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Transcript
The Sea Floor Marine Biology Chapter 2 A portion of the midAtlantic ridge about the sea surface in Iceland Water Planet • Oceans cover 71% of the globe – 61% of the Northern hemisphere – 80% of the Southern hemisphere • Regulates the climate and atmosphere Fig. 2.1 Ocean Basins • Classified traditionally into four large basins: –Atlantic Ocean –Pacific Ocean –Indian Ocean –Arctic Ocean Tab. 2.1 Which ocean is the largest and deepest? Which two are of average depth? Which ocean is smallest and shallowest? World Ocean • All four basins are connected • The interconnected system is referred to as the world ocean Southern Ocean • Oceanographers refer to the continuous body of water that surrounds Antarctica as the Southern Ocean. Fig. 2.2 Where did the Earth come from? • The Big Bang caused a cloud of dust to form. (13.7BYA) • The dust cloud forms into the Earth and the solar system (4.5BYA) • The universe began with a massive expansion • It is continuing to expand http://www.charliewagner.net/big.htm http://www.charliewagner.net/big.htm • Any particle with mass has an attraction for any other particle with mass. • Small pieces collide and clump into larger pieces. • Larger pieces clump together, forming rocks, meteors, and eventually planets or stars. DENSITY • So much heat was generated as the Earth formed that it was probably molten. • Matter settled within the planet based on density. • Density is the mass of a substance divided by its volume. Density density = mass volume www.johnniemoore.com/blog/im ages/icecubes.jpg www.ramdh When two substances are mixed, the denser material sinks and the less dense material floats. www.ramdhanyk.com Fig. 2.3 Earth’s Internal Structure • Core is mostly iron – Pressure a million times that at the surface – 4000oC (7,200oF) – Solid inner core, liquid outer core – Swirling motion in the outer core produces the earth’s magnetic field Mantle and Crust • Mantle solid but nearly at the melting point of rock – – – – Mantle slowly flows like a liquid Upper and lower layers Uppermost part is sold Outermost portion of mantle and crust form the lithosphere – Mantle layer just under lithosphere is the asthenosphere (relatively fluid) • Crust is the outermost layer – Relatively thin Tab. 2.2 The nature of the rock at any area in the crust determines the elevation and whether it is covered with water. Ocean and Continental Crusts • Both crusts are less dense than underlying mantle • Continents float on the mantle like icebergs • Ocean crusts float on the mantle, but not as high (why continents are dry) • Oceanic crust is much thinner than continental crust. Lithosphere and Aesthenosphere • Lithosphere includes the crust and the upper rigid mantle • Aesthenosphere – 100-250 km deep – Highly viscous, flexible – flows Continental drift • Alfred Wegener - geophysicist 1912 • Wegner’s hypothesis – All continents once joined together in a supercontinent – Pangaea – Began breaking up 180 million years ago – Did not explain how continents moved Evidence of Plate Tectonics • Continents on opposite sides of Atlantic fit together like puzzle pieces • Coal deposits and geologic formations on opposite sides of Atlantic match up • Fossils match • Mid-Ocean Ridges Fig. 2.4 Fossil evidence of supercontinent orange indicates the fossil remains of Cynognathus, a Triassic land reptile. Dark blue indicates fossil remains of the freshwater reptile Mesosaurus. Green indicates fossils of the fern Glossopteris, found in all of the southern continents. Brown indicates fossil evidence of the Triassic land reptile Lystrosaurus. (Map courtesy United States Geological Survey) Mid-Ocean Ridges • Discovered by sonar after World War II • A continuous chain of volcanic mountains • The world’s largest geological feature Fig. 2.6 Fig. 2.5 Deep depressions in the seafloor are called trenches. Fig. 2.7 Trenches and ridges are geologically active. Mid-Ocean Ridges • Associated with earthquakes and volcanoes • Sediments are thicker farther from ridge • rock older farther from the ridge • Bands of rock alternating between normal and reversed magnetism – Parallel to the ridge Fig. 2.8 Magnetic Anomalies Particles in molten rock act like tiny compasses. Field reversals occur naturally over hundreds of thousands of years. Fig. 2.9 Sea floor spreading and magnetic fields Fig. 2.10 The lithosphere is broken up into plates. Plates can contain oceanic crust, continental crust, or both. Lithospheric plates float on the aesthenosphere. Fig. 2.11 Some trenches are formed by the collision of oceanic and continental crusts. One plate slips below the other. Subduction • Plate descends into the mantle • Heat and pressure breaks up plate – causes earthquakes • More heat melts the plate • Some molten material rises to form volcanoes • Denser oceanic crust sinks – Rock recycled – Very old rocks are always continental Fig. 2.12 When two oceanic plates collide, one dips below the other Trenches curve due to earth’s spherical shape. Creates island arcs Aleutian islands Mariana islands Fig. 2.13 Mt. Veniaminof, part of Aleutian Island Chain Three types of plate boundaries • Mid-Ocean Ridges • Trenches • Shear boundaries – Two plates move past each other – Intense friction – Sometimes “locks”, builds pressure, then slips = EARTHQUAKE!! Fig. 2.14 San Andreas Fault is the largest example of a shear boundary. Fig. 2.15 Driving forces of tectonic plate movement: primarily “slab pull” and partially convection currents. Slab pull: cooling lithosphere gets denser, slips into mantle, pulling the rest of the plate behind it. Causes separation at mid-ocean ridge. Fig. 2.16 Geologic history of the Earth. Sediments The seafloor is carpeted with muddy sediment. Two most abundant types of sediment: Lithogenous made from rock physical and chemical breakdown Biogenous made from living things skeletons of diatoms, foraminiferans radiolarians and coccolithophorids Fig. 2.17 Fossil shell of a foraminiferan, used as an indicator of ocean temperature in the past. Types of Biogenous Sediment • Siliceous ooze – Made of silica (SiO2) – Glasslike • Calcareous ooze – Made of calcium carbonate (CaCO3) Fig. 2.18 Using the magnesium/calcium ratio of foraminiferans in core samples, scientists determine average sea surface temperatures. White and blue lines indicate glacial and interglacial periods. Fig. 2.19 The Continental Margin Continental Margins Part 1 • Continental shelf – Shallowest part of the margin – 8% of the ocean surface area – Composed of continental crust – 1 km – 750km wide • Shelf break – Slope abruptly gets steeper – Starts at 120-400m deep Continental margins Part 2 • Continental slope – Exact edge of the continent – Submarine canyons channel sediment to deep-sea floor. • Continental rise – Sediment moving through submarine canyon accumulates • Deep-sea fan Abyssal plain • • • • • Deep sea floor Oceanic crust Relatively flat Abyssal hills Seamounts – Submarine volcanoes – Guyots (gee-oh’s) are flat-topped seamounts Fig. 2.20 The continental shelf off of Atlantic City Scours are scars made by icebergs during the last iceage. Fig. 2.21 Multibeam sonar image of California continental margin. Arrow indicates Monterey Canyon Active and Passive Margins • Depends on plate tectonics • Active margins – Continental/Oceanic crust margins form trenches – Highly active geologically • Earthquakes, volcanoes • Passive margins – Continental edge is not a plate boundary Fig. 2.22 Fig. 2.23 Steep, rocky shorelin es are typical of active margins. California Coast at Monterey Fig. 2.24 A broad coastal plain with wide shelf, gentle slope indicates a passive margin (Atlantic Coast) Fig. 2.25 Plates pulling apart at mid-ocean ridge. In the center of the mid-ocean ridge is the central rift valley Central rift valley and Hydrothermal Vents • Cracks and crevices in floor and sides of valley • Sea water seeps down cracks • Sea water heated by hot mantle material • Heated water forces back up through crust as hydrothermal vents – Deep-sea hot springs Hydrothermal Vents • Discovered in 1977 • Warm water – 10-20oC (50-68oF) – Some 350oC (660oF) • Dissolved minerals – Sulfides – Rapidly cool and solidify – “black smokers” • Biologically rich Fig. 2.26 Fig. 2.27 Formation of the Hawaiian Islands • The Hawaiian Ridge Chain • Connected to the Emperor Seamount chain • Volcanoes progressively older northwest along the ridges • Newest island: Loihi – Not yet above the surface Formation of the Hawaiian Islands • Hot spot – Plume of hot magma rises from deep in the mantle – Pacific plate moves to north and east above the mantle. – New volcano forms above current location • The “debate” – Skeptics say volcanoes occur because of stress or weakness in tectonic plate causing cracking. 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