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The Sea Floor Physical Geology, Chapter 18 Origin of the Ocean Methods of Studying the Sea Floor Continental Shelves & Continental Slopes Submarine Canyons Passive Continental Margins Active Continental Margins Mid-Oceanic Ridges form? Fracture Zones Seamounts, Guyots & Aseismic Ridges Reefs Sediments of the Sea Floor Oceanic Crust & Ophiolites The Age of the Sea Floor The Sea Floor & Plate Tectonics Origin of the Ocean • Early formation ~ 4 b.y. • Degassing of Earth’s interior • Comet impacts Methods of Studying the Sea Floor • Rock samples – Rock Dredge – Corer – Sea Floor Drilling • • • • • • • • Submersibles Echo Sounder Multibeam Sonar Sidescan Sonar Seismic Reflection Profiler Magnetic Surveys Gravity Surveys Seismic Refraction Surveys Features of the Sea Floor • Passive continental margin (Fig. 18.5) – Continental shelf – Continental slope – Continental rise • Abyssal Plain • Active continental margin (Fig. 18.5) – Continental shelf – Continental slope – Trench • • • • • Mid-Oceanic Ridge Seamounts Fracture Zones Submarine canyons Aseismic ridges Continental Shelves & Continental Slopes • Continental shelf: shallow, submarine platform, 0.1º seaward dip • Continental slope: 4-5º steep slope from a depth of 100-200 m at edge of continental shelf Submarine Canyons Submarine canyons: V-shaped valleys that run across continental shelves & slopes Abyssal fans: fan-shaped deposits of sediment at base of submarine canyons Turbidity currents: masses of sediment-laden water pulled downhill by gravity Passive Continental Margins • Passive continental margin: – Continental shelf, continental slope, and continental rise. – Extends to abyssal plain at 5 km depth. • Continental rise: at base of continental slope, wedge of sediment from continental slope to deep-sea floor, slopes 0.5º. Passive Continental Margins Continental Rise: Types of deposition • Turbidity currents – flowing down slope • Contour currents – flowing along slope Passive Continental Margins Abyssal Plains • Abyssal plains: – Very flat regions at base of continental rise. – Composed of horizontal sediment layers probably deposited by turbidity currents. – Flattest features on the Earth. Active Continental Margins • Active continental margin: – Earthquakes, young mountain belt, and volcanoes. – Consists of continental shelf & slope, and oceanic trench. – Lacks continental rise and abyssal plain. – Associated with convergent plate boundaries. Active Continental Margins Oceanic Trenches • Oceanic trench: – Narrow, deep trough parallel to edge of continent or island arc – Continental slope steepens to 10-15º – Benioff seismic zone – Volcanoes landward – Low heat flow – Negative gravity anomaly Mid-Oceanic Ridges • Mid-oceanic ridge: – – – – – Undersea mountain range Basalt 80,000 km long 1500-2500 km wide 2-3 km above ocean floor • Rift Valley: – – – – – Crust extension Along ridge crest 1-2 km deep Several km wide Present in Atlantic & Indian Ocean, absent in Pacific Ocean Mid-Oceanic Ridges Geologic & Biologic Activity at the Ridges • Geologic Activity at the Ridges – Shallow focus (0-20 km) earthquakes – High heat flow decreasing away from the ridge – Basaltic eruptions – Hot springs (Black smokers) • Biologic Activity at the Ridges – – – – – – Mussels Crabs Starfish Giant white clams Giant tube worms Thermophilic bacteria Europe India North America • Fracture zones: – Major lines of weakness of the Earth’s crust – Cross MOR at right angles – Rift valley is offset – May extend onto continents South America • Seamounts: – Conical undersea mountains that rise 1000 m above the sea floor – Some are islands – 10,000 in W. Pacific Seamounts, Guyots, & Aseismic Ridges • Guyots: Flattopped seamounts • Aseismic ridges: Submarine ridges not associated with earthquakes. Reefs • Reefs: – Wave resistant ridges of coral, algae, & other calcareous organisms – Warm, shallow, sunlit, clean water – Reef types • Fringing reefs: Flat table-like, attached directly to shore • Barrier reefs: parallel to shore, detached by lagoons • Atolls: Circular reefs rimming lagoons, surrounded by deep water Sediments of the Sea Floor • Basaltic oceanic crust • Terrigenous sediment: – Land-derived sediment. – Turbidity & contour currents • Pelagic sediment: – Fine-grained clay & skeletons of microscopic organisms. – Absent on ridge crests. Oceanic Crust & Ophiolites • Oceanic crust is thinner (7 km) and denser (3.3 g/cm3) than continental crust. • Layer 1: Marine sediment (variable thickness & composition). • Layer 2: 1.5 km, pillow basalts overlaying basalt dikes (closely spaced, parallel, vertical). • Layer 3: 5 km sill-like gabbros. • Ophiolite: Slivers of oceanic crust emplaced on land represented by distinctive rock sequences Oceanic Crust & Ophiolites • Pillow basalt from a northern California ophiolite The Age of the Sea Floor & The Sea Floor and Plate Tectonics • The age of the sea floor – Younger than 200 m.y. • The sea floor & plate tectonics – Origin of most sea floor features related to plate tectonics (Chapter 19) End of Chapter 18…