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105_09-seafloor_part_1 - Introduction The Sea Floor Oceans...

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Unformatted text preview: Introduction The Sea Floor Oceans Oceans – Oceans and seas cover over 70% of Earth’s surface – Ocean basins are topographically lower, compositionally simple, and relatively younger than continents – Difficult to study Chapter 9 How are the Oceans Explored? Drilling, Drilling, dredging and coring to collect rock and sediment samples How are the Oceans Explored? Clamshell Clamshell samplers How are the Oceans Explored? Piston Piston corers – Piston cores up to 50 m long 1 Deep Sea Drilling Project (DSDP) Deep Sea Drilling Project (DSDP) 15-year program (1968-1983) 15(1968Operated Operated in waters up to 6,000 m deep Drilled Drilled >1,000 holes Glomar Challenger, 1968-1983 1968- 2 How are the Oceans Explored? Remote Remote controlled or manned submersibles allow detailed observation and sampling Submersible Alvin Alvin, taking a dive! 3 Shinkai 6500 (Dive 351): manipulator arm and push sampler On the summit of South Chamorrow Seamount/West Pacific Shinkai - Japanese submersible used for sea-floor studies. Echo sounder Multibeam sonar Oceanographic Research 1920 1920 technological breakthrough: – Echo sounder, known as sonar (sound navigation and ranging) – Transmit sound waves towards ocean bottom – receive reflected sound waves – Measure the time it takes for the signal to travel to the ocean floor and back – Velocity of sound waves in water: 1,500 m/s Oceanographic Research Multibeam Multibeam sonar – Array of sound sources and listening devices – Obtains a profile of a narrow strip of seafloor every few seconds – Profiles are recorded as vessel advances, building a continuous swath of relatively detailed coverage Oceanographic Research Echo Echo sounding, multibeam sonar and seismic profiling 4 Oceanographic Research Seismic Seismic profiling Seismic shooting from the Atlantis, 1940s Seismic shooting from the Atlantis, 1940s Early seismic profile Watergun ready to be deployed Oceanic Crust - Its Structure and Composition Formed Formed at spreading ridges, generally basalts and gabbro Oceanic Crust Pillow Pillow basalts – Lava erupts under water at spreading ridges Gabbro Gabbro – Plutonic version of the same magma 5 Oceanic Crust - Its Structure and Composition Ophiolites: Ophiolites: – Oceanic crust found on continents – Nature of oceanic crust was known well before actual observation by drilling Excellent Excellent example: – Oman ophiolite Gabbro Oman ophiolite: Layered Gabbro Mantle Oman ophiolite: Gabbro and dikes Oman ophiolite: Pillow basalts 6 Topography of the Oceans General General topography of seafloor established by sonar Continental Margins Area Area around continents where granitic continental crust changes to basaltic oceanic crust Types of Continental Margins Active Active margins: narrow, descend into a trench – develop at the leading edge of a continental plate, plate motion Passive Passive margins: wider, built up from sedimentation – found within plates rather than at a boundary Continental Shelf Continental Continental crust Gently sloping between the shore and the continental slope Gently Width Width of shelf varies greatly (up to 1000 km wide) 7 ...
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