By Charlene K. Smith
Waves can also be caused by underwater earthquakes and by the pull of gravity of the moon and the sun. While standing on the seashore on a windy day pay attention to the waves rolling toward you. Does the waves rolling toward you feel like the whole sea is moving forward? As the wind passes over the water, the top of the water moves up and down. The water only appears to move forward.
Now try this experiment to get an idea how waves move.
You and your friend are holding a rope---you are at one end, your friend at the other end now jiggle your end of the rope. You will see wave shapes pass along the rope toward your friend. It can't move move forward, because you're holding it. Only the wave shapes are moving. The top of the wave is called the crest. the curved distance between two crests is called the trough (trawf).
Sea captain loved to say "we'll sail with the tide." What does this mean? What is a tide? Tides happens twice a day. The regular, daily movement of the sea is what we call tide. In the morning the ocean beach may be dry, until about six hours later. You can build your sand castle and have lots of fun until about six hours later, then the whole beach will be covered with water. The sea has risen, or the tide is "high" or "in". In about six hours after that, you can again build your sandcastle. The sand will be wet and littered with driftwood and other things left as the water went back down, Now the tide is "low"or "out". The rise and fall of the sea is caused by the turning of the earth and the tug of gravity from the moon and sun.
There are many moving streams of water in the sea called currents. Some currents are Equatorial Currents. They run by the sea's surface and can be seen. There are other currents that flow below the surface of the sea. These deep currents affect the climate and speed of ships. Ancient sailors were delighted to sail with a current than against a current. Surface currents support many kinds of sea creatures that are eaten by other sea animals bringing food to other parts of the sea.
Currents move mainly because of wind. They are also helped by the heat of the sun and the rotation, or spin of the earth. Ocean currents start at the equator. Strong winds called trade winds, push water westward. When a current reaches a continent, it is turned toward the north or the south. The currents north of the equator flow in a clockwise direction. Those south of the equator flow counter-clockwise.
(pronounced tsoo-nah-mee) are often incorrectly called a tidal wave. A tsunami is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earth quakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can attack coastlines, causing devastating property damage and loss life.
The changing earth is in constant motion, bringing fresh energy to refuel the existing interior and exterior of our planet. This lesson will help inquiring minds understand some of the secrets that surround us.
Study Guide for Tides and Waves