GEOL 1301 Chapter Outline Chapter 12: The Ocean Floor

1. OCEANOGRAPHY is a science composed of other sciences as applied to the study of the oceans. It includes such major branches as:
   1. physical oceanography, study of ocean currents, tides, waves and propagation of various forms of energy in the ocean;
   2. chemical oceanography, study of chemical processes and features, and their variations in the ocean;
   3. biological oceanography or marine biology, study of life in the ocean; and
   4. geological oceanography or marine geology, study of the geology of ocean basins.
   
   
2. THE VAST WORLD OCEAN: The oceans cover 71% of the total surface area of the earth. This leaves 29% land (continents, islands, and so on).
   1. In the northern hemisphere, 61% of surface areas is ocean, while in the southern hemisphere 81% is ocean. Because the northern hemisphere thus includes about two-thirds of all the land, it is sometimes called the land hemisphere and the southern hemisphere by comparison would be the water hemisphere.
   2. The Pacific Ocean is by far the largest, making up almost half the total ocean area of the earth.
   3. Also, the average depth of the oceans below sea level (3,800 m) is almost five times as great as the average height of land above sea level (840 m).
   
   
3. MAPPING THE OCEAN FLOOR
   1. The voyage of the converted warship H.M.S. Challenger (1872-1876) was the first scientific expedition devoted specifically to study of all aspects of the oceans. Wireline soundings suggested the presence of mountain ranges in the middle of ocean basins.
   2. The echo sounder, now used routinely in oceanographic work, was first used on the voyage of the Meteor (1925-1927), a German expedition to assess the feasibility of "mining" gold from seawater.
   3. Several hundred vessels are used today for oceanographic studies. Notable among these are deep-diving submersibles such as Alvin, and drilling ships such as the Joides Resolution.
   4. Analyses of satellite orbits have aided detailed measurements of gravity and the true shape of sea level, now known to have bulges and depressions that reflect the density and topography of rock below.
   5. Major topographic divisions of the ocean floor are the continental margins, deep-ocean basins, and mid-ocean ridges.
   
   
4. CONTINENTAL MARGINS are the submerged edges of continents. Two types can be distinguished: active margins, which lie on plate boundaries (subduction zones, usually), and passive margins, which lie well away from a plate boundary (but were initially shaped as one side of a rift valley).
   1. Passive Continental Margins
      1. The continental shelf is almost flat (average slope 0.1°), from the shoreline to the edge of the continental slope. It amounts to submerged edge of the continent, mantled with marine sediment, and position of the shoreline dependent on sea level. Depth at the outer edge averages about 130 meters (425 feet). Width of shelves averages about 80 km, but varies greatly from one place to another.
      2. The continental slope is a drop-off at the edge of the shelf. It is not really a cliff, but with typical slope of 5° it is comparatively much steeper than a shelf. The main reason for the "drop-off" is the transition from continental crust to denser oceanic crust. Water depth at the base of the continental slope commonly is thousands of meters.
      3. The continental rise is a "levelling-off" zone that lies at the base of the continental slope. It is built by sediments accumulated at the base of the slope. Deep-sea fans are lobe-shaped sediment accumulations commonly seen at the mouths of submarine canyons (SEE below) that coalesce to give the overall wedge shape of continental rise deposits.
      4. Submarine canyons and turbidity currents
         1. Submarine canyons are valleys cut into the edge of the shelf, extending from near shore to the face of the continental slope. A few of these canyons appear to be extensions of major rivers emptying from nearby land, and may to have formed at times when sea level was lower (as during the ice Ages). Most others have no connection with rivers from the land, and seem to have formed by strictly oceanic processes such as turbidity currents. Sediment collected at the mouth of a submarine canyon commonly makes a submarine fan.
         2. Turbidity currents are considered to be the major way in which sand and other coarse sediments are moved into deep water. Deposits created by turbidity currents are known as turbidites: they are characterized notably by graded bedding (coarse sediment at base, with successively finer materials upward).
         3. The density of water is affected by sediment content (turbidity) as well as by salinity and temperature. Turbid water can be enough denser than clear water that it will flow downhill and reach speeds up to 80 km/hr (50 mph). The resulting turbidity currents may travel for hundreds of miles before coming to rest and dropping the sediment load they carry.
   2. Active Continental Margins - differ from passive margins by having:
      1. a deep-ocean trench at the edge of the shelf.
      2. an accretionary wedge, or chaotic assembage of rock material that represents "scrapings" from the subduction zone, being tectonically pushed against the trench wall.
   
   
5. FEATURES OF THE OCEAN BASIN FLOOR
   1. Deep-ocean trenches include the deepest parts of the ocean. The "walls" that they appear to have in some pictures are not really cliff-like (the drawings are usually exaggerated to make subtle features stand out). Trenches are associated with subduction zones, where crustal plates are driven downward. Trenches not near a continent commonly have a belt of volcanic islands located on overthrusting side. Edges of continents next to trenches also may include a volcanic belt.
   2. Abyssal Plains include the flattest topography found on earth, some areas being smoother than the ocean surface above them. Deposits on abyssal plains are dominated by the materials washed in by turbidity currents, but also include fine particles slowly settled from overlying water (organic remains plus wind-blown and ice-rafted debris from land), and chemical precipitates.
   3. Seamounts are the isolated peaks of seafloor volcanoes. Flat-topped seamounts are known as guyots, flattened by wave erosion before they were submerged into deeper water, some fringed by coral reefs or atolls before subsiding.
   
   
6. MID-OCEAN RIDGES
   1. The mid-ocean ridge system is a 70,000-km-long mountain range, with slightly more topographic relief than any present on-land mountain range. It makes up about 20% of the total surface of the earth.
   2. Mid-ocean ridges are much different from continental mountains, because of great differences in the processes going on inside. Continental mountain belts mostly are the result of crunching together and buckling of plates, whereas mid-ocean ridges mark zones of spreading, as new igneous material enters the crust from below and forces the older material aside.
   3. The crest or central axis of the ridge is marked by igneous activity. The abundance of hot rock near the surface causes high crustal heat-flow rates. Shallow-focus earthquakes are abundant, associated with the igneous activity that is pushing the crust apart. The rift valley found at the crest of a typical ridge is a strip of down-faulted rock, a result of the crustal stretching.
   4. The main reason for the elevation of mid-ocean ridges appears to be simply the contrast in density between the hot rocks near the ridge crest versus their cooler surroundings. This also fits nicely with the observation that trenches and subduction zones are mostly areas of low heat flow, where cooler, denser rocks are going down.
   
   
7. SEA-FLOOR SEDIMENTS: Continental-shelf areas receive most of the sediment eroded from land. Some are covered by 10 km or more of such deposits. The variety of material deposited on shelves is also great, because shorelines can migrate back and forth across a shelf as sea level fluctuates, or the continent uplifts and subsides. Except in trenches, sediment thickness and variety both tend to be much less in the deep ocean, where only turbidity currents can bring large amounts of land-derived sediment. The maximum known thickness of sediment in a deep ocean area other than a trench is about 1 km, and this rests on areas of old oceanic crust. The youngest oceanic crust, at mid-ocean ridges, has virtually no sediment on it.
   1. Types of Sea-floor Sediments: A special terminology is used for classifying sea-floor deposits:
      1. Terrigenous sediments are made from broken particles of preexisting rocks. This category includes turbidites, wind-blown dust, and volcanic ash.
      2. Biogenous sediment consists of the remains of oceanic plants and animals. It includes siliceous oozes from diatoms or radiolaria, calcareous oozes from a variety of calcite-secreting organisms, and phosphate deposits from some shells, also teeth, bones and scales. Cold, deep ocean water contains more carbon dioxide than warm water nearer the surface, thus is capable of dissolving calcite particles which sink into it. Thus calcareous ooze suggests shallower water depth of deposition (above the carbonate compensation depth) than siliceous, calcite-free ooze.
      3. Hydrogenous sediment is material deposited by direct precipitation from seawater. Manganese nodules are a notable example of this type of deposit, because of interest in mining them. Interest in mining stems from the fact that the U.S. has almost no manganese, which is important in steel making, while Russia and China have much of world's reserves. The nodules form in deep water, where they grow at rates of 0.0002 mm/year or less.
   2. Distribution of Seafloor sediments
      1. Terrigenous sediments are thick and comparatively coarse-grained on continental shelves. Fine-grained abyssal clay is notable in low-bioproductivity areas.
      2. Calcareous oozes are prevalent in shallower areas of the ocean. Siliceous oozes occur in areas of especially high bioproductivity. In deeper waters, calcareous materials tend to dissolve because the CO2 content of the water is high enough to cause dissolution of calcitic material.
      3. Hydrogenous sediment is generally minor, notable only in a few areas of the ocean
      4. Some regions are so sediment-starved that trace amounts of space-dust (cosmogenic marine sediment) are easier to find.
   3. Sea-floor Sediments and Climatic Change: Records of past global climatic conditions may be recovered from sedimentary layers deposited on the ocean floor.
      1. Climate affects the type and abundance of life in the upper ocean, thus influences what type of fossils the sediment accumulated at that time contains.
      2. Climate also affects the chemical composition of ocean water, and the rates at which some chemical reactions occur. Besides influencing marine life and resulting fossil assemblages, this also can influence deposition of some types of chemical sediments.
   
   
8. RESOURCES FROM THE SEAFLOOR
   1. Energy Resources
      1. Oil and natural gas - Represent most of the value of current marine resources. The rocks drilled frequently are similar to marine rocks on land that produce oil. A major difference is the methods of exploration and production technology for offshore work.
      2. Gas hydrates - are a form of methane hydrate (natural gas) trapped in sediments under cold-deep waters. Some experts view this as a major energy source for the future.
   2. Other Resources
      1. Sand and gravel - are important as construction aggregate. Some such deposits are placers that contain economic amounts of gold, diamonds, tin or other dense minerals.
      2. Evaporative salts - are extracted from seawater by confining it to evaporation ponds (the classic way to obtain salt besides hard-rock mining). A few other materials such as magnesium and bromine are chemically extracted from seawater.
      3. Manganese nodules - could come in handy as an alternative source of manganese, cobalt and other metals if the usual supplies are cut off. Manganese is important in steel-making, and The U.S. has no significant domestic supply. However, no economically feasible way of utilizing the nodules has been developed yet.
   
   
   Last revised November 23, 2003 by M.A.Jordan