BACKGROUND:

The ocean system is complicated. The water in the oceans is salty because as the surrounding rocks erode(land and under the oceans).  Erosion causes the release the minerals and elements that make up the rocks.  The minerals are dissolved in water and are  in solution until either chemical or biological reactions in the oceans precipitate the minerals.  For instance, radiolarians, a marine protozoa, can take Si + O dissolved in the marine water and create a siliceous skeleton of a water-based quartz (opaline SiO₂+ H₂O).   

Erosion of the land also brings sediments to the oceans bottoms via rivers.  The sediments drape the landscape of the ocean’s  bottoms, sometimes hiding the topography of the ocean’s floors.  The topography of the ocean floor is as varied as the continental topography. 

Although we have known that the underwater topography (bathymetry)  is varied since the 1870's, most people do not even think about the seafloor.  Students always see water and think that the bottom is like a bowl, smooth and curved.  It has been determined that the ocean basins have an average depth of almost four kilometers. Other topographic highs include the mountainous ocean ridges, deep-sea trenches, jagged, linear fracture zones,  abyssal hills, and   seamounts.  The continents are just the highest of the ridges.  Sea level is just the arbitrary point at which the oceans meet the land.  Above sea level we call it topography and below is called bathymetry. Topography is measured as positive height, whereby bathymetry goes deeper from sea level.

PROCEDURE:

1. Discuss with students the physical shape of the ocean's floor, pointing out that 70-71% of the surface of the Earth is covered with water.  Valleys and mountains can be found below the water.  Use the physiographic hydrologic globe to show students the bottom of the oceans.  The oceans are really just land  that is lower than sea level.  Use the worksheet on the ocean’s basins to see if you can find the ridges and trenches. 
     
2. Many elements are dissolved in seawater.  You may want students to guess which elements are in water.  Use the worksheet on “Elements in Seawater” to illustrate which ones  are dissolved in seawater.  Students will be surprised that there are so many elements.  Some of these elements come out of solution in various forms which can be used for economic purposes.  Students in lab will see manganese nodules which are potato-size masses of manganese, cobalt, and nickel.  Salt is also produced by solar evaporation of salt ponds, that precipitate sodium chloride. 
    
3. Answers: 

1. H, Na, Mg, K, Ca, C, O, S, Cl, Br;
2. Sr, B, Si, F
3. He, Ne, Ar, Kr, Xe, Rn, N, and O
4. oxygen
5. Pacific, Bering, Canada, Siberia, Eurasia, Greenland, Labrador, NFD, Wes Europe, Norway, North America, Iberia, Canary, Cape Verde, S. Leone, Mexico, Yucatan, Caribbeam, Guatemala, Peru, Chile, Bellingshausen, Argentine, Brazil, Guiana, Guinea, Angola, Cape, Agulnas, Mozambique, Weddell, Crozet, Somali, Mascarene, Arabian, Bengal, Wharton, Australia, Wilkes, Tasman, Philippine, South Cina, Japan, Okhotsk
6. Atlantic, mid-Atlantic Ridge
7. Fracture zones caused by uneven movement of the crust. 

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