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Plate Tectonics

Continental drift

Plate Tectonics

Continental Drift and Paleomagnetism Paleomagnetism Renewed interest in continental drift initially came from rock magnetism Magnetized minerals in rocks show the direction to Earth s magnetic poles at the time of rock formation Even as rocks move over geologic time, rock magnetism retains original alignment

Divergence and Magnetic Reversals When Molten Magma solidifies the iron minerals align to the earth s magnetic field. Magentic Reversals Every few 100,000 yrs. Create Pattern unique to age of oceanfloor. Unique pattern combined with sampling can be used to date & investigate oceanfloor structure and age.

Continental Drift and Paleomagnetism Apparent polar wandering Apparent movement of magnetic poles revealed in magnetized rocks indicates continents have moved Shows that Europe was much closer to equator when coal-producing swamps existed Polar wandering curves for North America and Europe have similar paths, but are separated by about 30 of longitude (Differences can be reconciled if continents are placed next to one another)

A Scientific Revolution Begins During 1950s & 60s technology permitted extensive mapping of ocean floor The seafloor spreading hypothesis As seafloor moves away from ocean ridges, newly formed oceanic crust replaces it At deep-ocean trenches older oceanic crust is consumed Geomagnetic reversals Earth's magnetic field periodically reverses polarity Geomagnetic reversals are recorded in the ocean crust In 1963 discovery of magnetic stripes in the ocean crust near ridges was tied to concept of seafloor spreading

A Scientific Revolution Begins Geomagnetic Reversals Magma at ocean ridges is magnetized with the polarity of the existing magnetic field When Earth s magnetic field reverses polarity, fresh oceanic crust records this with alternating stripes of normal (like present) and reversed polarity as seafloor spreads away from ridges The last piece of the puzzle Transform faults were discovered by J. Tuzo Wilson in the mid 1960s These faults connect the ocean ridge system that partly divides Earth s lithosphere into rigid plates Wilson was among the first to propose the theory of plate tectonics

Plate Tectonics

Plate Tectonics: The New Paradigm Much more encompassing theory than continental drift The composite of a variety of ideas that explain the observed motion of Earth s lithosphere through the mechanisms of subduction and seafloor spreading Became the basis for viewing most geologic processes Earth s major plates Associated with Earth's strong, rigid outer layer, the lithosphere Overlies a weaker region in the mantle called the asthenosphere Plates continually change shape and size Several plates include an entire continent plus a large area of seafloor All major interactions among individual plates occur along their boundaries

Testing the Plate Tectonics Model Plate tectonics and earthquakes Plate tectonics model accounts for the global distribution of earthquakes Deep-focus earthquakes are closely associated with subduction zones Absence of deep-focus earthquakes in oceanic ridges is consistent with plate tectonics theory The pattern of earthquakes along a trench provides a method for tracking the plate's descent

Testing the Plate Tectonics Model Evidence from ocean drilling Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment Age of sediment increases away from ridges Thickness of ocean-floor sediments increases away from ridges

Testing the Plate Tectonics Model Hot spots and mantle plumes Hot spots are caused by rising plumes of mantle material Volcanoes can form over them (Hawaiian Island chain) Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary

Hot Spots

Driving Mechanisms Initially thought that convection drove plates

The Driving Mechanism No one driving mechanism accounts for all major facets of plate tectonics Researchers agree that convective flow in the rocky 2900 kilometre-thick mantle is the driving force of plate movement Several mechanisms generate forces that contribute to plate motion; driven by the unequal distribution of heat in the mantle Slab-pull and ridge-push Slab-pull: relatively cool and dense oceanic crust sinks into the asthenosphere and pulls the trailing lithosphere with it Ridge-push (less important) results from oceanic lithosphere sliding down the flanks of ocean ridges under gravity

The Driving Mechanism Models of plate-mantle convection Any model describing mantle convection must be consistent with observed physical and chemical properties of the mantle Models Convective layering at 660 kilometres Whole-mantle convection Deep-layer model

Transform, Divergent, Convergent, Plate boundaries

Plate Tectonics Divergent, Convergent, Transform ~12 large rigid lithospheric plates + several smaller ones Move over weak ductile asthenosphere No deformation except at edges!

Divergent Plate Boundaries Seafloor plate separation Continental plate separation ~5 cm/yr separation Mid-Ocean Ridges Mid-Atlantic East Pacific Mid-Indian Iceland Ridge Continental Rifts East African Rift Rio Grande Baikal Rift Rhine Valley

Divergent Plate Boundaries

Convergent Plate Boundaries Ocean-Continent Accretionary wedge e.g. W. South America Ocean-Ocean Volcanic island arcs e.g. Japan, Marianas Continent-Continent Crust shortens or thickens e.g. Himalayas

Convergent Plate Boundaries

Transform Plate Boundaries One major fault or series of faults Shallow quakes They migrate and move terranes from one area to another Faults can connect ridge to trench or trench to trench

Transform Plate Boundaries

Measuring Plate Motion Possible with modern technology to directly measure relative motion between plates

Plate Tectonics It is the unifying theory of Earth Sciences (the current paradigm) it is a testable model, and so far tests of it have provided support for the general model (although in many cases causing revisions of specific elements) which is not the same as saying it is without problems, is being constantly revised at some scale, and could even be replaced by a new theory at some point

Cambrian 543-490 Ma

Ordovician 490-443 Ma

Ordovician 490-443 Ma Erosion of emerging Taconic Mountains to the NE Muds deposited in shallow tropical seas 420Ma

Silurian 443-417 Ma

Silurian 443-417 Ma Climate in Ohio was tropical similar to presentday Bahamas Silurian sea was largely mud-free Large reefs separated shallower waters across much of Ohio from deepter waters in the Michigan Basin to N and Appalachian Basin to E

Devonian 417-354 Ma

Devonian 417-354 Ma Early Devonian Shallow tropical seas cover Ohio Thick deposits of limestones on sea floor End of Devonian Erosion of the rising Appalachian Mountains to the E Muds deposited into oxygen-poor sea

Carboniferous 354-290 Ma

Carboniferous 354-290 Ma Early Carboniferous (Mississippian) Extensive marine deposits of muds and silts from eroding Appalachians to the east Late Carboniferous (Pennsylvanian) Development of large deltaic systems, ie nearshore marine to terrestrial conditions and only rarely oceanic conditions Coal-forming swamps present in many parts of E USA 345 Ma 325 Ma

Carboniferous 354-290 Ma Large parts of western North America were covered by water Terranes of various sizes were drifting towards the westcoast Carbonate sediments were deposited in shallow sea Eventually added to western North America, forming the Rocky Mountains 310 Ma

Permian 290-248 Ma

Triassic 248-206 Ma

Jurassic 206-144 Ma

Jurassic 206-144 Ma Atlantic Opens as Gondwana and Laurasia separate; ie Pangaea breaks up Starts from the north 195 Ma 180 Ma 150 Ma

Cretaceous 144-65 Ma

Cretaceous 144-65 Ma Western Interior Seaway Early to mid-cretaceous Arctic Ocean transgresses onto the continent and is joined by water from Gulf of Mexico from South separating the continent in half Shallow sea with abundant wildlife 115 Ma 75 Ma 65 Ma

Tertiary 65-2.6 Ma Oceans of today take shape Large parts of Eurasia are flooded Ocean separates North from South America Himalayas form as Tethys disappears Antartica becomes isolated as first Australia then South America drift north ---> Southern Ocean

Tertiary 65-2.6 Ma North & South America connect as subduction to the west forms volcanic islands; leads to island chain First islands above sealevel about 15 Ma Isthmus of Panama closes around 3 Ma changing sea circulation as until then Atlantic and Pacific exchanged water freely Greenland still connected to NA 8 Ma

Quaternary 2.6 Ma - present Oceans of today Caribbean forms as North & South America connect and subduction takes place at eastern edge Due to massive icesheets, sea-level is much lower for much of the Quaternary by 100-150 m exposing the shelves

The future + 50 Ma

The future + 150 Ma

The future + 250 Ma

Crustal deformation

Stress: Directed force/pressure

Divergent Plate Boundaries Constructive plate boundaries plates moving apart molten rock injected adding new rock to the trailing edge of the plates and creating ridges, mostly on the floors of ocean basins

Course Information

Rift valleys of North America Gulf of California - on-going

Convergent Plate Boundaries Destructive plate boundaries collision or, more commonly, subduction of denser oceanic plates below margins of less dense continental plates, creating oceanic trenches and mountain belts on the continental margins

Transform Plate Boundaries Conservative plate boundaries transform faults where plates slide past one another (e.g. San Andreas fault) Generally conserves or preserves landmass

Mapping Geological Structures Strike and dip Strike (bearing) compass direction of line produced by intersection of inclined rock layer or fault with horizontal plane Generally expressed as an angle clockwise from North Dip (inclination) angle of inclination of surface of rock unit or fault, as measured from horizontal plane Includes both inclination & direction toward which rock is inclined

Mapping Geological Structures