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Major Features of Ocean Basins

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Major Features of Ocean Basins 21/09/2020 ESCI 1006 & 1106 Oceanography Outline -Bathymetry - Classifications of the Ocean Floor - Continental Margins - Deep-Ocean Basins Major features of ocean basins Parts of chapters 3 & 4 Multi-beam systems Scale of earth observation varies combine many echo sounders Mega (space-based, satellite) Measures small variations in elevation of surface water Global sea surface elevation map from satellites The topography of ocean floors © 1997, David T. Sandwell 49 1 21/09/2020 Typical continental margins bordering the Distribution of elevations and depths on Earth edges of a continent. Not all transitions between oceanic and continental crust are Highest point on continents plate boundaries or active margins. Those that are not are passive margins (found along coastal areas that surround the Atlantic ocean (‘Atlantic-type’ margins) Active and passive continental margin Av. land elevation Av. depth of ocean basins Deepest point in ocean basins Active margins – lots of tectonic activity 51 Passive margins – not associated with volcanism & earthquakes Cross section of a typical ocean basin Continental margins composed of continental flanked by passive continental margins shelf, slope, & rise The transition to basalt (green) marks the true edge of the continent and divides ocean floors into two major provinces. The submerged outer edge of a continent is called the continental margin. The deep seafloor beyond the continental margin is properly called the ocean basin. Example of a passive continental margin Continental margins composed of continental Continental margins composed of continental shelf, slope, & rise shelf, slope, & rise Continental shelf Continental slope - True edge of the continent – Tapered, thinned, ‘flooded’ extension of the continent – Seaward edge of continental shelf – Highly variable in width (80 km avg.) – Relatively steep slope (5° avg., up to 25°) – Gently sloping submerged surface (~0.1°) – Relatively narrow (20 km avg.) 2 21/09/2020 Continental margins composed of continental Beyond the continental margins … are deep- shelf, slope, & rise ocean basins Major morphological features of deep- ocean basins are: - Oceanic ridges (mid-ocean ridges) - Transform faults and fracture zones - Abyssal hills - Abyssal plains - Seamounts, ocean islands and large plateaus Continental rise – Thick accumulation of sediments at base of slope. - Trenches and island arcs – Laterally extensive (hundreds of km) Fracture zones extend outward from the ridge axis, beyond the active transform fault Continental margin Oceanic ridge (Mid-ocean ridge) Fracture zone is inactive region. Segments of a lithospheric plate on either side of a transform fault move in opposite directions from each other, the plate segments adjacent to the outward segments of a fracture zone move in the same direction Abyssal hills Abyssal hills Mid-Atlantic Ridge - Relief of 50m to few hundred meters - Spacing ranges from 5 to 10 km -Form primarily by faulting (Multibeam data) 3 21/09/2020 Abyssal Plains are flat, featureless expanse of sediment on ocean floor • 3 to 6 km deep • Abyssal plains are Abyssal Plains extraordinarily flat. • Limited tectonic or volcanic activity • Sediments passively accumulate 63 63 Source: Hamblin and Christiansen, Earth’s Dynamic Systems A view of abyssal plains Seamounts - a conical underwater mountain formed by a volcano and rising 1000 meters or more from the sea floor water seds bedrock (basalts) Bathysnap time-lapse Porcupine Abyssal Plain (single beam echo sounder data) The process of forming Submarine, wave-cut seamounts are called guyots guyots (G) & seamounts (S) They were discovered using echo sounder data collected during the second world war in the western Pacific by Harry Hess Harry Hess Guyots are flat-topped seamounts that once were tall G and S are associated with mid-ocean enough to approach or penetrate the sea surface. ridges or hotspots 4 21/09/2020 Seamounts associated with East Pacific Rise Hot spots create linear volcanic chains with that progressively age in one direction Mantle plumes and hot spots remain stationary under moving plates. Plate movement above mantle plumes and hot spots. Seamounts associated with hot spots Hot Spots Emperor seamounts 59.6 Ma 48.1 Ma 43.4 Ma 7.2 Ma Hawaiian chain Loihi - 0 Ma A New Hawaiian Island: Loihi Trench is an arc-shaped depression in the deep- Island arc – curving chain of volcanic islands ocean floor that can be found at ocean-ocean and and seamounts. Found parallel to trenches. ocean-continent convergent plate boundaries. Subduction zones. Trenches and Island Arcs The Izu – Bonin- Mariana arc system *Challenger Deep ~10,900 m (Challenger Expedition estimated 8,184 m ) Average Pacific ocean * depth - 4,188 meters Mariana Trench: the They are the deepest features in the oceans, bottom of the ocean - BBC reaching ~11,000 meters in the Marianas Trench. Volcanic ‘arcs’ – named for arcuate shape Source: Hamblin and Christiansen, Earth’s Dynamic Systems 5 21/09/2020 Features of Earth’s solid surface shown as percentages of the Planet’s total surface Key morphological features of ocean basins: -Oceanic ridges (MOR) - Abyssal hills - Transform faults and fracture zones - Abyssal plains - Seamounts, ocean islands and large plateaus - Trenches and island arcs Summary • It is difficult to discover the shape of the seabed • Seafloor features result from tectonic activity, erosion, and deposition • The ocean floor can be divided into two regions: – Continental margins – Deep-ocean basins • Key morphological features of ocean basins are: oceanic ridges (MOR), transform faults and fracture zones, abyssal hills, abyssal plains, seamounts, ocean islands and large plateaus, trenches and island arcs 6.
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