Cause and Effect: Tides (2024)

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

sea

coast

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 thou

sands

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.