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movEmENts of ocEaN WatEr
The ocean water moves constantly. Both the
temperature and salinity are responsible for

its constant motion. Waves, tides and ocean
currents are the different kinds of movements
of ocean water.

1. WavEs

The surface of the earth’s oceans is in constant motion, moving up and down in the form
of waves. If you have been to a beach, you have witnessed waves first hand.
Waves form as energy is transferred from molecule
to molecule across the surface. In general, the water
molecules move very little. It is only the energy that
actually moves. However, as waves break across a
shore or crash in violent storms, they can be the
source for great mixing of an ocean’s water.

Everything from earthquakes to ship wrecks create
waves; however, the most common cause is wind.
As wind passes over the water’s surface, friction
forces it to ripple. The rise and fall of water takes
place in a wave. The rise is called the crest and
the fall is known as the trough. The horizontal distance from one crest to another or from
one trough to another is known as the wave length. The vertical distance from crest to
trough is known as the wave height.
Wave length is the horizontal distance, either between the crests or troughs of two consecutive
waves. Wave height is a vertical distance between a wave’s crest and the next trough. Wave period
measures the size of the wave in time. A wave period can be measured by picking a stationary point
and counting the seconds. It takes for two consecutive crests or trough to pass it.
In deep water, a wave is a forward motion of energy, not water. In fact, the water does not
even move forward with a wave. If we followed a single drop of water during a passing wave,
we would see it move in a vertical circle, returning to a point near its original position at
the wave’s end. These vertical circles are more obvious at the surface. As depth increases,
their effects slowly decreases until completely disappearing about half a wavelength below
the surface.

The largest tsunami ever measured was 150 m high. These waves travel at a speed of more than
700 km per hour. The tsunami of 2004 caused wide spread damage in the coastal areas of India. The
Indira Point in the Andaman and Nicobar island got submerged after the tsunami.

tidEs

Several times during the day, the level of water at the ocean shore changes. This regular
rise and fall of waters in oceans and seas is called a tide. Tides are caused by strong
gravitational pull exerted by the sun and the moon on the earth’s surface. Moon has the
greatest effect on tides whereas the sun has about one half of the effect.

Social Studies
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24 Social Studies-7-7