• 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

    Forces that contribute to tides are called tidal constituents. The Earth’s rotation is a tidal constituent. The major tidal constituent is 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.

    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. The high tide on the side of the Earth facing the moon is called the high high tide. The high tide caused by the bulge on the opposite side of the Earth is called the low high tide. 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
    One high tide always faces the moon, while the other faces away from it. Between these high tides are areas of lower water levels—low tides. 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 almost 390 times 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).

    Twice each month, the moon lines up with the Earth and sun. These are called the new moon and the full moon. When the moon is between the Earth and the sun, it is in the sun’s shadow and appears dark. 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 lined up, 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.

    While a tidal bore is a tidal wave, a tsunami is not. Tsunami is taken from the Japanese words for “harbor wave.” Tsunamis are caused not by tides, but by underwater earthquakes and volcanoes. Tsunamis are associated with tides because their reach surpasses the tidal range of an area.

    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 anything, 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). The lagoons function much like barrages, but are usually constructed out of more natural materials, like rocks. Tidal lagoons can sit along coasts and do not prevent the natural migration of wildlife.

    Geographic imaging systems (GIS) rely on tidal calculations. 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.

    Raking in the clams.

    A Really High Tide
    The same gravitational force that creates a high tide can create a black hole. The moons tidal force pulls in the Earths ocean, creating a tide. At the right distance, a black holes tidal force pulls in everything in its pathincluding light. (And once youre in a black hole, there is no low tide!)

    Watch Out
    Tidal flatsthe low-lying areas that are underwater at high tide and dry at low tidecan be dangerous places. In soft-bottomed intertidal zones off Alaskas Pacific shore, for instance, the mud is several feet thick. People have wandered out onto the tidal flats, gotten stuck in the mud and drowned when the tide rushed in.

    Surfing the Dragon
    In rivers with strong tidal bores, surfing is a popular recreational sport. The worlds strongest tidal bore is on the Qiantang River in southern China. This tidal wave can be 9 meters (30 feet) high and travel at 40 kilometers per hour (25 miles per hour). Surfers rarely remain upright for more than 10 seconds. Athletes call surfing the Qiantang surfing the dragon.

  • Term Part of Speech Definition Encyclopedic Entry
    acid Noun

    chemical compound that reacts with a base to form a salt. Acids can corrode some natural materials. Acids have pH levels lower than 7.

    adapt Verb

    to adjust to new surroundings or a new situation.

    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

    algal bloom Noun

    the rapid increase of algae in an aquatic environment.

    anchor Verb

    to hold firmly in place.

    aquaculture Noun

    the art and science of cultivating marine or freshwater life for food and industry.

    barrage Noun

    a low dam.

    bay Noun

    body of water partially surrounded by land, usually with a wide mouth to a larger body of water.

    Encyclopedic Entry: bay
    beach Noun

    narrow strip of land that lies along a body of water.

    Encyclopedic Entry: beach
    biodiversity Noun

    all the different kinds of living organisms within a given area.

    Encyclopedic Entry: biodiversity
    canal Noun

    artificial waterway.

    cargo Noun

    goods carried by a ship, plane, or other vehicle.

    carnivore Noun

    organism that eats meat.

    Encyclopedic Entry: carnivore
    celestial Adjective

    having to do with the sky or heavens.

    channel Noun

    waterway between two relatively close land masses.

    Encyclopedic Entry: channel
    coast Noun

    edge of land along the sea or other large body of water.

    Encyclopedic Entry: coast
    complex Adjective


    construct Verb

    to build or erect.

    consumer Noun

    person who uses a good or service.

    coordinates Noun

    a set of numbers giving the precise location of a point, often its latitude and longitude.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    dam Noun

    structure built across a river or other waterway to control the flow of water.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    dense Adjective

    having parts or molecules that are packed closely together.

    digest Verb

    to convert food into nutrients that can be absorbed.

    disrupt Verb

    to interrupt.

    dissolve Verb

    to break up or disintegrate.

    dramatic Adjective

    very expressive or emotional.

    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

    ebb tide Noun

    tide that flows from high tide to low tide.

    eject Verb

    to get rid of or throw out.

    electricity Noun

    set of physical phenomena associated with the presence and flow of electric charge.

    engineer Noun

    person who plans the building of things, such as structures (construction engineer) or substances (chemical engineer).

    estuary Noun

    mouth of a river where the river's current meets the sea's tide.

    Encyclopedic Entry: estuary
    exert Verb

    to force or pressure.

    exoskeleton Noun

    the hard external shell or covering of some animals.

    fluid Noun

    material that is able to flow and change shape.

    geoduck Noun

    type of large, burrowing clam.

    geographic information system (GIS) Noun

    any system for capturing, storing, checking, and displaying data related to positions on the Earth's surface.

    Encyclopedic Entry: GIS (geographic information system)
    Global Positioning System (GPS) Noun

    system of satellites and receiving devices used to determine the location of something on Earth.

    gravitational pull Noun

    physical attraction between two massive objects.

    gravity Noun

    physical force by which objects attract, or pull toward, each other.

    hard-bottomed Adjective

    shore or coast with a rocky lower layer.

    harness Verb

    to control or guide for a specific purpose.

    hermit crab Noun

    type of marine animal (crustacean) that uses found materials, such as other creatures' shells, as its shell.

    high high tide Noun

    tide created when the Earth directly faces the moon.

    high tide Noun

    water level that has risen as a result of the moon's gravitational pull on the Earth.

    high-tide zone Noun

    coastal area that is partially submerged at high tide.

    industry Noun

    activity that produces goods and services.

    inlet Noun

    small indentation in a shoreline.

    intertidal zone Noun

    region between the high and low tide of an area.

    lake Noun

    body of water surrounded by land.

    Encyclopedic Entry: lake
    liquid Noun

    state of matter with no fixed shape and molecules that remain loosely bound with each other.

    location Noun

    position of a particular point on the surface of the Earth.

    Encyclopedic Entry: location
    low high tide Noun

    tide created when the Earth faces away from the moon.

    low tide Noun

    water level that has dropped as a result of the moon's gravitational pull on the Earth.

    low-tide zone Noun

    mostly submerged coastal area that is only exposed at low tide.

    mapping Noun

    making and using maps.

    marine mammal Noun

    an animal that lives most of its life in the ocean but breathes air and gives birth to live young, such as whales and seals.

    marsh Noun

    wetland area usually covered by a shallow layer of seawater or freshwater.

    Encyclopedic Entry: marsh
    mass Noun

    unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion.

    microscopic Adjective

    very small.

    mid-tide zone Noun

    coastal area that is underwater at high tide, partially submerged as the tide ebbs, and exposed at low tide.

    migration Noun

    movement of a group of people or animals from one place to another.

    mist Noun

    clouds at ground-level, but with greater visibility than fog.

    Encyclopedic Entry: mist
    moon Noun

    natural satellite of a planet.

    Encyclopedic Entry: moon
    navigate Verb

    to plan and direct the course of a journey.

    neap tide Noun

    the lowest level of high tide when the difference between low and high tide is the least, occurring when the gravitational pull of the sun counteracts that of the moon.

    new moon Noun

    dark phase of the lunar cycle when the moon is invisible or barely visible, occurring when the moon passes between the sun and earth.

    nudibranch Noun

    brightly colored marine organism (gastropod), also called a sea slug.

    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    planet Noun

    large, spherical celestial body that regularly rotates around a star.

    Encyclopedic Entry: planet
    poisonous Adjective

    toxic or containing dangerous chemicals.

    pollute Verb

    to introduce harmful materials into a natural environment.

    pororoca Noun

    local term for tidal bore, or tidal wave, especially of the Amazon River.

    precise Adjective


    predator Noun

    animal that hunts other animals for food.

    predict Verb

    to know the outcome of a situation in advance.

    prey Noun

    animal that is hunted and eaten by other animals.

    radio astronomy Noun

    the study of outer space using radio waves.

    ray Noun

    flat-bodied fish with fins that appear to flap like wings.

    red tide Noun

    the rapid, dense accumulation of algae or other plankton that contain red or brown pigments; also called algal bloom.

    renewable resource Noun

    resource that can replenish itself at a similar rate to its use by people.

    rip tide Noun

    strong current that runs along the surface of the ocean from shore to open sea. Also called rip current.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    rotation Noun

    object's complete turn around its own axis.

    Encyclopedic Entry: rotation
    sand Noun

    small, loose grains of disintegrated rocks.

    sand bank Noun

    large underwater deposit of sand, often tall enough to reach the water's surface.

    sea Noun

    large part of the ocean enclosed or partly enclosed by land.

    Encyclopedic Entry: sea
    sea anemone Noun

    type of marine animal related to corals and jellies.

    sea star Noun

    marine animal (echinoderm) with many arms radiating from its body. Also called a starfish.

    seaweed Noun

    marine algae. Seaweed can be composed of brown, green, or red algae, as well as "blue-green algae," which is actually bacteria.

    semidiurnal Adjective

    occuring twice a day.

    shell Noun

    hard outer covering of an animal.

    shipping Noun

    transportation of goods, usually by large boat.

    shore Noun


    significant Adjective

    important or impressive.

    silt Noun

    small sediment particles.

    Encyclopedic Entry: silt
    soft-bottomed Adjective

    shore or coast with a soft lower layer of silt or sand.

    splash zone Noun

    coastal area above the high-tide zone that is only submerged during storms.

    spoil Verb

    to rot or ruin.

    spring tide Noun

    tide occuring during the times of full and new moon that "springs" to above-average highs and lows.

    stable Adjective

    steady and reliable.

    starfish Noun

    sea star. Marine animal with multiple arms that can cling to rocks or move about. Sea stars are not fish.

    stomach Noun

    organ in animals that helps digest food.

    surfing Noun

    the sport of riding down a breaking wave on a board.

    Encyclopedic Entry: surfing
    surpass Verb

    to go beyond a set limit.

    technology Noun

    the science of using tools and complex machines to make human life easier or more profitable.

    tentacle Noun

    a long, narrow, flexible body part extending from the bodies of some animals.

    terrestrial tide Noun

    the effect of the moon's tidal force on land surfaces of the Earth.

    tidal bore Noun

    tidal wave. Tide flowing upstream against the current of a river, forming a wave of water.

    Encyclopedic Entry: tidal bore
    tidal constituent Noun

    force that helps create a tide.

    tidal energy Noun

    energy produced as ocean waters surge in and out with tides.

    Encyclopedic Entry: tidal energy
    tidal energy generator Noun

    machine for turning tidal energy into electricity humans can use.

    tidal flat Noun

    coastal wetlands, often found within the intertidal zone, formed when mud is deposited by tides.

    tidal force Noun

    gravitational pull exerted by one object, such as the sun or moon, that raises tides on another object, such as the Earth.

    tidal lagoon Noun

    pool of ocean water that is partially cut off from the ocean by a barrier. Often used as a source of hydroelectric power.

    tidal range Noun

    the difference in height between an area's high tide and low tide.

    tidal stream Noun

    an ocean current produced by the tide.

    tide Noun

    rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.

    Encyclopedic Entry: tide
    tidelands Noun

    intertidal zone. Region between the high tide and the low tide of an area.

    tide pool Noun

    small pond created by an ebb tide and submerged by a high tide.

    tribe Noun

    community made of one or several family groups sharing a common culture.

    tsunami Noun

    ocean waves triggered by an earthquake, volcano, or other movement of the ocean floor.

    turbine Noun

    machine that captures the energy of a moving fluid, such as air or water.

    uniform Adjective

    exactly the same in some way.

    vertical zonation Noun

    the arrangement of species adapted to different, layered areas of a larger ecosystem.

    volcanic eruption Noun

    activity that includes a discharge of gas, ash, or lava from a volcano.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano
    volcanologist Noun

    scientist who studies volcanoes.

    wave Noun

    moving swell on the surface of water.

    wetland Noun

    area of land covered by shallow water or saturated by water.

    Encyclopedic Entry: wetland
    wildlife Noun

    organisms living in a natural environment.

    wind Noun

    movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

    wind energy Noun

    kinetic energy produced by the movement of air, able to be converted to mechanical power.