Unit 6: The Sea Floor
Turn to Page 62 in Introduction to the World s Oceans Study the bathymetric chart What features do you see? How do the Atlantic and Pacific Oceans differ? What do you think accounts for those differences?
Ocean Bathymetry: What do you see?
What is Bathymetry? Bathymetry is from the Greek: bathos = deep, and metry = measure is the study of ocean contours A bathymetric chart shows what the ocean floor would look like if all the water drained out
Bathymetry continued More is known about the surface of Venus, Mars, and the dark side of the moon than the ocean floor Only 5% of the oceans have been mapped with the same precision as the moon there are areas the size of Kansas where no ship soundings have been made and even well surveyed areas are based on widely separated survey tracks.
Early bathymetric studies were performed with a sounding line or lead line to study the depth of the ocean floor Sailors measured depth in fathoms (6 ft) by stretching the rope between two outstretched arms
Precision Depth Recorder (PDR) developed in the 1950s used a focused beam to measure depth to 1 m in accuracy
Multi-beam echo sounders and side-scan sonar allows mapping of swaths up to 60 km wide
Bottom Coverage & Data Density by Survey Method Leadline Single Beam Multibeam 1-2 K soundings per survey 500-750 K soundings per survey 400,000 1,000,000 K soundings per survey Image courtesy of NOAA & UNH
National Geographic Video http://video.nationalgeographic.com/video/p layer/news/environment-news/us-oceanfloor-mapping-vin.html
Satellite Altimetry measures differences in sea surface elevation A sea surface anomaly is caused by the different gravitational pull of ocean mounts and trenches
Satellite Bathymetry continued
Sea Floor Bathymetry as Measured by Satellite over 4.5 year period
NOAA Satellite Bathymetry animation http://www.youtube.com/watch?v=w1tzo bxguo4 http://www.youtube.com/watch?v=jkeglsq QMyQ&feature=related
USS San Francisco In 2005 the nuclear attack sub USS San Francisco collided with an uncharted sea mount at flank (maximum) speed The sub was sailing at a depth of 500 ft. in the South Pacific about 350 south of Guam The sub was almost lost They repaired it by cutting the bow off the retiring USS Honolulu and installing it on the San Francisco.
Why Study Bathymetry?
Ocean-Floor Topography Varies with Location Hypsographic Curve
What are the three classifications of ocean floor? Continental Margins the submerged outer edge of a continent Deep Ocean Basin the deep seafloor beyond the continental margin Mid-Ocean Ridges The submarine mountain range in the middle
Deepest not in the middle, but off to either side of the shallower mid-ocean ridge
Mid-Atlantic Ridge
Passive margins Active margins Continental Margins
active margin passive margin
Continental Margins Continental Shelf - shallow, submerged edge of the continent. Shelf break - abrupt transition from continental shelf to the continental slope. Continental Slope - transition between the continental shelf & the deep-ocean floor. Continental Rise - accumulated sediment found at the base of the continental slope.
Continental Shelves Are Seaward Extensions of the Continents The features of a passive continental margin: (a) Vertical exaggeration 50:1 (b) No vertical exaggeration.
Continental Margins Submarine Canyons
California s canyons
Submarine Canyons Form at the Junction between Continental Shelf & Continental Slope turbidity currents
How do canyons form? river erosion during previous glaciation periods turbidity currents: rapid movement of a mass of sediment down a slope ~ avalanche often caused by earthquakes
Topology of Deep-Ocean Basins Differs from That of the Continental Margin A mid-ocean ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.
Hydrothermal Vents hydrothermal vents: spring of hot, mineral- and gas-rich seawater near the spreading centers of mid-ocean ridges In 1977, Robert Ballard & J. Grassle discovered new life forms in Alvin, at 3000 m near the Galápagos Islands
Hydrothermal vents are sites where superheated water containing dissolved minerals & gases escapes through fissures, or vents.
Mid-Oceanic ridges & Hydrothermal Vents Warm-water vents are below 30⁰ C and emit clear water. White smokers emit water between 30 ⁰ and 350 ⁰. Water is white because of light-colored dissolved minerals. Black smokers are above 350⁰ C (662 ⁰F) and emit black water because of presence of dark metal sulfides. Question: Why don t they emit steam instead of water?
Volcanic Seamounts & Guyots Project Above the Seabed Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges.
Deep-Ocean Basins Trenches are depressions in the ocean floor caused by the subduction of a converging ocean plate.
Deep-Ocean Basins Island Arcs, chains of volcanic islands & seamounts, are usually found parallel to the edges of ocean trenches. (left) As two oceanic plates converge, an island arc is formed by volcanic activity.
Trenches & Island Arcs Form in Subduction Zones
Video Marianas Trench http://www.youtube.com/watch?v=byjygfrp 3F4
Coral atolls form when volcanic islands sink leaving behind the coral reef surrounding a lagoon where the volcano used to be.
Atolls Fanning Island
The Abyss abyssal plains: flat, featureless expanses of seafloor covered w/ sediment between continental margins & oceanic ridges 3700-5500 m Why is the abyssal plain so flat? It is flatter than any similar plain on land
Sea Floor Spreading Evidence In the 1950s, scientists discovered parallel patterns of magnetic stripes on the sea floor. This provides evidence of seafloor spreading and a record of magnetic pole reversals.
Global Distribution of Earthquakes Global distribution of earthquakes also provides evidence for theory of plate tectonics and seafloor movement.
Age of the Seafloor Late 1960s deep-sea drilling began on the Glomar Explorer Radiometric dating of ocean rocks to determine seafloor age Oldest seafloor is about 200 million years old compared to 3.5 billion years old for oldest rocks on land.
Age of Seafloor Symmetric pattern of age distribution - youngest at the ridges and oldest at the subduction zones
Plate Tectonics Theory Lithosphere tectonic plates that float on ductile asthenosphere Large scale geologic features occur at plate boundaries 2011 Pearson Education, Inc.
Plate Tectonic Processes
Global Plate Boundaries
Driving Forces of Plate Motion Slab Pull Theory: Older dense plates at the subduction zones sink into the underlying asthenosphere providing most of the driving force for plate motion.
Types of Plate Boundaries 2011 Pearson Education, Inc.
Divergent Boundary Features Plates move apart at the mid-ocean ridge forming a rift valley New ocean floor is created Shallow focus earthquakes happen here
Generation of a Divergent Boundary
Convergent Boundary Features Plates move toward each other Oceanic crust destroyed Trenches and volcanic arcs are found here Deep focus earthquakes
Three Types of Convergent Boundaries
Oceanic Continental Convergence Ocean plate is subducted Continental arcs generated Explosive andesitic volcanic eruptions
Oceanic Oceanic Convergence Oceanic crust is destroyed Friction of plates melts sediments and crust Molten magma erupts in form of island arcs
Continental Continental Convergence No subduction Tall mountains uplifted Himalayans
Transform Boundary Features Oceanic Transform Fault ocean floor only Continental Transform Fault cuts across continent All transform faults occur between mid-ocean ridge segments. 2011 Pearson Education, Inc.
Global Hot Spots Scattered around the earth are approximately 40 fixed areas of volcanic activity known as hot spots Hot spots remain stationary as the plates move and are useful for tracing plate movements As ocean plate moves over a hot spot, a chain of volcanos is formed.
2011 Pearson Education, Inc. Global Hotspot Locations
Hawaiian Islands Hot Spot