NR 150 1st Edition Lecture 5Outline of Last Lecture I. Continental DrifII. Earth’s Structure: 2 approaches to viewing the structureIII. New Technology leads to the explanation of Continental Drif TheoryOutline of Current LectureI. The ocean floor is mapped by BathymetryII. Ocean-floor topography varies with locationCurrent LectureI. The ocean floor is mapped by Bathymetrya. Bathymetry: the study of ocean floor contoursi. 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, seac. Advances in Bathymetryi. (single beam) Echo sounding: not always accurate because the speed of sound through seawater varies with temperature, pressure, and salinity; produces 2D imageryii. Multi-beam systems: can have as many as 121 beams radiating to the seafloor; produces 3D imageryiii. 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 depths1. An undersea mountain can “pull” water toward it from the sides, forming a mount of water measured by satellitesII. Ocean-floor topography varies with locationa. Ocean basins are not shaped like bath tubThese 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 seabedc. Continental margins: the submerged outer edge of a continenti. Passive: margins facing the edges of diverging plates, surround the Atlantic Ocean (Atlantic0type margins)1. Little earthquake or volcanic activity is associated with themii. Active: margins near the edges of converging plates OR near places whereplates are slipping past each other, surround the Pacific Ocean (Pacific-type margins)1. Characterized by earthquakes and volcanic activityiii. Continental shelf:1. Submerged extension of continents2. Underlain by granitic (continental) shelf3. Sediments from erosion of land build up the shelf4. Gently slopes seaward5. Water depth is only about 75m on averagea. Allows for offshore drilling relatively easily of oil and minerals6. 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 floor2. Sediments mainly come from the shelf3. Steeper than the continental shelfa. Slopes along active margins are generally steeper than those along passive margins4. About 20km wide and 3700m deep5. Bottom of the slope is the true edge of the continentv. Shelf break:1. Abrupt transition from continental shelf to continental slope2. About 140m deep3. Surprisingly constant depth word-wided. Submarine canyons form at the junction between continental shelf and continental slopei. A feature of some continental marginsii. Cut into the continental shelf and slope, ofen terminating on the deep-sea floor in a fan-shaped wedge of sedimentiii. Turbidity current1. A fast dense current of water that carries sediment offshore2. Occurs when turbulence mixes sediments into water above a sloping bottom3. Earthquakes along active margins can trigger bigger turbidity currentse. Continental risei. Forms as sediments accumulatesii. Ofen an apron of sediment that covers the bottom of the continental slopef. Features in deep-ocean floori. Oceanic ridges: a mountainous chain of young, basaltic rock at an active spreading center of an ocean1. Spreads 65000 km around the Earthii. Hydrothermal vents1. Hot springs on active oceanic ridges2. Sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents (white smokers or black smokers)iii. Abyssal plains and abyssal hills1. Abyssal plains:a. Flat, cold, sediment-covered ocean floor between continental rise and the oceanic ridge (usually deeper than3700m)b. Between continental margins and ocean ridges2. Abyssal hills:a. Small sediment-covered extinct volcanoes or rock intrusions; we think they are associated with seafloor spreadingiv. Seamounts and guyots1. Seamounts: volcanic mountains (bigger than abyssal hills) that risefrom the ocean floor and never grow tall enough to come close to the sea surface2. Guyots: flat-topped seamounts eroded by wave action (can be justbelow sea surface or sticking out of the water)v. Trenches: an arc-shaped depression in the deep-ocean