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Circulation in the Solid Earth & Plate Tectonics Geos 110 Lectures: Earth System Science Chapter 7: Kump et al 3rd ed. Dr. Tark Hamilton, Camosun College Wegener’s Pangea • Late Paleozoic (Permian – Early Jurassic) • Supercontinent closer to S. Pole • Permian glaciations • Jurassic supercontinent desert Geological & Geophysical Evidence for Pangea (Continental Drift & Plate Tectonics) • Widespread evidence for S. Hemisphere glaciation: (S.America, Africa, Antarctica, Australia & Antarctica) = Gondwanaland • Fossil ties between separated continents: Glossopteris – giant seed ferns + Mesosaurus ~ croc • Geological ties: belts of correlative Precambrian through Paleozoic geology: plutons, sediment basins • Fit of Continents – especially at 2000 m isobath • Paleomagnetism, inclinations, apparent polar wander • Now recognized to be caused by seafloor spreading Geophysical Evidence for Earth’s Interior Structure & Composition • Earthquakes occur in Earth’s outermost 700 km of cold, brittle, rigid, lithosphere • For the largest earthquakes, the earth “rings like a bell” and vibrates for days. • Elastic waves reflect from and refract through internal layers with different rigidity & density • Speed of sound in rock varies as a function of temperature, pressure and composition • This is useful to provide internal seismic tomography Seismic Evidence for Earth’s Interior • Big earthquakes occur on pre-existing faults • Elastic wave vibrations spread out, bounce & bend • Arrival times indicate distances, locations & physical properties Seismic P-Waves: Volume Compresses • Elastc waves have different modes of vibration, travel & velocities • Body Waves travel through by restorably deforming it or making it vibrate. 2 kinds: Primary – P waves and Secondary – S waves • Compressional waves (push-pull), the fastest body wave arrives 1st • Speed increases with depth because rocks become stiffer at higher pressure than they become denser denser M = K + 4µ/3 Seismic Shear S-Waves (shape deforms) • Elastc waves have different modes of vibration, travel & velocities • Body Waves travel through by restorably deforming it or making it vibrate. 2 kinds: Primary – P waves and Secondary – S waves • Secondary (Shear) waves side to side or up and down, 0.7 speed of P • Speed increases at a slower rate with depth than P does • Liquids have no shear strength or S waves, outer core shadow zone Seismic Tomographic Imaging of Earth • Transect under Nam • To 2700 km depth = base of Mantle • Dark (blue) = fast speeds or colder • Light (red) = slow speeds or warmer • Image shows the last of the subducted Farallon Plate under Texas Horizontal Motion Seismograph • Anchored to bedrock or pier • The mass stands still while the Earth moves around it • Magnet and pick up coil • Lateral motion versus time is a wave • Now this is done digitally except for temporary field instruments Seismic Damage: 1989 Loma Prieta Quake • October 17, 1989 RM = 6.9, San Andreas Fault biggest since 1906 • Collapsed 2 level highway CF880 Cypress viaduct • 68 dead, >4,000 injured • $7,000,000,000 property damages • To be a damaging earthquake it needs to be large >/= 6.4 and shallow • Each number on the richter scale is X 31.66 more energy release and x 10 higher amplitude of ground motion Seismograms: 1989 Loma Prieta Quake • Thick fill or unconsolidated sediment amplifies ground motion due to surface waves: local geology & proximity both affect amplitude • More ground motion, more & infrastructure building damage Earth’s Composition • Carbonaceous chondrites = most primitive solar system material: silicates, oxygen, rock forming elements, Carbon, water (oldest type of meteorite) • The Crust + Hydrosphere + Atmosphere is enriched in light elements • Mantle & Core are depleted in light elements and enriched in heavier and more refractory (high melting point) elements • While layering formed early in a few hundred Ma, the inner core and crust appear to have been growing throughout Earth history Seismology & Earth’s Interior • Concentric layers by mechanics & composition: Lithosphere: solid, strong, cold, brittle & Quakes • UM-Partial Melt~5, LM-Plastic Solid, OC-Liquid, IC-Solid Seismology & Earth’s Interior • Lithosphere = Crust plus brittle upper Mantle ~100 km thick – Mostly Intermediate density Igneous rocks (cooled from melts), lesser Metamorphic (strained, recrystallized in solid state) & Sedimentary in uppermost few km • Asthenosphere- weak upper mantle below the Moho is ~95% solid ultramafic (olivine) with partial melting and low strength. – Involved in convection & melt generation of basaltic magmas at spreading ridges & under volcanoes – Low velocity zone ~30-80 km is most active Seismology & Earth’s Interior • Lower Mantle = Mesosphere – 660 km discontinuity to ~2900 km – Most of Earths volume. Entirely hot, plastic, solid, ultramafic (Mg-Fe rich silicates) – Insulating, slowly convecting at ~15 cm year • Outer Core – 100% Liquid Fe-Ni, 2900-5150 km – Will not pass shear waves, shadow zone – Convects rapidly generating internal magnetic field • Inner Core – Solid, super high density Fe-Ni – Heterogeneous, E-W, layered, ~1 km scale variation – Growing as it cools, driving convection, spins > 1/day Magnetic Dynamo & Earth’s Interior • Outer Core – 100% Liquid Fe-Ni, 2900-5150 km • Differences in heat, spin & composition drive convection • Flowing conductor creates circular electromagnetic field Plate Tectonics & Cycling the Solid Earth • Ocean Basins are low -3.9 km deep & young <100 Ma • Continents are +800 m asl & old ~4 Ga, high Earth’s Geocentric Dipole Field • Dipole: Tan (I) = 2 Tan ( λ), Surveying by compass • Cannot tell longitude, can tell geomagnetic latitude • Rocks get thermally or chemically magnetized Detecting & Measuring Seafloor Spreading • Mid-Ocean Ridges ~2km deep, hot & normally magnetized • Magnetic Stripes are symmetric and a proxy crustal age Detecting & Measuring Seafloor Spreading • Mid-Ocean Ridges ~2km deep, hot & normally magnetized • Magnetic Stripes are symmetric and a proxy crustal age • Iceland a Hotspot on a Mid ocean ridge Paleogeographic Reconstruction • Paleogeographic Earth Reconstruction, Early Cambrian (540Ma), Dr. Ron Blakley (2010) Paleogeographic Reconstruction • Paleogeographic Earth Reconstruction, Early Devonian (400Ma), Dr. Ron Blakley (2010) Paleogeographic Reconstruction • Paleogeographic Earth Reconstruction, Early Permian (280Ma), Dr. Ron Blakley (2010) Paleogeographic Reconstruction • Paleogeographic Earth Reconstruction, Tertiary – Cretaceous Boundary (65 Ma), Dr. Ron Blakley (2010) • Inset 90 Ma Late Cretaceous Highstand Composition & Behavior of Earth’s Interior • Plate tectonic motions depend on materials strengths & rheology (response to forces, flow) • Continental & Ocean Crust and Mantle are compositional layers • Lithosphere & Asthenosphere are rheological discinctions • The Lithosphere is rigid, solid and buoyant plates on a convecting mostly solid but weak dense, ductile asthenosphere Plate Tectonics & Cycling the Solid Earth • Ocean Crust & Continental Crust both on many plates • 7 large plates & 3-4 smaller ones Plate Tectonics & Cycling the Solid Earth • Most Large Earthquakes > Rm 6 occur on plate boundaries • Little strain beneath or within plates 3 Types of Plate Boundaries • Mid Ocean Ridges & Continental Rifts (divergent) • Subduction zones: ocean crust sinks beneath a more buoyant plate margin (convergent) • Transform Faults connect the other 2 types and segment the ridge system allowing for lateral motion and shear 3 Types of Motions & Faults • Divergent Normal Faults, crust in tension atop a heat bulge in the upper mantle. – New lithosphere is made. • Convergent Thrust or Reverse Faults and crustal thickening plus uplift of overriding plate. – Old ocean lithosphere is recycled • Shear Lateral motion to right or left depending on offset of ridges or subduction zones Rifts Divide Plates: Make New Ocean Lithosphere • Slow ridges are steep and rugged, fast are wide & low • ~1 km3 magma per km ridge per year, quakes < 6.4 Rm Trenches Collect Sediments where Old Ocean Crust Subducts • Megathrust earthquakes > Rm 9.4, volcanic arcs, hydrothermal mineral deposits on upper plate Transform Faults Cut Mid Ocean ridges or Subduction Zones • Megathrust earthquakes < Rm 8.8 on active segments between ridge offsets (QCFault, San Andreas) Continental Rifts in East Africa (since Cretaceous) • Red Sea to Lake Victoria • Cradle of Human Evolution • Hot Springs • Salt Deposits • Atlantic looked like this 200 Ma ago Deep Sea Hydrothermal Vents on Ridges & Arcs • Chemosynthetic Life Web based on S, Fe • Larger animals eat bacteria, tube worms, clams, crabs • Hydrothermal massive sulphide deposits: Au, Ag, Cu, Zn, Pb, Ba • Axial Seamount Juan de Fuca, 21 North EPR, TAG, Galapagos, Kermadec Arc, Caribbean Plate Tectonics: MOR’s, Trenches & Fracture Zones • Most Large Earthquakes > Rm 6 occur on subducting plate boundaries • Ridge Length = Subduction Length & 3X active transform 3 Types of Convergent Plate Boundaries • Ocean Subducts under Continent (Andes, Cascades) • Ocean Subducts under Ocean Crust (Philippine, Indonesia, Aleutians, Antilles, South Scotia) • Continent-Continent Convergence, no Subduction (Himalayas-Urals-Appalachians) Oceanic Continental 7.19mislabelled in text Transform Faults Cut Mid Ocean ridges or Subduction Zones • Sovanco FZ connects Juan de Fuca and Gorda • Mendocino FZ connects Gorda & EPR Sea of Cortez Active 2 Kinds of Continental Margins Passive • Passive margin rifted at beginning of Wilson Cycle • Active Margin subducting, MOR in between Mantle Convection Drives Plate Motion • Whole mantle convection driven from heat in Outer Core, Hotspots require this at least Mantle Convection Drives Plate Motion • 2 Layer Mantle Convection, isolated compositional layers, depleted upper mantle, more irregular Possible Forces Affecting Plate Motion • Gravity sliding from thermal bulge under MOR • Negative buoyancy for old ocean crust at trenches Continental Orogenic Belts & Rock Cycle • Continents are made of older fragments that were once at plate margins • Cycles of accretion, rifting • Uplift, erosion, sedimentation, metamorphism, renewed igneous activity • Oldest rocks ~4Ga seds! Rock Cycle • Rocks and minerals & the lements which comprise them are recycled • Mainly at Convergent margins: buoyancy, thickening, erosion etc. Wilson Cycle (Supercontinent Formation) • • • • • Plates move towards Subduction Zones Continental crust is too thick and buoyant to subduct Continents re-collide and amalgamate every ~500Ma Pangea broke up at 205 Ma forming Atlantic Eventually the Pacific will dissappear (300-400 Ma)