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Chapter 18Chapter 18
Plate tectonics 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.