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Transcript
Oceanography CRN # 10053 Lecture 5a Debbie Reynolds Lecture Certification: 1 Lectures 1 a) b) Properties of sea water, ocean physics, ocean basins, explorers Meteorology, heat balance, geostrophic circulation Lectures 2 a) b) Surface ocean winds, currents, temperature and salinity Deep circulation, instruments Lectures 3 a) b) El Niño Climate change in the oceans, including sea level rise Lectures 4 a) b) Wind waves, rogue waves and tsunami Tides Lectures 5 a) b) Sea-floor spreading, continental drift − Plate Tectonics Ocean sediments, coasts and estuaries Lectures 6 a) b) Food web, plankton, invertebrate animals Vertebrate animals, communities tidal to abyssal 2 Introduction • • • • • Ocean crust is denser than the continental crust Ocean crust is much younger than the continental crust Thermal convection from the mantel drives continental drift and sea floor spreading − Plate Tectonics New ocean crust formed in rifts (for example the Mid-Atlantic Ridge) as the sea floor is pulled apart − Sea floor spreading Old ocean crust is forced down and under the continental crust to balance the spreading − Subduction 3 Global Bathymetry ● Abyssal Plains ● Trenches ● Ridges ● Continental Margins (Shelves & Rises) 4 Mariana Trench • • • • • Discovered by Challenger Expedition (1872-76) Deepest part of the world's oceans In western Pacific east of the Mariana Islands Long and narrow Reaches a maximum depth of 10,900 m (35,800 ft or 6.78 mi) 5 Earth Structure • • • In 1692 Edmund Halley thought the Earth consisting of a hollow shells Two inner concentric shells around an innermost core The shells and the core corresponded to the diameters of the planets: Mercury, Mars and Venus 6 Earth Structure 60 mi 120 mi Core Temperature 5,500°C 9,900°F 7 60 mi 120 mi Lithosphere Lithos – rock Asthenosphere Asthenes - weak 8 Earth Structure • Seismic Waves • P−wave (Primary) Compression waves • S−wave (Secondary) Transverse wave Can’t travel through liquid Slower than P−wave • Speed of wave affected by density 9 Earth Density Density (g/cc) Continental Crust (granite) ~ 2.7 g/cc Oceanic Crust (basalt) ~ 2.9 g/cc Water = 1.0 g/cc Iron = 7.8 g/cc Nickel = 8.9 g/cc • • • Earth model Density decreases from center Density jumps at layer boundaries Moho (1909) − named for Andrija Mohorovičić Radius (km) 10 Erosion & Isostatic Balance Isostatic → Gravitation equilibrium Half Dome 11 Alfred Wegener • • • • German Meteorologist Father of “Continental Drift” theory 1912: Theory rejected at symposium of the American Association of Petroleum Geologists 1915: Book “The Origins of Continents and Oceans” (1880 –1930) Winter in Greenland 1912-13 12 Continental Drift • Wegener noticed • • How continents fit together How geological and fossil features matched He could not explain how the continents could move All continents once part of super continent: “Pangaea” Fit of continents also noted by Leonardo da Vinci (1500s) & Francis Bacon (1620s) 13 Sea Floor Spreading • • In 1957 a map by Heezen and Thorp showed symmetry in Mid Atlantic Ridge In 1960 geologists Harry Hess and Robert Diaz speculated that new sea floor was being created at the mid-Atlantic Ridge and destroyed elsewhere 14 Magnetic Evidence • Earth’s magnetic poles irregularly reverse ~100,000 years • Magnetite orients its own magnetic field with the earth when a fluid and than holds that orientation once cooled • Magnetometers towed behind ships can measure the local magnetic changes due to magnetite 15 Magnetic Evidence Vine and Mathews (1963) 16 Radioactive Decay • • • Parent atoms radioactively decay to daughter atoms Decay rate is defined by the half life; the time it takes half of the parent atoms to decay Begin with 100 parent atoms; at the end of 1 half life, 50 parent atoms have decayed to 50 daughter atoms 17 Dating Evidence Using radioactive dating: Age of Earth 4.5 billion years Oldest rocks ~ 4 billion years Found in the Canadian Shield and in Australia, Africa Without radioactive heating Earth’s core would be cold • Uranium-238 (U238) decays with a half life of 4.5 billion years • The final product is lead-206 (Pb206), Half life 1017 years • There are 18 intermediate daughter products on path from uranium to lead with much shorter half lives • Measuring the ratio of U238 / Pb206 can determine the age 18 Dating Evidence Age of Ocean Basin in Millions of Years 19 20 Deep Sea Vent • • • • Ocean water circulates from ocean into sea floor and back out at vent Water becomes heated to ~350°C (660°F) Water picks up chemicals along path through rock Chemicals react with each other and some may be precipitated 21 Black & White Smokers 22 Mid-Atlantic Ridge: Iceland 23 Earthquake Evidence Pacific Ring of Fire ? 24 Plate Tectonics • Tectonics: from Latin “tectonicus” − pertaining to building Powered by heat from radioactive decay • Lithosphere: Crust plus rigid uppermost part of mantel • Asthenosphere and lower mantel are plastic • Lithosphere ‘floats’ on top of Asthenosphere Isostatic balance due to density differences Isostatic balance − gravitational equilibrium between the lithosphere and asthenosphere • Ocean Crust is more dense than Continental Crust • Boundary Interactions Divergent plate boundaries Convergent plate boundaries (determined by density) Transverse plate boundaries (strike-slip fault) 25 Convergent Plate Boundaries • Oceanic crust slips below the Continental crust Subduction zone Example: West Coast of South America 26 Convergent Plate Boundaries • Continental crusts collide Example: Himalayas • Oceanic crusts collide Example: Japan 27 Earth Convection Drives Plate Tectonics 28 Strike-Slip Fault • Plates slide by each other; side by side Example: San Andreas Fault, California 29 North Eastern Pacific 30 Hot Spots • Hot Spots: Volcanic regions underlying mantle that are anomalously hot • Tectonic Plates slide above the hot spots Example: Hawaii Lo'ihi: will break the ocean surface in 10,000 to 100,000 years 31 Hot Spot Emperor Seamounts: 70 − 30 million years 32 Development of Atoll Charles Darwin 33 34 Submarine Eruption 1. 2. 3. 4. 5. 6. 7. 8. Water vapor cloud Water Stratum Lava flow Magma conduit Magma chamber Dike Pillow lava 35 Davidson Seamount • Located off the coast of Central California, 80 mi (129 km) southwest of Monterey • 26 mi (42 km) long and 8 mi (13 km) wide • One of the largest seamounts • 7,480 ft (2,280 m) tall • Summit 4,101 ft (1,250 m) below the sea surface • Biologically diverse, with 237 species and 27 types of deep-sea coral 36 37 Transform Fault Transform faults occur because sea floor spreading cannot occur uniformly on a spherical earth 38 Major Tectonic Plates (~10) 39 Motion of Tectonic Plates • Maximum rates of spreading ~ 10 cm/yr (4 in/yr) • Faster spreading rates at the East Pacific Rise compare to the Mid-Atlantic Ridge • Pressure from India Plate to increase heights of Himalayas 40 Plate Tectonics Summary 41 Summary • • • • • Ocean crust is denser than the continental crust Ocean crust is much younger (less than 280 million years old) than the continental crust (up to 4 billion years old) Thermal convection from the mantel drives continental drift and sea floor spreading − Plate Tectonics New ocean crust formed in rifts (for example the Mid-Atlantic Ridge) as the sea floor is pulled apart − Sea floor spreading Old ocean crust is forced down and under the continental crust to balance the spreading − Subduction 42