Key Concepts
Water and Seawater
- Earth is the only planet in our solar system with liquid
water on its surface.
- All known life forms on Earth depend on water.
- Water is unique among substances.
- The high surface tension of water, which results in
water "piling up" on a flat surface, is just one of the unusual
properties of water. Water's characteristics are important in determining
how water interacts with other substances.
- Water can dissolve more substances and greater quantities
of these substances than any other liquid.
- Ocean water contains salts and minerals that make it
different from fresh water.
- Most of Earth's water (a little over 97%) is in the
oceans. Freshwater may be the most precious and scarce natural resource
supporting humanity.
Density and Layers in the Ocean
- Salts dissolved in water increase the
density of water.
- The concentration (quantity) of salts in seawater is
expressed as salinity.
- Very small changes in salinity can significantly alter
seawater density.
- Objects or substances that have a higher density than
water sink and ones that have a lower density rise and float.
- Liquid water can also rise or sink if its density is
different from that of the surrounding water.
- Most substances have higher density in the solid phase
than in the liquid phase. Water is an exception to this rule. Ice floats
because it is less dense than water.
- The world's oceans may be thought of as having three
layers where water conditions such as temperature, salinity, and density
are essentially the same:
- Surface (mixed) layer - This layer is 100 to 500 meters
thick and waters are well mixed by winds and waves. The salinity and
temperature of this layer changes seasonally. This zone contains 2%
of the total ocean volume. It has the warmest and least dense water
in the ocean.
- Intermediate layer - Water density changes markedly
in this layer. Its depth is dependent on temperature and salinity and
whether it's in the open ocean or near a coastal area.
- Deep (bottom) layer - About 80% of the ocean's volume
is found in this layer. Low temperature, high salinity, and high density
mark it.
- Many water movements in the ocean are driven by differences
in density.
Salinity-Temperature-Pressure
- Water density is controlled by changes in temperature,
and salinity (concentration of dissolved salts and minerals), and (to
a lesser degree) pressure.
- Temperature is a measure of the amount of heat gained
or lost by an object or substance.
- Decreases in seawater temperature cause density to increase
uniformly due to the salinity of the water.
- Water is resistant to compression, therefore, only very
slight increases in density occur with an increase in pressure.
Bathymetry (submarine topography)
- The sea bottom was thought at one time to be flat and
featureless. Parts of the ocean bottom, however, are as uneven and rugged
as the familiar mountain
topography of land.
- The part of the ocean nearest the continentents is divided
into three parts:
- The continental shelf,
- The continental slope ( it's here that we find submarine
canyons),
- The continental rise.
- Beyond the continental margins lie deep ocean basins.
The topography here is very varied. The following bathymetric features
can be found:
- Abyssal plains
- Abyssal hills
- Seamounts
- Deep-sea trenches
- Mid-ocean ridge
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