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
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.
- 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.
- 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.
- 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.