What Causes Tides?
Tides are created by the gravitational pull of the moon and, to a lesser extent the sun, upon the Earth's oceans. The sun has about half the effect that the moon has on the Earth's tides.
In most areas, typically two high tides and two low tides occur within a 24 hour and 50-minute lunar day. (A lunar day is 50 minutes longer than a solar day.) The time between each high and low tide (or low and high tide) is 6 hours and 12.5 minutes.
In most coastal regions, one of the daily high tides is higher than the other high tide and one of the low tides is lower than the other.
Some regions, such as the Gulf of Mexico, only have one high tide and one low tide each day.
During a new or full moon, both the sun and moon are in line with the Earth. The combined gravitational effect causes extremely high-high and low-low tides. The tides during this time are referred to as spring tides.
During the quarter moon portions of the lunar cycle, the sun and moon are at right angles in relation to the Earth and the resulting tidal range is smaller. The tides during the quarter moon cycles are called neap tides.
A region's wind and weather patterns can alter tide height.
The shape of coastlines can also affect tides. The Bay of Fundy, off Nova Scotia, features the greatest tidal range, which measures more than 15 m (49 ft.). This bay is funnel-shaped, which increases the magnitude of the tides.
Islands that lie the furthest from continents have the smallest tidal ranges of up to about 1 m (3.3 ft.).
The difference between the highest and lowest tides is called the tidal range. When people speak of the tide as being "in" or "out," they are referring to the cyclic changes of the water level within this tidal range. The intertidal zone is the shore area within the tidal range. This zone gets exposed to a wide extreme of conditions. Although the intertidal zone is a relatively limited area, a wide variety of plants and animals make their homes here.
Scientists have divided the intertidal area (also called the littoral zone) into four main zones:
- splash zone
- The splash zone is never covered by water and is only sprayed with saltwater during high tides.
- high intertidal zone
- The high tide zone is covered with saltwater only during high tides.
- middle intertidal zone
- The mid tide zone is covered and uncovered twice a day by the tides.
- low intertidal zone
- The low tide zone is only uncovered during the lowest of tides.
Below the intertidal zones lies the subtidal zone where the animals and plants are rarely exposed to air.
Tide pools can be found at any of the different intertidal zones; yet, most occur in the low intertidal.
If you visit the beach at low tide, you can observe the many different kinds of creatures that live in tide pools. The best time to explore a tide pool is daytime, during a minus tide—an unusually low tide that drops below the average low tide line.