chapter_07 - Circulation of the Solid Earth: Plate...

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Unformatted text preview: Circulation of the Solid Earth: Plate Tectonics Earth: Chapter 7 Dr. Emily Berndt Fundamentals of the Climate System EAS 253 a. Alfred Wegener was responsible for the Alfred underpinnings of plate tectonics underpinnings i. Pangaea – supercontinent 1. 2. 3. 4. Fit of Africa and the South American continental Fit shelves shelves Similar fossils on the continents Similar rock types and structures Glacial evidence I. Introduction I. ii. Continental Drift Pangaea Pangaea II. Anatomy of Earth II. a. Seismic probing of Earth’s Interior ii. Knowledge of the interior comes from . studying the behavior of earthquake waves waves iii. Seismology – the study of earthquakes i. and related phenomena and iii. Earthquake – a sudden release of i. stored energy as a result of rapid movement between two blocks of rock movement Elastic rebound Elastic I. Introduction I. II. Anatomy of Earth II. 1.Focus- Site of energy release below the Focussurface surface a.~700 km deep b.Energy radiates in all direction 2.Epicenter- location on Earth’s surface Epicenterabove the focus above 3.Fault- fracture in crust II. Anatomy of Earth II. b. Seismic waves i. Waves are recorded on a seismograph 1. the rate waves travel through Earth 1. depends on the properties of the material depends 2. calculate the average speed of waves 3. compare several seismographic records 3. around the Earth to construct a view of the seismic wave paths the Seismograph Seismograph A seismogram records wave amplitude vs. time amplitude The epicenter is located using three or more seismic stations or Time interval between arrival of P and S waves and A time-travel graph is used to find the distance to the epicenter the Possible seismic paths through the Earth through b. Seismic waves ii. Types of Seismic Waves 1. Body waves- travel through Earth’s interior a. Primary (P) Waves i. Result from compression ii. Compressions and expansions in direction of wave travel iii. Travel through solids, liquids, or gases b. Secondary (S) Waves i. Also called shear waves ii. Displacements perpendicular to direction of wave travel iii. Only travel through solids 2. Surface Waves- travel across Earth’s surface a. Transmit energy along surface b. Movement is constrained vertically c. Similar to a water wave II. Anatomy of Earth II. Compressional P Waves Compressional Primary (P) waves Primary Oscillation of S Waves Oscillation Secondary (S) waves Secondary Surface waves Surface I. Anatomy of Earth I. b. Seismic waves iii. Generalized Structure of Earth iii. Generalized Structure of Earth iii. 1. 2. 3. Crust, mantle (upper and lower) and core (outer and inner) Layers defined by seismic wave velocities MOHO- boundary between crust and mantle a. Increase in wave velocities b. Depth of 7-70 km Low Velocity Zone (LVZ)- slower velocities ~80-300 km depth Velocities increase again at the transition between upper and lower Velocities mantle mantle Lower mantle and outer core boundary a. Decrease in P wave velocities b. Disappearance in S waves P wave velocity increases again ~5150 km a. Deflects P waves b. Confirms solid inner core 4. 5. 6. 7. II. Anatomy of Earth II. c. The crust i. Varies in thickness and composition 1. Continental Crust thick, less dense older, granitic 2. Oceanic Crust thin, more dense younger, basaltic II. Anatomy of Earth II. c. The Crust iii. Igneous, Sedimentary, and i. Metamorphic Rocks Metamorphic 1. All rocks are composed of minerals 2. Mineral – a natural, inorganic solid, 2. with a definite crystal structure and chemical composition chemical II. Anatomy of Earth II. 3. Igneous Rock- forms by cooling and solidification of magma or lava a. b. Intrusive- form below surface (Magma) (Granitic) IntrusiveExtrusive- form above the surface (Lava) (Basaltic) 4. Rock exposed to Earth’s surface tend to decompose or weather into 4. sediments sediments a. b. c. d. Wind, water, gravity Sediments are deposited in layers Weight of sediments compact the layers below Spaces between sediments fill in with minerals (cement) 5. Sedimentary rock- forms by lithification (compaction and cementation of 5. sediments) sediments) a. b. Layered Sandstone, mudstone, shale Can form chemical or biologically 6. Metamorphic rock- forms as heat, pressure, or chemical active fluids change 6. the mineral and chemical composition of the rock the a. Marble and slate II. Anatomy of Earth II. c. The crust iii. Major Rock-Forming Minerals 1. Silicate Minerals 2. Felsic 3. Mafic II. Anatomy of Earth II. The Rock Cycle The II. Anatomy of Earth II. d. The Mantle i. Most of what we know is due to seismology ii. Relatively uniform in composition and made of silicate minerals silicate iii. Pressure and temperature increase with depth 1. Changes the structure and mineral composition of Changes silicates silicates 2. Changes the velocities of seismic waves iv. Link between LVZ and plate tectonics 1. Some molten rock present 2. Allows crust and upper mantle to move relative to lower Allows mantle mantle II. Anatomy of Earth II. e. The Core i. Iron and Nickel – much denser ii. Seismic waves bounce off the mantle- core Seismic boundary boundary iii. Source of Earth’s magnetic field iv. Magnetic dynamd -Transform energy from convection to -Transform electrical currents to create a magnetic field field 1. Convective fluids must be conductors 2. Outer core convects due to temperature gradient 3. Energy source could be radioactive decay III. The Theory of Plate Tectonics III. a. Seafloor-Spreading i. Resurgence of interest in Wegener’s ideas was the result of Resurgence investigating the seafloor in the 1960s investigating ii. Mapping revealed 1. Mid-ocean ridges and seafloor spreading a. Rift valley b. Linear volcanic chains c. New seafloor created d. Seafloor spreads to sides 2. Magnetic characteristics of seafloor a. Rock become magnetized as molten rock cools (Curie Rock Point) Point) b. Magnetized in direction of Earth’s magnetic field c. Reveals magnetic field reversals Ocean Seafloor Ocean III. The Theory of Plate Tectonics III. b. Paleographic Reconstructions III. The Theory of Plate Tectonics III. b. Paleographic Reconstructions i. Reverse time by rolling the seafloor back into the Mid-Atlantic Ridge ii. Clues from Sedimentary Rocks 1. Glacial deposits High paleolat 1. 1. 1. 1. 2. Reef limestones Tropical paleolat Tropical Salt deposits Subtropical paleolat Subtropical Fossils match across continents Angle of magnetic field relative to sedimentary layer a. Estimate angle of magnetic field WRT horizontal b. Angle give lat layers formed c. Parallel to layers equator Parallel III. The Theory of Plate Tectonics III. c. Lithosphere and Asthenosphere III. The Theory of Plate Tectonics III. i. To explain the drift of continents, mobile plates need to be To distinguished from layer below distinguished ii. Mantle and crust are recategorized by material strength 1. Lithosphere- brittle crust 2. Asthenosphere- region of the upper mantle that flows a. Weak sphere b. Flows plastically and deforms easily (rock close to Flows melting point) melting c. Top is coincident with low velocity zone d. Extends to ~700 km a. b. c. d. IV. Plates and Plate Boundaries IV. The lithosphere is divided into about 20 rigid plates Plates can contain both oceanic and continental crust Tectonic activity is concentrated at plate boundaries Divergent margins - Plates more apart Plates i. Constructive margins ii. Mid-ocean ridges and rift valley iii. East African rift iv. Red Sea Red a. a. Convergent Margins -Plates collide -Plates i. Earth is not increasing in size ii. Destructive margins iii. Deep sea trenches and subduction zones a. Ocean- Continent Margins i. ii. iii. iv. Denser ocean plate is subducted This creates a deep sea trench Friction generates seismic activity Forms volcanic arc IV.Ocean Marginsand Plate Boundaries IV. Plates g. Ocean1. 2. 3. 1. 2. 3. 4. Subduction Forms a volcanic island arc Marians islands, Philippine Islands Tall mountain ranges as buoyant crust is forced upward Any ocean crust is subducted India and Asia to form Himalayas Wedges of deformed sediment a. b. Underplated- attached to overlying plate Carried into Asthenosphere i. ii. iii. iv. Dehydration Decarbonization Transform mineral composition Reactions important to global recycling of carbon h. Continent- Continent Margins c. Volcanoes derive CO2 from this sedimentary source i. Transform Margins - Plates move horizontally past one another Plates i. Crust is neither created or destroyed ii. Transform faults 1. 2. San Andreas Fault Connect Mid-ocean ridge segments Lithospheric Plates Lithospheric Distribution of Earthquakes Distribution Types of Plate Boundaries Types Divergent boundaries are located mainly along oceanic ridges mainly Continental Rifting Continental An oceanic-continental convergent plate boundary convergent Distribution of the World’s Oceanic Trenches Oceanic An oceanic-oceanic convergent plate boundary convergent A continental-continental convergent plate boundary convergent The collision of India and Asia produced the Himalayas (before) (before) The collision of India and Asia produced the Himalayas (after) (after) Transform Faults connect spreading centers centers Transform faults connect MidTransform Atlantic Ridge segments a. V. What Drives Plate Tectonics? V. Heat from the deep i. Plate tectonics is the surface expression of the mechanisms by which Plate heat escapes Earth’s interior heat ii. The average geothermal heat flux is 0.06 W/m2 1. Small compared to the net solar flux absorbed 240 w/m2 2. Transported by convection (mantle to base of lithosphere) 2. 3. By conduction through lithosphere 4. Or by convection at mid-ocean ridges iii. Sources of interior heat 1. Radioactive decay 2. Residual heat from earth’s formation 3. Growth of inner core iv. Radioactive decay 1. 2. i. ii. Important elements Continuous Decay Potassium, Uranium, Thorium Half life of 100 of millions to billions of years V. What Drives Plate Tectonics? Tectonics? iv. Radioactive decay 1. Important elements i. ii. iii. iv. Potassium, Uranium, Thorium Half life of 100 of millions to billions of years Found in crust and mantle Generate heat 2. 3. 4. Continuous Decay or loss of material Abundance was grater in past Rate of heat production also greater in past V. What drives plate tectonics? tectonics? a. Heat from the deep iv. Other heat sources 1. heating events during Earth’s formation 2. accretion of clumps of matter for planetesimals 3. planetesimals collided to form a large, primitive 3. planet planet 4. heat was transferred during the collisions and 4. accreation accreation 5. separation of crust, mantle, and core released 5. gravitational energy in the form of heat 6. convection transferred this heat to the surface 6. V. What drives plate tectonics? tectonics? b. Mantle Convection ii. Mantle material is heated locally, . expands, becomes less dense, and rises expands, ii. Cooler, denser material sinks iiii. Material rises to the base of ii. lithosphere lithosphere iiv. Size of the convection cells is v. unknown unknown V. What drives plate tectonics? tectonics? iiv. v. Separate cells in upper and lower mantle a. Change in chemical composition of rocks b. Changes in density 2. Whole mantle a. Change in mineralogy b. Changes occur quickly i. Lithosphere- Asthenosphere interaction 1. Subducting plates drive downward motion 2. Divergent plates drive upwelling 3. Transform faults drive lateral motion 1. 1. Layered Mantle Convection Layered Whole Mantle Convection Whole V. What drives plate tectonics? tectonics? c. Forces acting on plates. c. V. What drives plate tectonics? V. c. Forces acting on plates i. Mantle drag- friction between Asthenosphere and Lithosphere In the past was considered the cause of plate motion ii. Gravitational push or ridge push iii. Slab pull- oceanic slab pulls the rest of the plate into subduction zone iv. Bending resistance- elastic resistance of oceanic plate to being bent Bending into subduction zone into v. Trench suction- tendency for overriding plate to be drawn towards the Trench subduction zone subduction vi. Friction vii. Negative buoyancy Dense oceanic plate sinks viii. Overall plate motion is a result of the balance of forces VI. Plate Tectonics through Earth History Earth a. Evolution of the driving force i. ii. iii. iv. Earth has been losing heat since its formation Earth 4.6 b.y.a. 4.6 Heat loss has occurred via mantle convection Heat during past 4 b.y. during Earth heat loss was several times what it is Earth today today Continents have moved great distances VI. Plate Tectonics through Earth History Earth c. Wilson Cycles ii. A pattern: supercontinent, breaks apart, . continents disperse, then reassemble continents ii. Cycle take ~500 m.y. VI. Plate Tectonics through Earth History VI. ii. Cycle take ~500 m.y. 1. 2. 3. 1. 2. Approximate duration equated to the time it takes a plate to move halfway around the Approximate Earth Earth Typical plate speed 4 cm/year (40 km/my) Half the circumference of Earth is 20,000 km Bound to collide on a finite globe Hypothesis stating the continents are drawn toward colder regions of the Asthenosphere a. Thick supercontinent acts as an insulator b. Slows the release of mantle heat c. Mantle temperature rises under continent d. Modifies pattern of convection e. Resulting surface tension rips continent apart f. Continents move toward colder regions g. Consistent with present day plate speeds and path of seismic waves h. Africa has moved little since the break up of Pangaea i. Still over hot mantle ii. East African rift zone iii. Underlying mantle still retains heat buildup during Pangaea’s existence Underlying existence iii. Why do continents come together? ...
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This note was uploaded on 04/11/2011 for the course EAS 253 taught by Professor Dr.emilyberndt during the Spring '11 term at Saint Louis.

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