12-1    Composition of sea water:

1. 3.5% dissolved mineral substances.

2. Salinity: a quality describe the content of dissolved salt, e.g. 3.5 %.

3. Major species: Cl^-, Na⁺, SO₄^2-, Mg²⁺.

4. Sources of dissolved salt?

   • Chemical weathering of rocks.

   • Volcanic eruption <== Out gassing.

12-2    Resources:

1. Salt: evaporation.

2. Water: desalination for sea water to be used.

3. Metals: from manganese modules.

12-3    Layers of sea water:

1. Surface zone: mixture of waters due to waves and turbulant current. ~ 450 m. Sun's maximum radiation depth.

2. Transition zone: T decreases as depth increases. Sometimes called thermocline. < 1500 m.

3. Deep zone: > 1500 m. T ~ 4 degrees. Constant.

12-4    Topography of ocean floor:

1. How to determine? Use echo sounder. Sound wave transmitted, reflected from the bottom. If we know the speed of sound wave, we measure the time when echo gets back, we can determined the depth.

   • Ocean floor map: Fig.12.10.

2. Continental margins. Fig. 12.12:

   • Continental shelf: extension of continent, gently sloping. Average 2 m/km. 130 m deep, 80 km wide, really flat.

   • Continental slope: 70 m/km.

   • Continental rise: gradual incline. ~ 4-8 m/km.

   • Submarine canyons: valley like grand canyon exist. Some are seaward extension of river valley. Others are deeper. A place where turbidity current occurs.

   • Turbidity current: 

     • Dense sediment-rich flow.

     • Originated from the continental slope. Form deep sea fans. Fig. 12.14.

     • The sediments called turbidites. Fig. 12.14.

     • Important characters: 

     • Evidence: study of the breakout of transatlantic cables showed a significant time lapse between the earthquake and the break. Also, deep sea sediment have fragments of shallow water plants and sediments.

3. Ocean basin floor:

   • Deep ocean trenches:

     • Narrow troughs

     • show plate tectonic activity

     • parallel to plate boundaries

     • parallel to the volcanic island arcs.

   • Abyssal plains:

     • deep > 4500 m

     • really flat

   • Seamounts: volcanic peaks isolated on ocean floor.

   • Guyots: flat-topped seamounts.

4. Mid-Ocean Ridge:.

   • Occurred in the center of oceans.

   • A place where magma activity and faults very common.

   • A place where new oceanic crust is formed.

   • Sea floor spreading center.

   • Symmetric in Atlantic Ocean, but not in Pacific.

   • 2500-3000 m above ocean floor. Maybe 500-5000 km wide.

5. Coral reefs and atolls:

   • Coral live in warm water, need Sun. Therefore, reef-building coral at top 45 m. 

   • Atools: above sea level at rim, below in center (lagoon).

   • Depth of up to 1000 m.

   • How to form? Volcanic island subside, coral grow on top of it. Process can be see in Fig. 12.18.

6. Sea floor sediments:

   • Terrigenous sediments: derived from land. Accumulated slowly. Why> because sediments settle near shore.

   • Biogenous: calcareous ooze and siliceous ooze.

   • Manganeses nodules.

Homework:

• Read chapter summary on p.343.

• Use your own word to explain the key terms on page 344.

• Answer the review questions on page 345.