1st Edition NR 150 Lecture 5 Outline of Last Lecture I Continental Drif II Earth s Structure 2 approaches to viewing the structure III New Technology leads to the explanation of Continental Drif Theory Outline of Current Lecture I The ocean floor is mapped by Bathymetry II Ocean floor topography varies with location Current Lecture I The ocean floor is mapped by Bathymetry a Bathymetry the study of ocean floor contours i Early studies were ofen performed using a weighted line measure the depth of the ocean floor lead weight soundings b Sund old English meaning swimming water sea c Advances in Bathymetry i single beam Echo sounding not always accurate because the speed of sound through seawater varies with temperature pressure and salinity produces 2D imagery ii Multi beam systems can have as many as 121 beams radiating to the seafloor produces 3D imagery iii Satellite altimetry measure the sea surface height from orbit using radar pulses focus on variations in the elevation of surface water rather than directly measuring ocean depths 1 An undersea mountain can pull water toward it from the sides forming a mount of water measured by satellites II Ocean floor topography varies with location a Ocean basins are not shaped like bath tub These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute b Tectonic forces shape the seabed c Continental margins the submerged outer edge of a continent i Passive margins facing the edges of diverging plates surround the Atlantic Ocean Atlantic0type margins 1 Little earthquake or volcanic activity is associated with them ii Active margins near the edges of converging plates OR near places where plates are slipping past each other surround the Pacific Ocean Pacifictype margins 1 Characterized by earthquakes and volcanic activity iii Continental shelf 1 Submerged extension of continents 2 Underlain by granitic continental shelf 3 Sediments from erosion of land build up the shelf 4 Gently slopes seaward 5 Water depth is only about 75m on average a Allows for offshore drilling relatively easily of oil and minerals 6 Shaped by tectonics passive vs active ocean currents fast vs slow flowing and sea level low vs high sea level iv Continental slope 1 Transition between the continental shelf and the deep ocean floor 2 Sediments mainly come from the shelf 3 Steeper than the continental shelf a Slopes along active margins are generally steeper than those along passive margins 4 About 20km wide and 3700m deep 5 Bottom of the slope is the true edge of the continent v Shelf break 1 Abrupt transition from continental shelf to continental slope 2 About 140m deep 3 Surprisingly constant depth word wide d Submarine canyons form at the junction between continental shelf and continental slope i A feature of some continental margins ii Cut into the continental shelf and slope ofen terminating on the deepsea floor in a fan shaped wedge of sediment iii Turbidity current 1 A fast dense current of water that carries sediment offshore 2 Occurs when turbulence mixes sediments into water above a sloping bottom 3 Earthquakes along active margins can trigger bigger turbidity currents e Continental rise i Forms as sediments accumulates ii Ofen an apron of sediment that covers the bottom of the continental slope f Features in deep ocean floor i Oceanic ridges a mountainous chain of young basaltic rock at an active spreading center of an ocean 1 Spreads 65000 km around the Earth ii Hydrothermal vents 1 Hot springs on active oceanic ridges 2 Sites where superheated water containing dissolved minerals and gases escapes through fissures or vents white smokers or black smokers iii Abyssal plains and abyssal hills 1 Abyssal plains a Flat cold sediment covered ocean floor between continental rise and the oceanic ridge usually deeper than 3700m b Between continental margins and ocean ridges 2 Abyssal hills a Small sediment covered extinct volcanoes or rock intrusions we think they are associated with seafloor spreading iv Seamounts and guyots 1 Seamounts volcanic mountains bigger than abyssal hills that rise from the ocean floor and never grow tall enough to come close to the sea surface 2 Guyots flat topped seamounts eroded by wave action can be just below sea surface or sticking out of the water v Trenches an arc shaped depression in the deep ocean floor