Lecture 13

# Lecture 13 - Chapter 13 Divergent Boundaries Origin and...

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Chapter 13 Divergent Boundaries: Origin and Evolution of the Ocean Floor Mapping the ocean floor Bathymetry ( bathos = depth, metry = measurement) is the measurement of oceanic depth and the charting of the shape=topography of the ocean floor Depth was originally measured by lowering weighted lines overboard Nowadays bathymetric techniques use a sound energy Echo sounder (also referred to as sonar : so und na vigation and ra nging) is a name of device, which is sending a sound towards the ocean floor Invented in the 1920s Primary instrument for measuring depth Transmit a sound wave into the water in order to produce an echo when it bounces of an object Receiver is a second device . It intercept the echo reflected from the bottom. A clock precisely measures the travel time to fractions of a sec. The velocity of sound waves in water is known, it is about 1500 m/sec. Using a simple equation: Distance (to bottom ) = Velocity x Time / 2 High-resolution multibeam sonar is a modern device Employs an array of sound sources and listening devices Obtains a profile of a narrow strip of seafloor Viewing the ocean floor from space Satellites use radar altimeters to measure subtle differences of the ocean surface A variation in sea surface elevation, which is caused by gravitational attraction and mimics the shape of the seafloor. The sea surface anomaly is the difference between the measured and theoretical ocean surface. Seismic Reflection Profiles of the Oceans To view the rock structure beneath the sediments that blanket much of the seafloor marine geologists use a seismic reflection profile . To construct such a profile , strong low-frequency sounds are produced by explosions (depth charges) or air-guns. The sound waves penetrate beneath the seafloor and reflect off the contacts between rock layers and fault zones, just like sonar reflects off the bottom of the sea. Three major provinces of the ocean floor 1. Continental margins : passive and active 2. Deep-ocean basins : deep oceanic trenches, abyssal plains, seamounts, guyots or tablemounts oceanic plateaus 3. Oceanic (mid-ocean) ridges : rift valleys

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Fig. 13.6 Major provinces of the North Atlantic and a profile from New England to the coast of North Africa Continental margins That portion of the seafloor adjacent to the continents. It include the continental shelf , continental slope and continental rise Passive  continental margins Found along most coastal areas that surround the Atlantic ocean Not associated with plate boundaries Experience little volcanism and few earthquakes Fig.13.7 Schematic view showing the major features of a passive continental margin Features comprising a passive continental margin Continental shelf Flooded extension of the continent Varies greatly in width Gently sloping Contains important mineral deposits
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Lecture 13 - Chapter 13 Divergent Boundaries Origin and...

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