Chapter 4 The Way the Earth Works Plate Tectonics.pdf

Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 1 The Way the Earth Works: Plate Tectonics Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Tectonics ! The paradigm of ?How the Earth Works.? " Earth?s outer shell is broken into rigid plates that move. ! A case study of a scientific revolution. udy of ienti ic re lution. " A powerful idea based on multiple lines of evidence. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Tectonics ! Tectonic theory evolved in the 1960s. ! Previous research foundation. " Wegener (1915) ? Evidence supporting continental drift. " Hess / Dietz (1960) ? The sea-floor spreading hypothesis. ! By 1968, evidence for tectonics was overwhelming. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Tectonics ! Provides a unified mechanism explaining: ro " Igneous, sedimentary, and metamorphic rocks. " The distribution of earthquakes and volcanoes. " The origin of continents and ocean basins. " The distribution of fossil plants and animals. " The genesis and destruction of mountain chains. " Continental drift. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Lithosphere ! Tectonic plates are fragments of lithosphere. " Lithosphere is made of both crust and the upper mantle. " The lithosphere is in motion over the asthenosphere. ! Lithosphere bends elastically when loaded. ! Asthenosphere flows when loaded. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Buoyancy ! First described by Archimedes more than 2.2 ka. ! Floating solids displace water equal to their mass. ! An iceberg ic rg ?sinks? until the mass of water it displaces is equal to the total mass of the iceberg. ! This concept applies to lithospheric plates. " Continental ? Floats higher. " Oceanic ? Sinks lower. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 2 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak 2 Types of Lithosphere ! Continental ~ 150 km thick. " Granitic crust. #35-40 km thick. #Lighter (less dense) . #More buoyant ? Floats higher. ! Oceanic ~ 7 to 100 km thick. " Basaltic crust. #7-10 km thick. #Heavier (more dense). #Less buoyant ? Sinks lower. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate BoundariesPlate Boundaries ! Lithosphere is fragmented into ~ 20 tectonic plates (~ 9 major plates). ! Plates move continuously at a rate of 1 to 15 cm/yr. ! Plates interact along their boundaries. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak ! Locations on Earth where tectonic plates meet. " Identified by concentrations of earthquakes, volcanoes. ! Plate interiors have few earthquakes. Plate Boundaries Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Continental Margins ! Where land meets the ocean. " Margins near plate boundaries are ?active.? " Margins far from plate boundaries are ?passive.? ! Passive margin continental crust thins seaward. " Transitions into oceanic crust. " Traps eroded sediment. " Develops into the continental shelf. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Plate Boundaries: 3 Types ! Divergent ? Tectonic plates move apart. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Plate Boundaries: 3 Types ! Convergent ? Tectonic plates move towards one another. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 3 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Boundaries: 3 Types ! Transform ? Tectonic plates slide sideways. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Divergent Boundaries ! Sea-floor spreading progression. " Early stage #Rifting has progressed to mid-ocean ridge formation. #Forms a long, thin ocean basin with young oceanic crust. " Example: The Red Sea Time 1 Note: This diagram only depicts the crust, not the entire lithosphere. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Divergent Boundaries ! Sea-floor spreading progression. " Mid-stage #Ocean begins to widen. #New seafloor is added at the mid-ocean ridge. #Continents move farther apart. " Example: Greenland and the North Atlantic Time 2 Note: This diagram only depicts the crust, not the entire lithosphere. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Divergent Boundaries ! Sea-floor spreading progression. " Late stage #Mature, wide ocean basin.ture, de ocean basin. #Increase in age with distance from central ridge. #Edge of ocean basin - oldest; ridge proximal - youngest. " Example: The Atlantic Ocean Time 3 Note: This diagram only depicts the crust, not the entire lithosphere. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Mid-Ocean Ridges ! Linear mountain ranges in Earth?s ocean basins. ! Example: The Mid-Atlantic Ridge " Snakes N-S through the entire Atlantic Ocean. " Elevated ridge (1,500 km wide) 2 km above abyssal plains. " Axial rift valley. #500 m deep. #10 km wide. #Symmetric. #Site of eruptions. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Mid-Ocean Ridges ! Sea-floor spreading opens the axial rift valley. ! Rising asthenosphere melts, forming mafic magma. ! Pooled magma solidifies into oceanic crustal rock.into oc nic ustal "Pillow basalt ? Magma quenched at the sea floor. "Dikes ? Preserved magma conduits. "Gabbro ? Deeper magma. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 4 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Mid-Ocean Ridges ! ?Black smokers? are found at some MORs. " Water entering fractured rock is heated by magma. " Hot water dissolves minerals and cycles back out of rock. " When water reaches the sea, minerals precipitate quickly. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Ocean Crustal Age ! Oceanic crust spreads away from the ridge axis. " New crust is closer to the ridge; older crust farther away. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak ! Aging ocean crust moves away from heat source "Cooling, increasing in density, and sinking. "Accumulating increasing thicknesses of sediment. Oceanic Lithosphere Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Convergent Boundaries ! Lithospheric plates move toward one another. ! One plate dives back into the mantle (subduction). ! Subduction recycles oceanic lithosphere. " Subduction is balanced by sea-floor spreading. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Subduction ! Old oceanic lithosphere is more dense than mantle. ! Leading edge sinks like an anchor. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Convergent Boundaries ! The subducting plate descends at an average of 45!. " Plate descent is revealed by Wadati-Benioff earthquakes. #Mark frictional contact and mineral transformations. #Earthquakes deepen away from trench. ! Quakes cease below ~ 660 km. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 5 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Fate of Subducted Plates? ! Plate descent continues past the earthquake limit. ! The lower mantle may be a ?plate graveyard.? Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Subduction Features ! Subduction is associated with unique features. " Accretionary prisms. " Volcanic arcs. " Back-arc basins. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Convergent Boundaries ! Accretionary prisms ? Deformed sediment wedges. " Sediments scraped off subducting plates are smeared and welded onto the overriding plates. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Convergent Boundaries ! Volcanic arc ? Volcanic belt on an overriding plate. " The descending plate partially melts at ~ 150 km depth. " Magmas fueling volcanic eruptions. ! Arc type depends on overriding plate. " Continental crust ? Continental arc. " Oceanic ? Island arc. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Convergent Boundaries ! Back arc basin ? A marginal sea behind an arc. " Forms between an island arc and a continent. " Stretches crust behind the arc. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Transform Boundaries ! Lithosphere slides past. " Many transforms offset spreading ridge segments. " Some transforms cut through continental crust. ! Characterized by? " Earthquakes. " Absence of volcanism. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 6 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Oceanic Transforms ! The MOR axis is offset by transform faults. " Offset of linear MOR is geometric necessity on a sphere. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Transform Boundaries ! Oceanic transforms ? Offsets along the MOR. " Older interpretation ? Faulting occurs after MOR forms. " Modern interpretation ? Faulting occurs with the MOR. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Transform Boundaries ! Continental transforms. " Example: The San Andreas Fault. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Triple Junctions ! Places where 3 plate boundaries coincide.re ! Triple junctions migrate and change across time. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Hot Spots ! Volcanic plumes independent of tectonic plates. " Mafic magmas derived from the lower mantle. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Hot Spots ! Hot spot volcanoes create seamounts. " Seamounts age away from originating hot spot. " Age change marks direction of overriding plate motion. Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 7 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Tectonic Boundaries Evolve ! Plate boundaries change over geologic time. ! Oceanic plates. " Created at MOR spreading centers. " Destroyed at subduction zones. ! Continental plates. " Torn apart at rifts. " Joined during collision. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak ! Continental lithosphere can break apart. Contine tal lithos he n br rt. " Lithosphere stretches and thins. " Brittle upper crust faults. " Ductile lower crust flows. " Asthenosphere melts. " Sea-floor spreading. Continental Rifting Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Continental Rifting ! Example: East Africa. " The Arabian plate is rifting from the African plate. " Rifting has progressed to sea-floor spreading in? #The Red Sea #The Gulf of Aden " East African Rift ? On-going rift. #Thinned crust. #Elongate trough. #Volcanoes. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Collision ! Subduction consumes ocean basins. ! Ocean closure ends in continental collision. "Buoyant continental crust resists subduction. "Suture zone (mountains). Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Plate Collision ! Plate tectonic collision may involve? " Two continents. " A continent and an island arc. ! Collision ?sutures? the convergent plate boundary. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak Driving Mechanisms ! Old Model: Plates are dragged atop a convectingcon ng mantle; passive Earth: Portrait of a Planet, 3 rd edition 2008, by Stephen Marshak © W. W. Norton & Company Chapter 4 The Way the Earth Works: PlateTectonics PowerPoint slides prepared by Ronald L. Parker Earlham College, Department of Geosciences Richmond, Indiana parkero@earlham.edu 8 Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Driving Mechanisms ! Modern: 2 other forces drive plate motions. " Ridge-push ? Elevated MOR pushes adjacent lithosphere. " Slab-pull ? Gravity pulls a subducting plate downward. " Convection in the asthenosphere. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Velocities ! Absolute plate velocities may be mapped by? " Plotting plate motion relative to a fixed spot in the mantle. " Measuring volcano ages / distance along a hot spot track. GPS sensor records satellite observations in California Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Plate Velocities ! Plate vectors are determined GPS measurements. " Global Positioning System (GPS) uses satellites. " Knowledge of plate motion is now accurate and precise. Chapter 4: The Way the Earth Works: Plate TectonicsEarth: Portrait of a Planet, 3rd edition, by Stephen Marshak The Dynamic Planet ! Plate tectonics: The key to understanding geology. " Mantle is transferred to the surface and back down again. " The interior and surface of Earth are in constant motion. " Explains earthquakes, volcanoes, and continental drift. wolflor Microsoft PowerPoint - Ch04_Lecture_Earth3.ppt