chapter 3- plate tectonics and the ocean floor...2012/09/07 · bathymetry of seafloor: mid-ocean...
TRANSCRIPT
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�Thin rigid plates on surface Earth
�Plates move� Toward each other
� Away from each other
� Slide past each other
�Plate motions create major geological features
Fig. 3-14
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�Alfred Wegener (1921)� Puzzle-like fit of
continents� Edward Bullard
(1960) fit continents at continental slope
Fig. 3-39b
� Matching rock structures and ages across different continents
� Extensions of mountain belts from one continent to the other
Fig. 3-39c
�Glaciated rocks and glacial deposits� Direction of glacial movement
� Glacial ice in what is now temperate regions
�Other climate evidence� Different climates, different latitudes
�Distribution of fossils� Land fossils in separate continents without
“land bridges”
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�Continents cannot “plow” through ocean basin rocks
�Proposed energy source not likely
�Not gravity and tides
�Earth’s magnetic field and paleomagnetism� Magnetic minerals
� Magnetic field at time cooled below Curie point (600oC)
� Magnetic dip (inclination)
�Latitude
�Apparent polar wandering
Fig. 3-7a
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�Normal and reversed�Ages of changes known�Magnetic anomalies across seafloor�Symmetric about axis of mid-ocean ridge�Stripe patterns trend north-south�Henry Hess (1960)
� Bathymetry of seafloor: mid-ocean ridge and deep ocean trenches
� Ocean crust created at mid-ocean ridge
� Ocean crust destroyed at ocean trench
Vine and Matthews (1963)� Magnetic stripes
represent changes in Earth’s magnetic field
�Polarity reversals
�Symmetric pattern caused
by spreading at mid-ocean ridge
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Age of seafloor by magnetic stripes
Fig. 3-12
�Radiometric age dating of sea floor reveals sea floor youngest at mid-ocean ridge, oldest toward continents
�Heat flow is higher at mid-ocean ridge�Most earthquakes occur at plate
boundaries� Different types of earthquakes at different
plate margins
�Chemical composition� Crust: surface to 30 km (average), less
dense, mainly silicates
� Mantle: 30 km to 2900 km, more dense, mainly Fe and Mg silicates
� Core: 2900 km to 6370 km, densest, Fe and Ni
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�Physical properties� Lithosphere: surface to 100 km, rigid
and brittle, cooler temperature, less pressure
� Asthenosphere: 100 km to 700 km, plastic, partially molten; high temperature
� Mesosphere: 700 km to 2900 km, both plastic and rigid behavior; high pressure
� Outer core: liquid� Inner core: rigid
Fig. 3-18
�Gravity� Density of layers
�Seismology� Earthquakes produce seismic waves
�P and S
�Seismic tomography
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Fig. 3-16 a, b
Oceanic crust Continental crustBasalt Granite
Avg thickness 8 km Avg thickness 35 km
�Highly viscous material�Flows slowly�Oceanic crust and continental crust
“float” on asthenosphere� Oceanic denser, floats lower� Continental less dense, floats higher
� Isostatic adjustments
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Fig. 3-10
Fig. 3-20
�Plates move away from each other
�Mid-ocean ridge
�Rift filled with upwelling magma
�Early separation produces narrow, linear sea� Red Sea, e.g.
�Later separation wider ocean
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�Faster spreading rates
�Broad, gentle mid-ocean ridge with less intense earthquakes
�Slower spreading rates
�Narrow, steep mid-ocean ridge with more intense earthquakes
�Plates move towards each other
�Ocean floor destroyed by subduction into mantle at ocean trench
�Earthquake zone may extend from near surface to 670 km
�Ocean plate subducted beneath continental plate
�Continental arc
�Andesite volcanic rocks
�Explosive volcanic activity
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Fig. 3-28b
�Denser oceanic plate subducted beneath other oceanic plate
� Island arc
�Basaltic volcanic rock
�Two continental plates meet
�Neither is subducted
�High mountain ranges
�Deformed rocks
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�Plates slide past each other
�Between segments of mid-ocean ridge
�Oceanic transform faults are perpendicular to mid-ocean ridge
�Continental transform faults cut across continent
� Intraplate volcanism centered on columnar areas of magma
�Volcanoes on sea floor occur in sequence
� Island chains record plate motion�Mantle plumes may extend to mantle-
core boundary�Few occur near mid-ocean ridge
Fig. 3-32
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�Seamounts=conical underwater volcanoes
�Tablemounts=flat-topped underwater volcanoes
�Formation and erosion
�Moved away from mid-ocean ridge
�Sink farther from sea level
�Charles Darwin, coral reefs, and subsiding volcanic islands
�Large calcium carbonate structures
�Fringing reef
�Barrier reef
�Atoll
Fig. 3-34