The regular rise and fall of the ocean’s waters are known as tides. Along coasts, the water slowly rises up over the shore and then slowly falls back again. When the water has risen to its highest level, covering much of the shore, it is at high tide. When the water falls to its lowest level, it is at low tide. Some lakes and rivers can also have tides.

Causes of Tides

Tides are a result of the moon’s gravitational pull on the Earth. The closer objects are, the greater the gravitational force is between them. Although the sun and moon both exert gravitational force on the Earth, the moon’s pull is stronger because the moon is much closer to the Earth than the sun is. As a result, the moon’s gravitational pull causes the oceans to bulge on both the side of Earth closest to the moon and the side furthest from it, creating tides.

The moon’s ability to raise tides on the Earth is an example of a tidal force. The moon exerts a tidal force on the whole planet. This has little effect on Earth’s land surfaces, because they are less flexible. Land surfaces do move, however, up to 55 centimeters (22 inches) a day. These movements are called terrestrial tides. Terrestrial tides can change an object’s precise location. Terrestrial tides are important for radio astronomy and calculating coordinates on a global positioning system (GPS). Volcanologists study terrestrial tides because this movement in the Earth’s crust can sometimes trigger a volcanic eruption.

The moon’s tidal force has a much greater effect on the surface of the ocean, of course. Water is liquid and can respond to gravity more dramatically.

High Tides

The tidal force exerted by the moon is strongest on the side of the Earth facing the moon. It is weakest on the side of the Earth facing the opposite direction. These differences in gravitational force allow the ocean to bulge outward in two places at the same time. One bulge occurs on the side of the Earth facing the moon. This is the moon’s direct tidal force pulling the ocean toward it. The other bulge occurs on the opposite side of the Earth. Here, the ocean bulges in the opposite direction of the moon, not toward it. The bulge may be understood as the moon’s tidal force pulling the planet (not the ocean) toward it.

These bulges in the ocean waters are known as high tides. In the open ocean, the water bulges out toward the moon. Along the seashore, the water rises and spreads onto the land.

Low Tides and Ebb Tides

As Earth rotates along its axis, landmasses pass through the bulges in ocean waters where high tide occurs as well as the lowest points between these bulges, where low tide occurs. The flow of water from high tide to low tide is called an ebb tide.

Most tides are semidiurnal, which means they take place twice a day. For example, when an area covered by the ocean faces the moon, the moon’s gravitational force on the water causes a high high tide. As the Earth rotates, that area moves away from the moon’s influence and the tide ebbs. Now it is low tide in that area. As the Earth keeps rotating, another high tide occurs in the same area when it is on the side of the Earth opposite the moon (low high tide). The Earth continues spinning, the tide ebbs, another low tide occurs, and the cycle (24 hours long) begins again.

The vertical difference between high and low tide is called the tidal range. Each month, the range changes in a regular pattern as a result of the sun’s gravitational force on the Earth. Although the sun is much farther away from the Earth than is the moon, its high mass still affects the tides.

Because the Earth’s surface is not uniform, tides do not follow the same patterns in all places. The shape of a seacoast and the shape of the ocean floor both make a difference in the range and frequency of the tides. Along a smooth, wide beach, the water can spread over a large area. The tidal range may be a few centimeters. In a confined area, such as a narrow, rocky inlet or bay, the tidal range could be many meters. The lowest tides are found in enclosed seas like the Mediterranean or the Baltic. They rise about 30 centimeters (about a foot). The largest tidal range is found in the Bay of Fundy, Canada. There, the tides rise and fall almost 17 meters (56 feet).

When the moon is between the Earth and the sun, the side facing us is not visible. This is the new moon. When the Earth is between the sun and moon, the moon reflects sunlight. This is the full moon. When the sun, moon and Earth are all aligned during the new moon and full moon, the sun’s tidal force works with the moon’s tidal force. The combined pull can cause the highest and lowest tides, called spring tides. Spring tides happen whenever there is a new moon or a full moon and have nothing to do with the season of spring. (The term comes from the German word springen, which means “to jump.”)

In the period between the two spring tides, the moon faces the Earth at a right angle to the sun. When this happens, the pull of the sun and the moon are weak. This causes tides that are lower than usual. These tides are known as neap tides.

Tidal Features

Tides produce some interesting features in the ocean. Tides are also associated with features that have nothing to do with them.

A tidal bore occurs along a coast where a river empties into the ocean or sea. The tidal bore is a strong tide that pushes up the river, against the river's current. This is a true tidal wave. The huge tidal bore of the Amazon River is called the pororoca. The pororoca is a wave up to 4 meters (13 feet) tall, traveling at speeds of 15 kilometers (9 miles) per hour. The pororoca travels 10 kilometers (6 miles) up the Amazon.

So-called “red tides” also have nothing to do with actual tides. A red tide is another term for an algal bloom. Algae are microscopic sea creatures. When billions of red algae form, or “bloom,” in the ocean, the waves and tides appear red.

Finally, rip tides are not a tidal feature. Rip tides are strong ocean currents running along the surface of the water. A rip tide runs from the shore back to the open ocean. Rip tides can be helpful to surfers, who use them to avoid having to paddle out to sea. Rip tides can also be very dangerous to swimmers, who can be swept out to sea.

Intertidal Life

The land in the tidal range is called the intertidal zone. The intertidal zone is often marked by tide pools. Tide pools are areas that are completely underwater at high tide but remain as pockets of seawater when the tide ebbs. Tide pools are home to some of the ocean’s richest biodiversity.

The intertidal zone can be hard-bottomed or soft-bottomed. A zone with a hard bottom is rocky. A zone with a soft bottom has silt or sand. Wetlands and marshes are often soft-bottomed intertidal zones. Different creatures have adapted to different types of intertidal zones. Hard-bottom zones often have barnacles and seaweeds, while soft-bottom zones have more sea plants and slow-moving creatures like rays.

Intertidal zones are marked by vertical zonation. Different organisms live in different zones in the tidal range, depending on how much water reaches them. This zonation can often be seen vertically, with dry plants near the top of the tidal zone and seaweeds near the bottom.

The intertidal zone can be broken into four major mini-zones. The highest is called the splash zone (1). This area is splashed by water and mist during high tide, but is never fully underwater. Barnacles live on rocks in the splash zone. Many marine mammals, such as seals and sea otters, can live in the splash zone.

The high-tide zone (2) is pounded by strong waves. Animals that live in the high-tide zone often have strong shells and are able to cling tightly to rocks to avoid being swept out to sea. These animals include mussels and barnacles. Crabs, which have tough exoskeletons and can hide under rocks, also live in the high-tide zone.

The mid-tide zone (3) is usually the busiest part of the intertidal zone. This is where tide pools usually form. Animals from the high- and low-tide zones come here to feed. Animals that live in the mid-tide zone are still tough, but can have softer bodies than their neighbors in the high-tide zone. Brightly colored sea anemones, which are soft-bodied but strongly anchored to rocks, live in tide pools. Snails and hermit crabs use shells to protect their soft bodies.

Sea stars (sometimes called starfish, although they are not related to fish at all) are perfectly adapted to life in tide pools. They have a tough, leathery body that can withstand strong tides and waves. They have thousands of tiny, tube-like legs that help them stick to rocks or put them on the move for prey. Sea stars are carnivores, and will eat other animals, such as fish, snails, or crabs. They especially love mussels. The way sea stars eat is unusual. Sea stars move over a mussel and use their arms to pry open the mussel’s shell. Then, the sea star ejects its own stomach to surround the mussel. The sea star’s stomach contains powerful acids that dissolve the mussel and make it easy to digest when the sea star pulls its stomach back into its body.

The low-tide zone (4) is only dry at the lowest tide. Nudibranchs, a type of sea slug, live in tide pools in the low-tide zone. Like the sea star, this animal is a carnivore. Nudibranchs eat sponges, barnacles and other nudibranchs. Nudibranchs can also eat sea anemones, because they are immune to its poisonous tentacles.

People can be very active in the low-tide zone. Simple nets can catch fish here, and fishers can collect animals like crabs, mussels, and clams. “The tide is out, our table is set,” is a traditional saying among the Tlingit nation (tribe), who live along the Pacific Northwest coast in Alaska and Canada.

In the low-tide zone of the Puget Sound in the U.S. state of Washington, people practice tidal aquaculture. Aquaculture is the breeding, raising, and harvesting of plants and animals that live in the water. One of the most harvested animals is a giant clam called a geoduck. Geoduck farms have been set up in the Puget Sound tidelands, which are areas covered by the intertidal zone. On the farms, geoducks live in plastic pipes. Environmental groups worry about the impact of these pipes on the environment. Tools of aquaculture, such as unsecured pipes, nets, and rubber bands, can be washed away by tides. This debris can pollute the ocean, beach, and natural tide pools.

Tides and People

Tidal energy is a renewable resource that many engineers and consumers hope will be developed on a large scale. Now, small programs in Northern Ireland, South Korea, and the U.S. state of Maine are experimenting with harnessing the power of tides.

There are three different types of tidal power. All of these use tidal energy generators to convert that power into electricity for use in homes and industry.

In most tidal energy generators, turbines are put in tidal streams (1). A turbine is a machine that takes energy from a flow of fluid. That fluid can be air (wind) or liquid (water). Because water is more dense than air, tidal energy is more powerful than wind energy. Placing turbines in tidal streams can be difficult, because the machine disrupts the tide it is trying to harness. However, once the turbines are in place, tidal energy is predictable and stable.

Another tidal energy generator uses a type of dam called a barrage (2). A barrage is a low dam where water can spill over the top or through turbines in the dam. Barrages can be constructed across tidal rivers and estuaries. Turbines inside the barrage can harness the power of tides the same way a dam can harness the power of a river. Barrages are more complex designs than single turbines.

The final type of tidal energy generator is a tidal lagoon (3), which are bodies of water created by man-made enclosures built around parts of tidal bays or inlets. The lagoons function much like barrages, but are usually constructed out of more natural materials, like rocks.

Geographic information systems (GIS) must account for tides when mapping, especially when mapping the ocean floor. Tides affect the report on an area’s depth.

Predicting tides is very important for shipping and travel across oceans. Ships decide which channels they may navigate by calculating their own weight, the depth of the ocean and an area’s tidal range. Errors in navigation can strand ships along shores or on sand banks. Cargo can sit and spoil while waiting for a tide. This was not a significant problem after the 2004 tsunami in Southeast Asia. Even though the tsunami destroyed kilometers of coastline, GIS technology helped disaster-relief agencies get aid to victims in Indonesia, Thailand, and Sri Lanka.