Chapter 3 Bathymetry Bathymetry Measures the vertical distance from the oceans surface to mountains valleys plains and other sea floor features Topography shape of the ocean floor Poseidonus conducted the first recorded attempt to measure the ocean s depth a sounding 85 BC Stuck a probe out until the line hit the bottom Fathom standard unit of measurement for measuring ocean depth HMS Challenger first systematic measurements Echo Sounder sends a sound signal ping from the ship downward into the ocean where it produces echoes when it bounces off any density difference Most of our knowledge of ocean bathymetry has been provided by the echo sounder Precision Depth Recorder PDR uses a focused high frequency sound beam to measure depths to a resolution of about 1 meter Seabeam first multibeam echo sounder made it possible for a survey ship to map the features of the ocean floor along a strip up to 60 km Seismic reflection profiles sounds penetrate benath the sea floor and reflect off the boundaries between different rock or sediment layers used to measure ancient bathymetry Measuring Ancient Bathymetry air guns strong low frequency sounds details ocean structure beneath the sea floor Ocean Floor Provinces continental margins deep ocean basins mid ocean ridge the provinces are formed by the plate tectonic process Hypsographic Curve shows relationship between height of land and depth of ocean 70 8 of Earth covered by oceans Average ocean depth is 3729 meters average land elevation is 840 meters uneven distribution of areas of different depths elevations Variations suggest plate tectonics at work Continental Margins shallow water areas close to continents Deep Ocean Basins deep water areas farther from land Mid Ocean Ridge composed of shallower areas near the middle of an ocean Continental margins classified as either passive or active depends on Continental margin features continental shelf shelf break continental slope proximity to plate boundaries continental rise Passive margins not close to any plate boundary embedded within the interior of lithospheric plates and are therefore not in close proximity to any plate boundary usually lack major tectonic activity Ex East Coast of USA Active margins associated with lithospheric plate boundaries and are marked by a high degree of tectonic activity Two types Convergent active margins associated with oceanic continental convergent plate boundaries Transform active margins less common and are associated with transform plate boundaries Offshore faults usually parallel the main transform plate boundary fault and create linear islands banks and deep basins close to shore Continental Margins Continental shelf defined as a generally flat zone extending from the shore beneath the ocean surface to a point at which a marked increase in slope angle occurs geologically part of the continent type of continental margin determines the shelf features passive margins have wider shelves California s transform active margin has a continental borderland Shelf break marked increase in slope angle on a continental shelf Continental borderland high degree of relief that marks a continental shelf Continental Slope Continental slope lies beyond the shelf break where the deep ocean basins being Similar to mountains Greater slope than continental shelf marked by submarine canyons Submarine canyons Narrow deep v shaped in profile Steep to overhanging walls extend to base of continental slope Carved by turbidity currents Turbidity currents underwater avalanches mixed with rocks and other debris sediment from continental shelf moves under influence of gravity earthquakes etc sediments deposited at slope base Continental Rise Continental rise transition zone between the continental margin and the deep ocean floor continental crust and oceanic crust comprised of a huge submerged pile of debris The huge pile of debris is there because of turbidity currents Graded bedding suspended material that has settled out in a distinctive type of layering grades in size upward Turbidite deposits stacks of graded bedding Deep sea fans or submarine fans deposits at the mouths of submarine canyons are fan lobate or apron shaped the fans create continental rise when they merge together along the base of the continental slope Distal ends of submarine fans become flat abyssal plains The largest deep sea fan Indus Fan passive margin fan that extends 1800 Features that exist in Deep Ocean Basins km Abyssal Plains Abyssal plain flat depositional surfaces with slopes of less than a fraction of a degree that cover extensive portions of the deep ocean basins some of the deepest regions on Earth Formed by fine particles of sediment slowly drifting onto the deep ocean floor Suspension settling produces a thick blanket of sediment as fine particles accumulate on the ocean floor with time they cover most irregularities of the deep ocean The type of continental margin determines the distribution of abyssal plains few abyssal plains are located in the Pacific Ocean most occur in the Atlantic and Indian Oceans the deep ocean trenches found on the convergent active margins of the Pacific Ocean prevent sediment from moving past the continental slope the trenches act like a gutter that traps sediment transported off the land by turbidity currents Volcanic Peaks of the Abyssal Plains The volcanic peaks poke through the sediment cover of the abyssal plains they extend to various elevations above the ocean floor Some extend above sea level and form islands Seamounts volcanic peaks that are below seal level but ride more than 1 kilometer above the deep ocean floor and have a pointy top like an upside down ice cream cone Tablemount if a volcano has a flattened top Abyssal hill seaknolls Volcanic features on the ocean floor that are less than 1000 meters tall one of the most abundant features on the planted many created by stretching of crust during the creation of new sea floor at the mid ocean ridge Abyssal hill provinces extensive regions dominated by abyssal hills Ocean Trenches and Volcanic Arches Ocean Trench deep linear scars in the ocean floor caused by collision of two plates along convergent plate margins convergent active margin slopes descend into ocean trenches Ex Marianna Trench deepest trench Volcanic arc landward side of trench rises may produce islands or volcanic mountain range Pacific Ring of Fire occurs along the margins of the Pacific Ocean home to the majority of Earth s active volcanoes
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