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OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS Sources: www.google.com en.wikipedia.org Thompson Higher Education 2007; Monroe, Wicander, and Hazlett, Physical Geology TCNJ PHY120 2013 GCHERMAN OCEANS, SEAFLOOR, AND CONTINENTAL MARGINS • Introduction • Methods Used to Study the Seafloor • Oceanic Crust ‐ Its Structure and Composition • The Continental Margins • Features Found in the Deep‐Ocean Basins • Sediments on the Deep Seafloor • Reefs—Rocks Made by Organisms • Resources from the Oceans TCNJ PHY120 2013 GCHERMAN OCEANS cover more of Earth’s surface than land •71% of Earth’s area consists of oceans and marginal seas like Caribbean and Mediterranean Seas •Most of the ocean waters are in the are in the Southern Hemisphere, about 81 % of the surface water. TCNJ PHY120 2013 GCHERMAN EARTH’S SURFACE COVER Continental margins lying below sea level separate the continents from the ocean basins. TCNJ PHY120 2013 GCHERMAN OCEAN BASINS • Of the 3 major oceans (Pacific, Atlantic, Indian), the Pacific is the largest, contains over half of the water by volume, and has the greatest average depth — 3940m • The average depth of all ocean basins is ~3800 meters (12,500 ft). TCNJ PHY120 2013 GCHERMAN MAPPING THE SEAFLOOR • • • • • • Satellite measurements Echo sounding profiles Side‐scan sonar Seismic profiles using sound waves Manned and unmanned submersibles Drilling and collecting core samples TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR On June 27,1667, Captain John Wood, on the H. M. Speedwell, recorded that he saw the bottom while sounding at a depth of 80 fathoms (480 feet). His position was approximately 74°30'N, 63°E (to the east of Novaya Zemlya). He records that he "could see the shells at the bottom very plain." His observation is considered by many to be valid, making this the record for ocean water visibility. EOS, March 1, 1994. Misconception: The ocean floor is flat, except where islands protrude above the surface. Fact: The ocean floor has far more varied topography than the land areas, with higher and longer mountain ranges, deeper canyons and trenches. TCNJ PHY120 2013 GCHERMAN EARLY MAP AND PERSPECTIVE PLATO, ATLANTIS, AND AMERICA CIRCA 1664 TCNJ PHY120 2013 GCHERMAN CORE SAMPLE Scientific study of the ocean basins began in the late 1700's. Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling. They sample the sea floor using clamshell samplers and piston corers. TCNJ PHY120 2013 GCHERMAN SEAFLOOR DEPTH • first measured by lowering weighted lines overboard In the 1920’s depth was first determined using an Echo sounder (sonar) • Primary instrument for measuring depth • Reflects sound from ocean floor • Multibeam sonar • Employs an array of sound sources and listening devices • Obtains a profile of a narrow strip of seafloor TCNJ PHY120 2013 GCHERMAN MODERN METHODS OF STUDYING THE SEAFLOOR TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR Drilling core samples through the ocean floor and other remote sensing devices like seismographs are used to map the subsurface geology below the floor. TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR Deep sea drilling and studies of isolated exposures on active margins of continents and within ancient mountain ranges indicate that the sequence of oceanic rock types from top to base is: basaltic pillow lava, sheeted basalt dikes, and gabbro. TCNJ PHY120 2013 GCHERMAN DEEP‐SEA‐ DRILLING PROJECT (DSDP) AND OCEAN‐ DRILLING PROJECT (ODP) DRILL CORE SITES TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR (topography) TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR 1 2 TCNJ PHY120 2013 GCHERMAN 3 2 1 MID‐OCEAN RIDGES ‐ the site of sea‐floor spreading, are found in all major oceans and represent more than 20 % of Earth’s surface (it is where new crust is being formed). • They nearly encircle the world but are offset at intervals by large fracture zones TCNJ PHY120 2013 GCHERMAN MID‐OCEAN RIDGES Most of the geologic activity on ridges occurs along a narrow region on the ridge crest—called the rift valleys, where magma from the asthenosphere moves upward to create new sections of oceanic crust (on land Iceland). TCNJ PHY120 2013 GCHERMAN MID‐OCEAN RIDGES • Oceanic ridges are long, continuous submarine mountain ranges composed of volcanic rock which has been extruded on the sea floor. • Elevated position (higher than the surrounding area) • Extensive faulting (makes sense, earth is being pushed apart here). • Volcanic structures on newly formed mafic, oceanic crust • Are characterized by shallow‐focus earthquakes, and basaltic volcanism. TCNJ PHY120 2013 GCHERMAN MID‐OCEAN RIDGES are characterized by shallow‐focus earthquakes, and basaltic volcanism. • Black smokers and tube worms • Some ridges possess a central deep rift in which volcanism is frequent. TCNJ PHY120 2013 GCHERMAN OCEANIC RIDGES nearly encircle the world but are offset at intervals by large fracture zones TCNJ PHY120 2013 GCHERMAN PACIFIC OCEAN BASIN Hawaiian seamount chain aseismic ridges • Seamounts and guyots are common, widespread features of the sea floor. • Seamounts rise more than a kilometer from the ocean floor and are remnants of oceanic ridge volcanoes; Guyots have the same origin but are flat‐topped . • Aseismic ridges consisting of seamounts and guyots extend in a perpendicular orientation from many spreading ridges. TCNJ PHY120 2013 GCHERMAN INDIAN OCEAN BASIN abyssal plain aseismic ridges TCNJ PHY120 2013 GCHERMAN ATLANTIC OCEAN BASIN abyssal plain mid‐ocean spreading ridges aseismic ridge mid‐ocean spreading ridges abyssal plain TCNJ PHY120 2013 GCHERMAN aseismic ridge TWO TYPE OF CONTINENTAL MARGINS Continental margins lying below sea level separate the continents from the ocean basins. TCNJ PHY120 2013 GCHERMAN The continental margins include the continental shelf, continental slope, and in some places a continental rise. BLOCK DIAGRAM OF A PASSIVE MARGIN • A broad shelf leading to a slope characterize passive continental margins • The continental rise is typified by the overlapping deposits of submarine fans composed of turbidity current‐transported sediments. TCNJ PHY120 2013 GCHERMAN PASSIVE CONTINENTAL MARGINS •Void of active plate boundaries, exhibit few volcanoes and earthquakes. •Sediments eroded from the adjacent landmass (continent) accumulate to form thick undisturbed layers. • Submarine canyons are typical features of the continental slope, but commonly extend into the shelf. • Some may have formed by stream erosion during the Pleistocene when sea level was lower, but others, not associated with landward streams, were probably eroded by turbidity currents. TCNJ PHY120 2013 GCHERMAN SUBMARINE CANYONS TCNJ PHY120 2013 GCHERMAN Some may have formed by stream erosion during the Pleistocene when sea level was lower, but others, not associated with landward streams, were probably eroded by turbidity currents. ACTIVE MARGINS • A narrow shelf and a slope descending directly into an ocean trench characterize active continental margins. •These margins are also characterized by volcanism and earthquakes. TCNJ PHY120 2013 GCHERMAN ACTIVE MARGINS • Oceanic trenches are the surface expressions of subduction zones. • They are long, narrow, features which reach the greatest oceanic depths, and are characterized by low heat flow and negative gravity anomalies. TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR EXPOSED TCNJ PHY120 2013 GCHERMAN SIX TYPES OF OCEANIC SEDIMENT Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays which was derived from continents and oceanic islands. TCNJ PHY120 2013 GCHERMAN THREE TYPES OF SEDIMENT ON CONTINENTAL MARGINS Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays which was derived from continents and oceanic islands. TCNJ PHY120 2013 GCHERMAN TERRIGENOUS OR LAND‐DERIVED SEDIMENT consist primarily of mineral grains that are weathered and transported from continental rocks into the ocean. They accumulates at very‐ very slow rates— 5k to 50k years to deposit 1 cm of sediments, and are red or brown due to the oxidation of iron in sea water • 95% of the terrigenous material entering the ocean is delivered by rivers, and most of it is deposited on or near the continental margin. • Only a minor amount is deposited in the deep ocean. TURBIDITY CURRENTS AND BOUMA SEQUENCES TCNJ PHY120 2013 GCHERMAN CONTINENTAL SHELF, MARGIN, SLOPE, AND RISE DEPOSITS TCNJ PHY120 2013 GCHERMAN GLACIAL‐MARINE DEPOSITS THREE TYPES OF DEEP‐SEA (PELAGIC) SEDIMENT Consist largely of the carbonate and siliceous skeletons of microscopic organisms (ooze), and pelagic clays derived from continents and oceanic islands. PELAGIC SEDIMENT TCNJ PHY120 2013 GCHERMAN PELAGIC CLAY or pelagite is a fine‐grained sediment that accumulates as the result of the settling of particles to the floor of the open ocean, far from land. • These particles consist primarily of either the microscopic, calcareous or siliceous shells of phytoplankton or zooplankton; clay‐size siliciclastic sediment; or some mixture of these with trace amounts of meteoric dust and minor amounts of volcanic ash. • There are three main types of pelagic sediments based on composition: siliceous and calcareous oozes, and red clays • The composition of pelagic sediments is controlled by three main factors. 1) the distance from major landmasses, which affects their dilution by terrigenous, or land‐ derived, sediment. 2) water depth, which affects the preservation of both siliceous and calcareous biogenic particles as they settle to the ocean bottom. 3) ocean fertility, which controls the amount of biogenic particles produced in surface waters. BIOGENOUS SEDIMENT (OOZES) Diatoms – plants SiO2 Foraminifera – animals • Consist of shells and skeletons of floating animals and plants. • Some organisms having calcareous skeletons, whereas other have siliceous (SiO2) skeletons. • These sediments are fine grained and called oozes (very fine mud). CaCO3 Coccolithofores – plants TCNJ PHY120 2013 GCHERMAN SiO2 Radiolarian – animals CaCO3 CARBONATE COMPENSATION DEPTH (CCD) is the depth in the oceans below which the rate of supply of calcite (calcium carbonate) lags behind the rate of dissolution, such that no calcite is preserved. • Calcium carbonate is essentially insoluble in sea surface waters today. • Shells of dead calcareous plankton sinking to deeper waters are practically unaltered until reaching the lysocline where the solubility increases dramatically. • By the time the CCD is reached all calcium carbonate has dissolved according to this equation: TCNJ PHY120 2013 GCHERMAN HYDROGENOUS SEDIMENT • Includes minerals that crystallize directly from sea water through various chemical reactions, such as manganese nodules, that form as lumps on the seafloor and have the potential to contain large amounts of economic metals. • They are usually 20 % manganese, with the remainder other minerals. TCNJ PHY120 2013 GCHERMAN DEEP‐SEA SEDIMENT • Studies of this deep sea material can provide important information for comparison of the climatic histories of the land and sea. • Research along this line has indicated that significant chemical weathering began at the Eocene‐Oligocene boundary and continued for at least 20 million years before physical weathering became dominant, in the middle or late Miocene. • This shift appears to have been related to uplift in the Himalayas and North America (and probably elsewhere, as well). GSA Today, August, 1993. TCNJ PHY120 2013 GCHERMAN OCEAN BASINS GUYOUTS AND SEAMOUNTS TCNJ PHY120 2013 GCHERMAN ATOLLS AND CORAL REEFS • Atolls are coral islands consisting of a continuous or broken ring of coral reefs surrounding a central lagoon. • Atolls can form on the flanks of eroding and sinking volcanic islands on the oceanic crust as it moves, where coral continue to grow and build reef complexes upward while the island slowly sinks. TCNJ PHY120 2013 GCHERMAN ATOLLS AND CORAL REEFS • Atolls are coral islands consisting of a continuous or broken ring of coral reefs surrounding a central lagoon. TCNJ PHY120 2013 GCHERMAN CORAL REEFS organisms. There are fringing, barrier, and atoll reefs. TCNJ PHY120 2013 GCHERMAN are wave resistant structures built by corals, and other CORAL REEFS TCNJ PHY120 2013 GCHERMAN ATOLLS AND CORAL REEFS • Are constructed over thousands of years primarily from the accumulation of skeletal remains and secretions of corals and certain species of algae. • Coral Reefs and Atolls occur in warm, sunlit waters usually in the Pacific and Indian Oceans. Corals only live in waters above 65o F. and in the photic zone above 150 ft depth below the surface. TCNJ PHY120 2013 GCHERMAN Exclusive Economic Zones The United States claims all rights to the natural resources within 200 nautical miles of its coastline, including petroleum, natural gas, gravel and various metals. TCNJ PHY120 2013 GCHERMAN EARTH’S SEAFLOOR TCNJ PHY120 2013 GCHERMAN (IMPACT FEATURES?)