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Tides are the daily or twice daily rise and fall of the oceans. Tides are caused by the gravitational pull of the moon and sun on the ocean.

Tidal diagram
Click to view larger and see the legend.

  1. Sun
  2. Earth
  3. Moon
  4. Pull of the Moon
  5. Pull of the Sun

The difference between high tide and low tide is called the tidal range. The biggest tidal range is found in the Bay of Fundy, Canada where sea level rises and falls as much as 16 m (53 feet) in just over 6 hours. The smallest tidal ranges are less than 1 m (3 feet).

Most of these sand bars are covered with water at high tide and exposed at low tide. River Tyne estuary, UK

The highest tides, called spring tides, are formed when the earth, sun and moon are lined up in a row. This happens every two weeks during a new moon or full moon.

Smaller tides, called neap tides, are formed when the earth, sun and moon form a right angle. This causes the sun and moon to pull the water in two different directions. Neap tides happen during a quarter or three-quarter moon.

The width of the shoreline strip that is affected by waves depends on the tidal range. A large tidal range means that a wide strip of land might be subjected to the force of the waves. If the tidal range is very small, all of the wave's energy will be concentrated in the same place.

Over time these rocks will be broken down into smaller and pieces by the force of the waves.  West Australian coast. © Richard Burt

The rise and fall of tides causes water William to move in and out of estuaries, bays and harbours. This movement is called a tidal current. When the tide is rising, water flows from the ocean into the bay creating a flood current. When the tide falls, water flows from the bay back into the ocean creating an ebb current.


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