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Oceans GLY 2010 – Summer 2015 Lecture 16 1 Voyage of H.M.S. Challenger • Route sailed by Challenger from 1872 - 1876 2 Bathymetry 3 Use of Sonar 4 Seismic Reflection Profile 5 North Atlantic Ocean 6 Passive Continental Margins • Not located along an active plate boundary • Little or no seismic activity and volcanism • Found mainly in the Atlantic and most of the Indian oceans • Weathering and erosion of continental material produces a wide, thick deposit of undisturbed sediments • The coast of Florida is a passive margin 7 Passive Margin Features • Continental Shelf • Continental Slope • Continental rise 8 Continental Shelf • Gently sloping regions adjacent to continents – about 0.1% on average • Built by transport of sediment from the continents to the ocean Much of this material is felsic, and adds to the offshore region of the continent 9 Continental Shelf • Edge of the continental shelf, at about 130 meters depth (average), is in many ways the true edge of the continent • Continental shelves are much wider on passive margins (Florida) than active margins (Washington, Oregon), and may be non-existent on some margins 10 Importance of Continental Shelves • 7.5% of ocean surface, but contain much of the wealth of the oceans, including petroleum, natural gas, mineral resources, and huge sand and gravel deposits • Contain important fishing resources, although these are largely over-exploited and in danger of failure 11 Geology of Continental Shelves • Contain many glacial deposits, from ice ages when sea-level was lower • Submarine valleys are often seaward extensions of river valleys on the continent 12 Continental Slope • Steeply sloping region connecting the shelf and the deep ocean 13 Features of the Continental Slope • Averages about 20 kilometers in width Average slope is 5° (50x continental shelf), but reaches 25° in some places Marks boundary between continental crust and oceanic crust 14 Continental Rise • The area between the slope and the deep ocean floor, where the slope is much less 15 Geology of Continental Rise • Slope is about 1/3 degree • Thick deposits of mud, delivered by turbidity currents • When currents emerge from a canyon mouth, a deep-sea fan is formed 16 Active Continental Margins • Occur where oceanic lithosphere is subducted beneath a continental edge • Margin is narrow, with a veneer of highly deformed sediments • Parallel deep-ocean trenches around the circum-Pacific margin, and along Sumatra in the Indian Ocean • Volcanoes and SID earthquakes are common 17 Geology of an Active Margin • Ocean-floor sediments are mixed with oceanic crust and scrapped from the descending plate • Produces a chaotic mixture called an accretionary wedge • Prolonged subduction leads to a large accretionary wedge, for example of Honshu Island in Japan 18 Old and Cold • When the oceanic plate is old and cold, subduction angle is steep, and no accretionary wedge is formed – all sediments are subducted19 Deep Ocean Basin • Region between the continental margin and an ocean ridge • About 30% of the surface area of the earth, comparable to the continents 20 Features of the Deep Ocean Basin • • • • Abyssal Plains Deep-ocean trenches Oceanic plateaus Seamounts and guyots 21 Abyssal Plains • Among the flattest places on the planet • Deep accumulations of sediment bury everything except high volcanic peaks • Comes from a (without) & byssus (bottom) 22 Deep-Ocean Trenches 23 Ocean Trenches 24 Puerto Rico Trench Click to play animation 25 Marianas Trench Click to play animation 26 Oceanic Plateaus • Resemble the flood basalt provinces found on the continents • Created by mantle plume volcanism producing copious lavas which cover and smooth large areas of the ocean floor • Rock consists primarily of pillow lava, which may reach or exceed 30 kilometers in thickness • Examples include the Ontong Java and Rockwall Plateaus, shown on the next slide 27 Oceanic Plateaus 28 Seamounts • Tens of thousands of volcanic peaks dot the ocean floors – estimates range from 22,000 to 55,000 • Many rise hundreds of meters, but a few are larger • The largest form islands, like the Azores, Ascension, and St. Helena • As they move away from spreading centers, the plate beneath the volcano cools and contracts 29 Gulf of Alaska Seamounts • Some seamounts a occur in chains, formed as plates move over hotspots 30 Guyot • Volcanic islands are worn away by weathering, landslides, stream erosion, and wave action • In the surf zone, the island is worn flat, and becomes a guyot 31 Mid-Ocean Ridges (MOR) • Characterized by: • Rift valleys • Heat flow • Age of ridge 32 MOR Dimensions • Exceed 70,000 kilometers in length • Cover 20% of earth’s surface • Typical height is 2-3 kilometers above the ocean floor • Width ranges from 1000 to 4000 kilometers, although most are 2000-3000 kilometers 33 Not Mountains • MOR’s are high like mountains but are formed in an area of tension, not compression • The ridges are buoyantly lifted piles of hot basaltic crust • Some segments of the ridge have well developed rift valleys, named for the resemblance to the East African Rift • Flanks rise very gradually, with slopes of less than one degree, toward the ridge axis 34 Oceanic Rifts • Average fifty kilometers in width • May be two kilometers deep 35 Slow-Spreading Ridge Topography • Slow spreading leads to steep profiles and rift valleys 36 Fast-Spreading Ridge Topography • Median rift valleys are usually absent • Topography is much smoother 37 Supercontinent-Cycle • Pangaea was the most recent supercontinent, but not the only one • Reconstructions of plate positions before Pangaea are very difficult, because most older oceanic crust was destroyed by subduction • By matching geologic structures, paleoclimate records, and apparent polar-wandering curves, some reconstructions are possible, as shown on the following slides 38 600 MYBP • During the breakup of Rodinia 39 510 MYBP • Formation of Gondwana 40 430 MYBP • Collision of the northern continents 41 230 MYBP • Pangaea is assembled 42