Water Cycle - Oceans (2)
Pre Lab 

  • Investigating the Earth’s water cycle.  
  • Interpreting how water is recycled.
  • ocean
  • recycle
  • resource
  • salinity

Students use a worksheet to look at the world’s oceans.


The rain that falls on the land is slightly acidic due to carbonic acid (which forms from carbon dioxide and water). The water erodes the rock and the acid helps breaks down the rocks and carries it along in a dissolved state as ions. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.  The two ions that are present most often in seawater include chloride and sodium. These two ions are over 90% of all dissolved ions in seawater.  The concentration of salt in seawater (salinity) is about 35 parts per thousand. In other words, about 35 of 1,000  (3.5%) of the weight of seawater comes from the dissolved salts.

The water that forms the oceans is only part of the reason we have oceans.  Plate Tectonics or the moving of the Earth’s crust cause low and high parts of the Earth’s surface.  The low portions are the basins of the oceans.  The high portions of the surface are the continents.  Remember about 70% of the surface of the Earth is covered with water.  

Through geologic time ocean levels and its salinity have changed, as the Earth’s crust have moved.  Polar ice also takes fresh water out of the system, but when they melt they add more liquid.  The Earth’s water is a dynamic, changing system.

  1. Children may ask why there will never be any more water than we have now.  Why can't we just make more water?  Water is a naturally occurring substance that we can't make.  The amount of water on Earth will remain relatively constant in our life time.  Children may ask, "Where did the water come from?"  It basically came from steam that was emitted from volcanoes when the Earth was forming millions of years ago.  Our atmosphere trapped the gases and the water cycle started.
  2. Explain that although we may drink some of the same water that a dinosaur drank millions of years ago (the dinosaur would have to urinate and the water would evaporate), the water has gone through several states since that time.  It has evaporated into water vapor, condensed into clouds and fallen back to the Earth as rain and snow.  A natural recycling program, that requires time to clean the water.   In the process of “cleaning” salts are left behind.  Salt cannot evaporate.  So our oceans become salty.
  3. Students should use the worksheet to look at the different oceans.  Use the hydrographic globe to show students that the ocean bottoms have depth and topography. 

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