2. Sea floor spreading: proposed by Harry Hess in 1962. • He proposed that the sea floor moves away from the crest of a mid ocean ridge and finally disappears beneath a continent or an island arc (subduction). • Mantle convection is responsible for ocean spreading. Plates and plate boundaries • Rigid lithospheric plates move over plastic asthenosphere. • Plate boundaries: 3 types; 1. Divergent plate boundary: plates move away from one another. Also known as Constructive P.B. or spreading center. 2. Convergent P.B: plates move towards one another. Also known as Destructive P.B. 3. Transform P.B: plates move horizontally past one another. Also known as Conservative P.B. Do plates really move? • Plate motion is very slow but predictable. 1-10 cm/year. • Movement is measured by satellites, lasers and GPS. Do plates really move? • The magnetic anomalies at the sea floor and movement along a transform fault indicate plate motion. Divergent Plate Boundaries • Two plates move away from one another. • Can occur within a continent or an ocean. If it occurs in a continent— rifting. Divergent Plate Boundaries • Generally basaltic magma erupts and spreads on either side of the fractures causing the plates to push away from one another. • Examples: –Red Sea. –East African Rift • A passive continental margin forms when one of the moving portions of the plate are covered by sediments. • A mid ocean ridge is formed when divergent P.B. occurs in the middle of an ocean—mid Atlantic ridge. Convergent P.B. • Two plates move towards each other and collide forming mountains. Convergent P.B. • Three types of convergent p.b; –Ocean-ocean convergence –Ocean-continent convergence –Continent-continent convergence • Ocean-ocean convergence: one plate subducts under the other, a trench and a volcanic island arc are formed. • Ocean-continent convergence: oceanic plate subducts beneath the continent and an active continental margin is formed. • Continent-continent convergence: neither plate subducts and a collisional mountain chain is formed. 3. Transform P.B: plates slide past one another an no significant material is created or destroyed. • Transform faults generally connect two divergent p.b. or two trenches. Why plates move? • Mantle convection. • Ridge Push • Trench Pull Mantle plumes and Hot Spots • Mantle convection moves towards the earth surface in the form of a plume and appear on the surface as a Hot Spot. Plate tectonics and ore deposits • Valuable metallic ores are associated with divergent boundaries and volcanism on the sea floor.