oceans and margins

Oceans and their Margins


Contents of Entire Course

The Oceans and /their Margins
Ocean Circulation
Ocean Tides
Ocean Waves
Shaping of Coasts
Coastal Deposits and Landforms
Coastal Evolution
Coastal Hazards
Protection from Shoreline Erosion

adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University



The Oceans

  • Cover about 71% of the surface of the Earth.

  •  
  • The greatest ocean depth of 11,035 m occurs in the Mariana Trench

  •  
  • Have an average depth of 3,800 m.

  •  
  • Have a present volume of about 1.35 billion cubic kilometers, but the volume fluctuates with the growth and melting of glacial ice.

  •  
  • Salinity, a measure of amount of dissolved ions in the oceans, ranges between 33 and 37 parts per thousand.

  •  
  • The dissolved ions have been concentrated in seawater as a result of chemical weathering (Na, Ca, Mg, S, K, Br, and HCO3 ) and degassing of the mantle by volcanic activity (Cl & S).

  •  
  • Seawater would contain higher concentrations of dissolved ions if some were not removed by chemical precipitation, plants and animals, and absorption onto clay minerals.

  •  
  • Salinity varies in the oceans (see figure 13.3 in your text) because:

  •  
  • Surface waters evaporate, rain and stream water is added, and ice forms or thaws.

  •  
  • Salinity is higher in mid-latitude oceans because evaporation exceeds precipitation

  •  
  • Salinity is higher in restricted areas of the oceans like the Mediterranean and Red Seas (up to 41 parts per thousand).

  •  
  • Salinity is lower near the equator because precipitation is higher.

  •  
  • Salinity is low near the mouths of major rivers because of input of fresh water.

  • The temperature of surface seawater varies with latitude, from near 0o C near the poles to 29oC near the equator. But restricted areas can have temperatures up to 37oC.
    Ocean Circulation
     
    Surface Ocean currents are result of drift of the upper 50 to 100 m of the ocean due to drag by wind. Thus, surface ocean currents generally follow the same patterns as atmospheric circulation with the exception that atmospheric currents continue over the land surface while ocean currents are deflected by the land. The surface currents have the following properties: Tides are due to the gravitational attraction of moon and to a lesser extent, the sun on the Earth. Because the moon is closer to the Earth than the sun, it has a larger effect and causes the Earth to bulge toward the moon. At the same time, a bulge occurs on the opposite side of the Earth due to inertial forces.
    These bulges remain stationary while Earth rotates.  The tidal bulges result in a rhythmic rise and fall of ocean surface, which is not noticeable to someone on a boat at sea, but is magnified along the coasts. Usually there are two high tides and two low tides each day, and thus a variation in sea level as the tidal bulge passes through each point on the Earth's surface. Along most coasts the range is about 2 m, but in narrow inlets tidal currents can be strong and fast and cause variations in sea level up to 16 m Because the Sun also exerts a gravitational attraction on the Earth, there are also monthly tidal cycles that are controlled by the relative position of the sun and moon. The highest high tides occur when the Sun and the moon are on the same side of the Earth (new moon) or on opposite sides of the Earth (full moon). The lowest high tides occur when the Sun and the moon are not opposed relative to the Earth (quarter moons). These highest high tides become important to coastal areas during hurricane season and you always hear dire predications of what might happen if the storm surge created by the hurricane arrives at the same time as the highest high tides.


    Ocean Waves
     
    Waves are generated by winds that blow over the surface of oceans. In a wave, water travels in loops. But since surface is the area affected, the diameter of the loops decreases with depth. The Diameters of loops at the surface is equal to wave height (h) Wave Base

    Motion of waves is only effective at moving water to depth equal to one half of the Wavelength (L/2). Water deeper than L/2 does not move. Thus, waves cannot erode the bottom or move sediment in water deeper than L/2. This depth is called wave base. In the Pacific Ocean, wavelengths up to 600 m have been observed, thus water deeper than 300m will not feel passage of wave. But outer parts of continental shelves average 200 m depth, so considerable erosion can take place out to the edge of the continental shelf with such long wavelength waves.

    When waves approach shore, the water depth decreases and the wave will start feeling bottom. Because of friction, the wave velocity (= L/P) decreases, but its period (P) remains the same Thus, the wavelength (L) will decrease. Furthermore, as the wave "feels the bottom", the circular loops of water motion change to elliptical shapes, as loops are deformed by the bottom. As the wavelength (L) shortens, the wave height (h) increases. Eventually the steep front portion of wave cannot support the water as the rear part moves over, and the wave breaks. This results in turbulent water of the surf, where incoming waves meet back flowing water. Longshore currents
     - Since most waves arrive at the shoreline at an angle even after refraction. Such waves have a velocity oriented in the direction perpendicular to the wave crests (Vw), but this velocity can be resolved into a component perpendicular to the shore (Vp) and a component parallel to the shore (VL). The component parallel to the shore  can move sediment and is called the longshore current. Beach drift
    - is due to waves approaching at angles to beach, but retreating perpendicular to the shore line. This results in the swash of the incoming wave moving the sand up the beach in a direction perpendicular to the incoming wave crests and the backwash moving the sand down the beach perpendicular to the shoreline. Thus, with successive waves, the sand will move along a zigzag path along the beach. Offshore Transport and Sorting
    Particles picked up by wave motion move downslope, but the deeper the water, the less energy is involved in wave motion, so smaller and smaller particles are moved farther off shore. This results in size sorting of sediment, with grain size decreasing away from coast.
     

    Shaping of Coasts
     
    Coast represents the boundary between sea and land. When waves hit the coast, they can erode by breaking up rocks into finer particles and abrading other rocks by flinging rocks, sand and water against them. Over time, the effects can be large. Sediment is moved and redeposited to increase the size of continental shelves. The effects on the land surface can be seen by examining the shore profile.
    Coastal Deposits and Landforms
    Coastlines represent a balance between wave energy and sediment supply. If wave energy and sediment supply are constant, then a steady state is reached. If anyone of these factors change, then shoreline will adjust. For example, winter storms may increase wave energy, if sediment supply is constant, fine grained beach sand may be carried offshore resulting in pebble beaches or cobble beaches. Due to input of sediment from rivers, marine deltas may form, due to beach and longshore drift such features as spits, bay barriers, and tombolos may form.
    Depositional Features along coasts.

    Coastal Evolution
    The shape of coast is controlled mainly by tectonic forces and meteorological conditions. Examples:
    The southern coast of Australiais a rugged coast with many sea cliffs and raised wave cut benches (marine terraces). This is what is called an emergent coastline and in this case is due to a recent episode of uplift of the land relative to the sea by tectonic forces.
    The rest of eastern coast, on the other hand, is a submerged coast, and shows submerged valleys, barrier islands, and gentle shorelines, all due to rise of sea level since last glaciation age (during glacial ages, seawater is tied up in ice, and sea level is lower; when the ice melts sea level rises).


    Coastal Hazards
    Protection from Shoreline Erosion
     
    Seacliffs, since they are susceptible to landslides due to undercutting, and barrier islands and beaches, since they are made of unconsolidated sand and gravel, are difficult to protect from the action of the waves. Human construction can attempt to prevent erosion, but human engineering cannot always protect against abnormal conditions. Humans construct such things as sea walls, breakwaters, and groins in an attempt to slow coastal erosion, but sometimes other problems are caused by these engineering feats. For example, construction of groin (a wall built perpendicular to the shoreline) can trap sand and prevent beach drift and longshore drift from supplying sand to areas down current along the coastline. These down current areas may then erode, causing other problems.

    free hit counter