Plate TectonicsSummary of Important ConceptsPLATE TECTONICS - A POWERFUL UNIFYING THEORYPlate tectonics is a relatively new scientific concept, introduced some 30 years ago, but it has revolutionized our understanding of the dynamic planet upon which we live.PLATE TECTONICS INTRODUCEDEarth’s lithosphere, which consists of the earth’s crust and upper mantle, is cut up into roughly 20 plates that move relative to one another atop of the asthenosphere.Plates Interact: They converge, diverge or slide horizontally past one another.CONTINENTAL DRIFTAlfred Wegener in the early 1900’s proposed that the continents were once joined together in a single large land mass he called Pangea (meaning “all land” in Greek). He proposed that Pangea had split apart around 200-250 million years ago moving gradually to their present positions - a process that became known as continental drift.Wegner’s Evidence for Continental DriftContinents fit together like a puzzle….e.g. the Atlantic coastlines of Africa and South America.The Revival of Continental Drift 1940’s & 1950’sWork in the 1940’s and 50’s set the stage for the revival of Wegner’s work.During the 1950’s, intense oceanographic research and technological advancements provided maps of the sea floor showing mid-ocean ridges and deep sea trenches.Harry Hess and Sea Floor Spreading 1960’sEvidence of Sea Floor Spreading PaleomagnetismThroughout earth’s time the magnetic north and south have switched RANDOMLY and sporadically every 1000-10,000 years. When magnetism switches, its called a REVERSAL.Rocks record the direction of the earth’s magnetic field at the time the rocks form. Small magnetite (Fe) crystals in cooling magma act like compass needles that record the direction of the earth’s magnetic field when the magma solidifies.Frederick Vine and Drummond Matthews found that alternating normal and reversely polarized rock form a symmetrical stripe-like pattern parallel to the ridge crest.At MOR new sea floor is added and spreads laterally from the axis. As the magma cools and the iron bearing minerals crystallize they align themselves parallel to the lines of force of the earth’s magnetic field.Therefore, the sea floor is a ticker tape recording of the earth’s magnetism through geologic time. (Only for about ~200million years….why?)Evidence of Sea Floor Spreading Ocean Floor AgesEvidence of Sea Floor Spreading Hot Spot Volcanic IslandsA hot spot is a persistent volcanic center located directly above a rising plume of hot mantle rock.Hot spot mantle plumes remain stationary while the lithosphere moves over it. This process forms a chain of volcanic islands. The chain of islands formed indicates the direction of plate movement over the hot spot.Evidence of Sea Floor Spreading Interactions at Plate BoundariesThe majority of earthquakes and volcanic eruptions are concentrated in belts or linear chains at the boundaries of the lithospheric plates.DIVERGENT PLATE BOUNDARIESAlso called spreading centers and rifts; occurs where two plates move apart horizontally and new oceanic lithosphere is created.Continent-Continent Divergent BoundariesMid-ocean ridges where rising basaltic magma convects upward forming new ocean floor.Examples include: Mid-Atlantic Ridge spreading ~1 cm/yr; East Pacific Rise spreading ~6 cm/yr.CONVERGENT PLATE BOUNDARIESDevelop where two plates are moving horizontally toward each other and therefore are colliding. Can result in orogenic events (mountain building) or volcanism and deep ocean trenches. Depends on the plates involved.Continent-Ocean Convergent BoundariesOceanic crust is denser (more Fe and Mg) than continental crust. When they collide, the denser oceanic plate will SUBDUCT beneath that of the lower density continental plate.Volcanism. Melting of the subducting plate generates a (mafic) magma. The magma, being less dense than the surrounding solid mantle, rises up through the continental (felsic) crust. The end result is a volcanic arc on the continent paralleling the oceanic trenchEarthquakesExamples include the Andes Mountains in South America, the Cascade Mountains in Western USContinent-Continent Convergent Plate BoundariesWhen two plates carrying continental crust converge (after all the oceanic crust separating them is consumed by subduction) neither plate will subduct because of low densities.Orogeny. The result of collision is the construction of large scale high pointy mountain chains.EarthquakesExamples include the Appalachian Mountains formed during the formation of Pangaea, the Himalayas from the collision of India with Asia.Ocean-Ocean Convergent Plate BoundariesCollision of two oceanic slabs will result in the descent of one below the other initiating volcanic activity in a similar manner to ocean-continent collision. In the case of two oceanic plates colliding, the older (colder, denser) oceanic crust subducts.Volcanism. Forms volcanic island arc.Earthquakes.Examples include Japan, Aleutian islands, Caribbean islands, Mariana Islands,TRANSFORM PLATE BOUNDARIESOccurs when two lithospheric plates slide past one another horizontally.Not associated with volcanism or mountain building.Lots of shallow earthquakes.Driving force is not just convection? Ridge Push and Slab PullRidge Push: Gravity pulls the plates away from the mid-ocean ridge crests.Slab Pull: Gravity pulls the plates into the mantle. Plates tied to subducting limbs spread faster. Examples include: Mid-Atlantic Ridge spreading ~1 cm/yr; East Pacific Rise spreading ~6 cm/yr.Plate TectonicsSummary of Important Concepts- Plate tectonics refers to the concept that the earth’s outer rigid rock shell, called the lithosphere, is divided up into a number of separate pieces, called tectonic plates, that are shoved around by the slow convection of hot rock in the underlying asthenosphere. - The movement of the tectonic plates causes many of the major geologic features of the earth’s surface, including earthquakes, volcanoes, mountain ranges, and major features of the ocean floor like mid-ocean ridges and oceanic trenches. The movements of these plates causes the continents to move (drift) over time, opens up new ocean basins, and closes old ones. - Alfred Wegener, in the early 1900’s, was the first person to gatherevidence that the continents had moved - a process called continental drift. His evidence suggested that the continents on both sides of the Atlantic Ocean had once been joined into a single
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