1 The Ocean System

by E. Tucker Hirsch and Amanda P. Jaksha

T
he ocean covers approximately phenomena and may use such terms square miles (350 million sq. km). In
three-quarters of the surface interchangeably. Ways that students comparison, the United States only
of planet Earth and makes the are often taught about the ocean can covers about 6 million square miles
planet look distinctly blue from afar. create areas of confusion. For example, (9 million sq. km), and all of North
Phrases such as “the blue planet” and memorization of the names of different American only comes in at about 15
“the water planet” make it obvious ocean basins can lead to confusion million square miles (24 million sq. km)!
how important the ocean is to life and misunderstanding about the The ocean is a large and vast system
on Earth. Without the ocean, life interdependence of all oceans. This that dominates processes on Earth and
as we know it would not exist. This chapter will explore these concepts governs our experiences on land.
chapter explores some of the physical and areas of confusion, as well as If you were to drain all of the water
properties and processes that happen provide an overall understanding of out of the ocean, you would see a
throughout the ocean and along its the interconnected system and physical landscape of valleys, plains, basins, and
borders. Many students use terms such processes that govern the movement of mountain chains that looks similar to
as currents, tides, and waves to explain water on our ocean planet. the landscapes above the ocean. When
processes that take place in the ocean. talking about topography below the
Although these are commonly used Ocean or Oceans? surface of the ocean, we use words that
terms, students do not fully understand The area of water that covers the ocean may be less familiar, such as trench,
the processes that create such ocean basins spans more than 215 million ridge, and seamount. When you look

10 The Ocean System (v. 3)

 GRADE STANDARD EEI UNIT point in the ocean, which plunges to
more than 10,000 meters, or 6.7 miles.
Grade 3 3.1.d The Atlantic Basin is the second largest
3.1.1 The Geography of Where We Live in area. It has a large mountain chain,
Grade 4 4.1.4 known as the Mid-Atlantic Ridge,
running north-south through its center.
Grade 5 5.3.a Earth’s Water The Mid-Atlantic Ridge is more than
16,000 kilometers (9,900 miles) long,
Grade 6 6.2.c
6.3.a which is more than five times as long as
6.4.a; 6.4.d the Rocky Mountains chain! The Indian
Basin, bordered mostly by Africa,
Grade 7 Asia, and Australia, is the third largest.
8.8.a-d Finally, the Arctic Basin surrounds the
Grade 8
North Pole. The water that flows around
the edge of Antarctica is sometimes
closely at the topography of the ocean it Boundaries, such as water temperature considered the Southern Ocean,
becomes clear that all of the “individual” and density, exist in the ocean and can, encompassing the southern part of the
oceans that students memorize are, in therefore, restrict the movement of Indian, Pacific, and Atlantic Basins.
fact, part of one large interdependent marine organisms. As mentioned previously, a common
global ocean. Just as we create artificial Just as the continents have misconception among students is that
boundaries between countries on land, distinguishable characteristics, there are separate oceans. Traditionally,
we create artificial boundaries between the topography of the four main students learn names for oceans—
ocean basins. Water in the ocean flows ocean basins have distinguishable Pacific, Atlantic, Indian, and Arctic—and
between basins, through trenches and characteristics as well. they learn to locate these oceans in
over ridges, just as air moves around The four main ocean basins are the particular places on a globe. Naming
and through our mountains and Pacific, Atlantic, Indian, and Arctic. and locating oceans in such a way causes
valleys on land. In the same way that The Pacific covers the widest area and students to focus on the differences
seeds and pollen are carried by wind, has the largest average depth of the between them, as opposed to viewing
so too are some ocean species moved four basins. The Pacific contains the Earth as having one connected ocean
through patterns of ocean circulation. Mariana Trench, the deepest known with different basins.

CHAPTER OVERVIEW

The ocean is a global, interconnected system that has regions, or basins. In the Classroom:
Water and ocean life move between these basins and up and down the How Big Is the Ocean? 13
water column. Ocean water circulates around the globe through ocean In the Classroom:
currents that are either wind-driven currents or density-driven currents. Density 20
The circulation of water that is driven by differences in density is called
Student Thinking:
thermohaline circulation, also known as the global conveyor belt. What Causes Tides? 24
Additionally, waves are phenomena that do not move the ocean water;
Student Thinking:
rather, they are a result of wind energy acting on the surface of the ocean.
Ocean Currents, Tides,
Unlike currents and waves, tides are not caused by wind, density, or other and Waves 25
actors on our planet. Tides are caused by the gravitational pull from the Pictures of Practice:
sun and moon. Explaining Waves 26

This chapter explores these physical phenomena in the ocean and students’
common ideas about ocean currents, tides, and waves.

The Ocean System 1111

 Our ocean contains
trenches that are
miles deep and
seamounts that
rise miles above the
seafloor. The seafloor
is anything but flat!

One way to help students see that deep,” but may have trouble imagining gym and calculate how many gyms it
the oceans are interconnected is to the depth. Students’ ideas of “very deep” would take to equal a specific depth in
challenge them to decide where one may range from a few hundred feet to the ocean. Make sure to reinforce with
ocean ends and another begins. Use tens of thousands of miles. Students may students that while they are exploring
students’ questions during this task to lack experiences that help them visualize horizontal distances, some distances in
develop a dialogue about the arbitrary distances. Providing experiences with the ocean are measured on a vertical
borders that have been designated in distance can be helpful. plane.
oceans. Ask students to think about why One way to help students gain Using well-known features on land
these distinctions were made in the first experiences with distance is to have them may help students visualize depths as
place, how they may help us, and how measure off different distances. Using well. For example Mount Everest is
they may limit our understanding of one your school’s gym or outdoor areas, have only 8.9 kilometers (5.5 miles) high,
world ocean. students walk or run distances you have in comparison to the Mariana Trench,
In addition, it is important to help talked about in class. See if students which is 10.8 kilometers (6.7 miles)
students understand that the ocean can run or walk 30.5 meters (100 feet), deep. As most students have not seen
basins are not featureless plains, but 61 meters (200 feet) or more! Older Mount Everest, local examples may be
rather contain mountains, plains, students may be able to go even farther. even more powerful. You may want
canyons, trenches, hills, and more. Ask students if any of them have ever to compare depths to local mountain
Have students compare the features walked or run 11 kilometers (7 miles). ranges or tall buildings in cities close
found in the ocean to features found If so, was it easy? You can also use maps by that students may have visited. Some
on land. Encourage students to look for of the area surrounding your school to examples of ocean depths that students
similarities and differences. Students demonstrate these distances to them. can explore are that the average depth
likely realize that the ocean is “very Try having students measure the school’s of the ocean on the continental shelf is
about 150 meters (500 feet), the average

Teaching depth of the ocean overall is about 4,000

meters (13,000 feet), and the deepest
Tip places in the ocean are about 11,000
meters (36,000 feet) or slightly more
than 11 kilometers (just shy of 7 miles)!
As you cover the concept of Earth having one large, interconnected
Ocean Color. What causes the
ocean with many features, make sure you are consistent in your
ocean to appear blue? Is the water itself
language. Making sure to say Pacific Ocean Basin or Pacific Basin
blue? Through experience, students
instead of Pacific Ocean can exert a major influence on students’
understand that water is usually clear.
understanding of this difficult concept.
They associate the color of the ocean
with what they have seen, rather
than sediment, plankton, or other

12 The Ocean System

 In the
Classroom How Big Is the Ocean?

A
big challenge for teachers is helping students grasp just how vast the ocean really is. Students may struggle
with understanding the sheer size of the ocean, just as they struggle with large numbers and distances.
Activities such as the one following can help students visualize the ocean’s expanse. Teachers could
ask students to do research comparing the ocean or ocean basins to things with which they have experience. For
instance, how many Olympic-sized swimming pools would it take to fill up the Atlantic Ocean Basin? How long would
it take for a student to swim across the Indian Ocean Basin? How many times larger is the Indian Ocean Basin than
their home state? While questions like these still have large numbers in their answers, students can see that the ocean
is larger than anything else they are familiar with.

Materials
• Blow-up globe (beach-ball style)
• Space to toss a ball
• Chart and art supplies to record data

Directions
Students will toss a globe to one another and should be in a space that allows
free movement. Either standing or sitting in a circle or sitting on top of desks
works well.

Select an amount of time to record data (e.g., three minutes or once everyone
touches the ball five times). During that time, students should individually
count how many times one of their fingers (e.g., everyone’s right thumb)
touches an area of the ocean or an area of land. Alternatively students may count both sets of fingers for the
number on land versus water, for example, seven fingers may touch the ocean while only three fingers touch land.

At the end of the period, students add up and record, collectively, how many “ocean hits” and “land hits”
they observed.

Repeat the activity as time allows. For a variation, compare finger touches of land and the four different ocean
basin areas.

This is a good visualization activity for all ages, especially younger students. For older students, this activity can
also provide a good discussion about data collection, interpretation, and analysis. By repeating the activity and
taking averages, teachers can point out important patterns. The data can then be used to create pie charts, bar
charts, or other visual data representations. Students may also calculate ratios (7:3; ocean:land) and percentages
(70 percent ocean; 30 percent land).

Discuss
Did the data match your expectations? How did the data support or not support your ideas about the ocean size?

Look at how the ocean water connects. Why is it more accurate to refer to one ocean than to many oceans?

The Ocean System 13

 VISIBLE LIGHT IN OCEAN tone of the Yellow Sea. In the ocean, physical properties
Understanding why the ocean often distinguish water masses from each
sunlight appears blue requires that students other similar to air masses. Temperature
understand the visible light spectrum, and salinity as well as dissolved and
how different items transmit, reflect, suspended solids determine the density
0m
red refract, or absorb light wavelengths. of water. Water of different densities
infrared
For example, picture a glacial lake in creates layers, which are less prone to
orange

Colorado or the azure waters of the mix until disturbed. Ocean circulation
ultraviolet

50 Caribbean Sea. Deep, clear water, systems move water through the ocean
yellow

lacking in sediment and plankton, horizontally and vertically and on

appears a brilliant blue. Of the colors both local and global scales. The water
of the visible light portion of the in different basins may have different
100
electromagnetic spectrum, those characteristics, including temperature,
violet

green

with longer wavelengths, on the red density, and salinity.

portion of the spectrum, are most easily Wind-Driven Circulation. As
150 absorbed. The blues are most easily wind moves water along the ocean’s
scattered by water molecules, reflecting surface, it facilitates both horizontal
back into our eyes. Sediment, plankton, and vertical water movement. Vertical
and other particles also result in mixing is when water moves from the
200
scattering of light. For example, coastal depths of the ocean to the surface or
waters, which tend to contain high vice versa. Students may not realize
blue

concentrations of phytoplankton, appear that water in the ocean mixes vertically.

250 green due to the scattering of yellow and Vertical mixing brings cold, nutrient-
green light from these organisms. rich water from the depths of the ocean
The quality and quantity
of sunlight decreases with to the surface. This process is called
depth. Red, orange, and yellow Ocean Currents upwelling. Upwelling can occur along
spectrums are absorbed within Ocean water flows between basins at coasts of continents and is commonly
meters of the surface, while
blue penetrates the farthest. the surface and at various depths under seen along the coast of California.
the surface of the water. Although the Winds blowing from north to south
ocean is one body of water, water masses along the coast cause surface water to
contributors. For example, students within different basins may behave move offshore and away from land.
may have observed patterns in water in different ways. An analogy can be As surface water is blown away from
color related to depth, with lighter made to the air to better understand the land, water from below the surface
color typically indicating shallow water this. The air “fills up” the space above moves up and into the area that was
and dark blue meaning deeper water. our heads and in the sky and can be vacated, like on a conveyor belt. Coastal
Students can grasp that suspended thought of as one body of air. However, upwelling is very important to marine
algae; particles of sand, mud, or Los Angeles is infamous for its smog, ecosystems, providing nutrients that
dirt; and ice can impact the ocean’s and the Rocky Mountains are known help support robust food webs. Wind,
color. Once they discover that some for their clean, crisp mountain air. You therefore, not only affects the surface
phytoplankton—tiny photosynthetic can watch the movement from a breeze of the ocean but also has a significant
organisms—floating in the sea are as it flows across a prairie, a lake, or the influence on how water moves up and
green, as is common in the productive grass in your front yard. The open space down in the vertical water column.
near-shore regions, while others are red above your head is not uniform, and Prevailing winds can move ocean
(from which the Red Sea gets its name), L.A.’s smog and the Rocky Mountains’ surface waters long distances and
they understand relative coloration. air could be thought of as different air determine the paths of many of
Additionally, explaining that sediments masses: Each mass has its own distinct the ocean’s surface currents. These
exist in a variety of colors will help them properties and can stay somewhat local currents are so consistent that early
understand why water is sometimes but, when prompted, can move and mix sailors, explorers, and voyagers sailed
very unusual colors, such as the mustard with other surrounding air masses. in them to cross large expanses of

14 The Ocean System

 WIND-DRIVEN CURRENTS

Wind-driven upwelling
and downwelling currents

ocean and reliably arrive in the desired change. These changes can influence West Coast of the United States is the
location, long before GPS and accurate organisms that live in the ocean, from California Current, which is an eastern
navigational tools. Even today, ocean the smallest phytoplankton to the largest boundary current. This type of current
currents are integral to the shipping blue whale. The map on pages 18–19 is usually shallower and moves more
routes and trade industry that move shows some of the most significant slowly than the western boundary
goods all over the planet. ocean currents. These currents have currents. The California Current carries
Prevailing winds and ocean surface well-known characteristics. For example, cold, nutrient-rich waters south along
currents aid in horizontal mixing of the dominant current near the East the western coast of the United States
water masses. This mixing influences Coast of the United States is the Gulf from British Columbia until it reaches
the physical characteristics of the ocean, Stream, which is largely driven by wind. the southern California bight, or bend,
life in the ocean, and the global climate. It is a western boundary current, at Point Conception. There, the coastline
As water masses mix, their salinity, which is a type of current that is usually bends eastward, which keeps the current
density, and temperature also mix and deep, warm, and fast flowing. On the offshore as it continues to flow south.

PREVAILING WINDS

Prevailing winds
control how air
circulates around the
globe. However, the
prevailing winds are
affected by ongoing
moving air masses
that pass through the
area as shown in the
second figure.

The Ocean System 15

 A countercurrent (called the Davidson THERMOCLINE DIAGRAM
Current) flows from south to north
along the coast, hugging the coastline
and the Channel Islands, and bringing
warmer water from the waters near Baja.
Because of these very different currents,
the climate of northern California can
be vastly different from the climate of
southern California and Baja. Many
argue that the point where the two
currents meet, near Point Conception,
is the distinction between northern
California and southern California.
North of Point Conception, coastal
communities such as San Francisco
and Monterey experience cooler, foggy
summers due to the influence of the
cold California Current. South of Point
Conception, the warmer, Davidson
Current can keep the fog at bay, and the
summers are generally warm and sunny.
As water depth increases, the temperature decreases. This diagram shows
Density-Driven Circulation.
the thermocline layer that separates the warmer surface water from the
Density of ocean water can also drive colder deep water.
ocean circulation. Temperature and
salinity are two major factors that
determine water’s density. Cold water is water column. While much of the ocean particular depth, that difference is
more dense than warm water and salt is constantly in motion, a snapshot of called the halocline. The halocline is
water is more dense than freshwater. the water column at any given moment the point in the water column at which
Waters of different densities only mix if will show distinct layering. the water changes salinity significantly:
physical changes occur, such as changes The movement and mixing of The upper layer is less salty and,
in temperature or salinity. For example, water layers, driven by density, is therefore, less dense than the deeper,
as surface water cools, it becomes called thermohaline (heat and salt) denser, saltier layer. Sometimes, when
more dense and sinks through the circulation. A thin layer of surface snorkeling or diving in the tropics, a
water column, causing it to mix with water tends to be relatively more stable, swimmer can actually see the zone at
surrounding water. On the other hand, warmer, and less salty than the water which the water may be slightly blurry
if surface water warms, it can result in below it. When there is a distinct because the less salty and saltier layers
evaporation, which will lead to in an difference between the temperatures are trying to mix, but because of the
increase in both salinity and density, of water at a particular depth, the their different densities and salinities,
which also causes the water mass to boundary layer between them is the layers remain separate. This
sink in the water column. Water masses called the thermocline. A thermocline separation is sometimes referred to as
will continue to sink or rise in the water is the point in the water column at stratification. A strong thermocline or
column until they reach equilibrium which the water temperature changes halocline can prevent the layers from
(i.e., when the water below is denser, dramatically. Often, swimmers can mixing. In the ocean, stratification from
and the water above is less dense). If feel this in a lake or the ocean; their temperature or salinity can prevent the
new water is added—from sinking, arms and head are warm enough, but transfer of nutrients or biomass unless
upwelling, wind, and so on—that water their feet, down deeper, are much more a disturbance event occurs, such as a
will spread horizontally. Differences in chilly! storm or significant seasonal changes.
water density (resulting from salinity When there is a distinct difference Often the depth of the thermocline and
and temperature) create layers in the between salinities of water at a halocline will vary with the seasons for

16 The Ocean System

a particular ocean region, which can Teaching
impact local climate and the organisms
living, feeding, or breeding in that area.
Tip
Global Currents Before discussing density-driven ocean circulation, make certain your
Wind and water density drive the global students have a solid grip on the concept of density. Density can be very
current system that circulates the water difficult for students to comprehend because it relates to something
in the ocean all over the globe. These happening on the molecular level­—something they cannot see. Some
currents circulate not just the water simple activities to increase student comprehension of density include
but also anything that is in the water, showing students images of molecules in salt water at different salinities
such as plankton and debris (both or having students add salt to a cup of water and describe why the
natural and introduced), throughout the density is increasing (they are putting more “stuff” into the same volume
ocean. Although these currents remain of water). There are many other lessons that exist to help reinforce this
constant they are not always fast. It takes student thinking, such as the Cartesian diver activity. Once the concept
approximately 2,000 years for one drop of density and its relation to temperature and salinity are solidified,
of water to travel the entire ocean. While density-driven ocean circulation becomes much more easily grasped. See
the global currents are complex, there activity ideas at http://www.bigelow.org/shipmates/density.html or
are specific currents that remain stable, http://www.pbs.org/wgbh/nova/teachers/activities/2402_titanic.html.
as can be seen in the following image.
This image shows the major surface and
deep currents in the ocean basins. From
this diagram you can see how water and density and, thus, drive the global southern end of the globe! When it
everything in it can easily travel from current system. Water in the North reaches the Southern Ocean it mixes
one side of the planet to the other. Atlantic Basin is cold, salty, and dense. with more deep, cold, salty water from
The physical and chemical This water mass sinks and travels along Antarctica. Some of this water upwells
characteristics of water, such as the bottom of the Atlantic Basin away because it follows the contours of the
temperature and salinity, influence from the ice caps, all the way to the ocean bottom and rises at the edges

GLOBAL OCEAN CURRENTS

The ocean is in constant motion, driven by surface winds and controlled by water temperature and density. Great
landmasses guide the direction of water movement, creating an enormous conveyor-belt effect.

The Ocean System 17

18 The Ocean System
The Ocean System 19
 In the
Classroom Density

T
his activity, in which students investigate water masses, density, and mixed surface layers, will only be
successful if students follow the process carefully. In this activity, students can see the effects of density
on different water masses. Cold, salty, blue water will separate from warm, fresh, red water, and students
will be able to see two distinct layers.

Materials
• Two small, clear beakers per group
• One large, clear container per group
• Eye droppers, pipettes, or small turkey basters (one per group)
• Blue and red food coloring
• Salt
• Spoon
• Ice
• Water
• Straws

Directions
Provide students with hot water and ice water in two separate cups or beakers (about ½ liter, or 2 cups, in each
container). Add red food coloring to the hot water and blue food coloring and salt to the cold water, stir until the salt
dissolves. The salt water should be fully saturated (i.e., add salt until no more can dissolve in the cup).

Have students pour the red, hot, freshwater into the large clear container.

Have students add the blue, cold, salty water into the large clear container. The water must be added slowly so it does
not mix with the freshwater. Students can use a pipette or eye dropper, or they can carefully pour the water, letting it
run down the inside of the beaker. Another option is to set up a titration-type scenario in which the blue water gets
dripped or slowly streams against the side of the large, clear container. The slower the better! Teachers should practice
this beforehand so they have a sense of how slowly students need to work.

View the stratified water masses on a white surface and background to clearly see the distinction between
the two layers. The cold saltwater is more dense. The hot freshwater is less dense.

Using the straw, students can blow air across the surface of the water. Because of the different colors, students will
be able to see the top layers moving away from the straw, see it contact the wall of the container, and reflect away
from the wall and down into the bottom layer. Students can watch the layers mix.

Discuss
Ask students to describe in their own words what happened.

Ask them what would have happened if they had used the blue food coloring for the hot water and the red food
coloring for the cold water. Would the experiment still work? Would the results be the same? What if the hot water
was salty and the cold water was fresh—would that impact their results?

Ask students why it was important to pour the water so slowly.

20 The Ocean System

of the continent of Antarctica. Some Teaching
of it continues to move around the
globe or into the Indian Ocean Basin.
Tip
At the northern parts of the Indian
Basin, this cool, salty, North Atlantic What do rubber duckies, messages in bottles, and Nike shoes have
water mixed with the Southern Ocean in common? They have all helped scientists learn more about ocean
upwells as it comes into contact with circulation and ocean currents after being accidently lost by cargo ships.
Africa and India. As the water masses Ask students to think about what might happen to a box of rubber duckies
move away from the Poles and toward lost near Hawaii? What about near England? How far might these bath
the warmer waters of the Equator, they toys float before they make land? Encourage students to trace the paths
gradually warm and continue to rise to that such items could take using maps of ocean currents. Explain to
the surface due to the decreased density students that currents cause marine debris, such as plastic, to build up
of warm water. On the surface, they are in particular locations in the ocean. Encourage students to find out more
pushed through different ocean basins about this phenomena by researching the Great Pacific Garbage Patch
by currents from prevailing winds and online. Although there are other places in the ocean where this same
tides. As the water masses on the surface phenomenom occurs, this is one of the largest and most well-known cases.
return to the Poles, wind traveling over
the ice sheets cools the water and aids
in freshwater evaporation, driving it
back down and completing the global gravitational force. That force is felt all are all aligned and have created even
current. This is a simplified model of the the way through the planet. Therefore, larger bulges in line with the sun and
actual mixing and movement of water water in line with that pull on either moon, leading to deeper troughs where
that occurs, but it demonstrates how side of the planet bulges. While on the the water is pulled away. Therefore,
water, driven by differences in density far side of Earth, opposite the moon, when the gravitational forces of the
and the wind, moves through the ocean. it looks like the water is pulling away moon and sun are in the same plane,
Scientists refer to this simplified model from the planet, that bulge is an artifact there are extremely-high high tides, and
of the global ocean currents as the of the moon’s pull, a result of inertia. extremely-low low tides. (This occurs
global conveyor belt. Therefore, there are two points of high when we see a full or new moon.) This
tide at any moment: both points in line phenomenon is called a spring tide
Tides with the moon’s pull—one on each side (although it has nothing to do with
While currents are driven by physical of the planet. the spring season). When the sun and
and chemical properties of the ocean, As the moon pulls on the water in line moon are in line, the ocean experiences
tides are driven by forces acting on the with its gravitational force, the water the highest high tides and lowest low
entire planet. The moon and sun exhibit opposite the moon’s pull must follow, tides of the tidal cycle.
gravitational pulls on Earth. At the leading to an area of low tide. As there As the moon travels around Earth,
same time, the spinning and orbiting are two points directly opposite the the bulge of water follows the moon’s
of Earth create centrifugal force. These moon’s pull, there are two low tides at pull. When the moon and sun are at a
opposing forces push and pull on Earth’s any moment—each at approximate right 90 degree angle, the bulges are pulled
surface, creating ocean tides. As the angles to the high tide if looking down in opposite directions and not as
relative positions of Earth, moon, and on the planet from space. exaggerated. This scenario occurs during
sun change, the ocean bulges, creating This is the most basic tide. On the side the quarter-moon phase and produces
the tides. The gravitational pull of the of Earth closest to the moon, and on the what we call neap tides. During neap
moon is the dominant force in these side opposite the moon, the tide is high. tides the difference between the high and
interactions. Because of this, the tides Other places on Earth experience low the low tides can be small.
cycle along with the moon. or intermediate tides at the same time The level of the tides between different
Imagine Earth as a flattened sphere, depending on their relative locations. tidal scenarios, such as spring and neap
covered by a uniformly deep layer of To understand the effect of the sun in tides, is not static. As the angles between
water. Now imagine that the moon is this model, imagine the sun in line with Earth, moon, and sun are constantly
pulling the water toward it through the moon. Now the gravitational forces changing, so are the tidal levels. Just

The Ocean System 21

 Day 28
Spring tide
Sun
New Moon

Day 21
Neap tide
Last quarter

Day 14
Spring tide
Full Moon
Day 7
Day 0
Neap tide
Spring tide
First quarter
New Moon

Tides are caused by gravitational

forces from the sun and moon. The
highest tides occur when the sun,
moon, and Earth align.

as we can predict the moon cycles, we of more than 38 feet (10 m) between and shallow for long distances the tidal
can predict the tidal cycles. Because the high and low tides can occur on a daily range can be drastic. For example, the
tidal cycles follow the moon cycles, they basis. Other areas experience nearly northern end of the Gulf of California
repeat approximately every 28 days. imperceptible tidal changes every day. can have tidal changes up to 7 vertical
These images and descriptions Many areas, including California and meters (23 vertical feet). In comparison,
represent a very basic explanation much of the East Coast of the United the maximum tidal range for San Diego
of the tides and explain the driving States, have semidiurnal tides: a high is 2.4 vertical meters (8 feet). Although
force behind tides. However, the tide and a low tide occur twice daily. it is hard to see, tides affect the open
actual heights and timing of tides Other areas, such as the northern Gulf ocean as well. However, the effects are
vary throughout the ocean and along of Mexico, have diurnal tides in which not as significant.
our coastlines. If the perfect sphere only one high and one low tide occur
mentioned previously increases in daily. These cycles are predictable and Waves
complexity, with underwater mountain are often published in Tidelogs for a Most waves are created by the wind
chains, deep canyons, volcanoes, and year’s tides for a specific area. moving across the surface of the
other features, the bulge of the ocean Tides are most evident along water. The energy from the wind is
cannot move as smoothly around the coastlines, where rocks, sand, and being transferred to the water, and
sphere. This is the effect that continents other coastal features are visible during that energy moves through the water
and the topography of ocean basins low tides and are covered with water in waves that we can see. In the open
have on the tides. The depth, size, and during high tide. The shape of harbors, ocean, these waves are often swells and
underwater features of the ocean basin bays, and other coastal features also may not break until they come into
affect tidal cycles, as do the shape of influence the tidal range, the vertical contact with land. Closer to land, the
the coastline and the surrounding distance between low and high tide, as momentum of the swell as it travels
land features. Some areas experience measured along the coast. In some areas toward the coastline is often disturbed
extremely drastic tides—differences where the coast is extremely narrow by a sudden or gradual decrease in the

22 The Ocean System

depth of the ocean floor. This slows the Teaching
bottom part of the wave, but the upper
portion of the wave carries forward
Tip
with its momentum, causing the wave
to increase in height and eventually fall The use of a Slinky can provide students with a simple visual representation
forward, or break. This action could be of how energy travels through waves. Add a small piece of tape to the Slinky
compared to a person tripping. Imagine somewhere in the middle. Tell students the tape represents a single water
tripping on a doorstep that you did molecule. Have two students hold the spring. One student should play the
not see—your feet slow down and your role of the wind and move the spring up and down. As the Slinky moves,
head lunges up and then falls forward. students will be able to see the wave motion of the spring and view how the
As a wave approaches the shore water molecule (i.e., tape) moves in a small circle. This simple activity can
and breaks, water is moving vertically help alleviate the common confusion that waves transport water molecules
beneath the surface. The water in a from Japan all the way to California where they break upon the shore.
wave moves in a circular motion,
while the energy travels horizontally
and eventually reaches the beach. The
energy from a wave is released onto the of different conditions, including the mathematical equation that takes into
beach, creating sea spray and movement depth of the water, the speed of the consideration the height of the wave,
of rocks and sand. If you watch a bird wind, the length of time the wind has the depth of the water, and the speed
sitting on the sea surface, you can see been blowing, and the fetch of the wind, the wave is traveling.
it move up and down with the vertical or the distance over which it blows.
motion of the water, but the bird does These characteristics influence the wave
not experience horizontal motion. in the open ocean and as it approaches
At sea the height of a wave, referred the shore. The point at which a
to as a swell, is determined by a number wave breaks can be described by a

HOW WAVES ARE MADE

storm

wash zone
wind

Wind makes waves.

Individual water molecules
in the waves do not travel far.
Instead they move in circular
motions in a relatively small
water particle circulation
area, passing energy onto
adjacent water. This is what
we see as a wave. breaking wave

The Ocean System 23

 What Causes Tides?

S
tudents, especially those living near a coast, experience daily changes in their ocean.
They routinely hear about high tide and low tide, but students still struggle with
understanding what mechanisms cause tides to occur (Ballantyne 2004). They also
question why tides are sometimes more dramatic than at other times and why tides happen at
different times of day.

Scenario
You are teaching a set of lessons on tides to your students. Halfway through the lessons you hear
students sharing incorrect ideas during small-group work. You decide to conclude the day’s lesson
with a quickwrite because you want to see how many students understand that the gravitational
pull of the moon (and partially the sun) causes tides. The following is a sample of responses you
received from students.

Question
Explain as much as you know right now about what causes tides.

Scientific Answer
The primary mechanism that causes tides is the gravitational pull of the moon. The gravitational pull of the sun is
also a factor, as is the rotation of Earth. When the two pulls are aligned, there are more dramatic differences in tides
(tides are amplified). When the two pulls are offset, the difference in tides is less dramatic. Other factors, such as
local coastline and physical topography of the marine and land environment, will affect tidal activity.

Student Answers
Ryan: The tides change because Earth moves on its axis. And tides that are also created by tremors, underwater
volcanoes, things that set-off automatically.

Caleb: I think the gravity from the sun and the Earth moving around the sun makes the tides lower or higher, and
then when we get farther from the sun they’re lower, and when we are closer to the sun, the tides get higher. When
we’re spinning around the sun in the wintertime, the tides are normally lower because we’re farther away from the
sun, and in the summertime they’re higher because we’re closer to the sun.

Leslie: The tides are drawn by the moon.

Alice: I don’t really know a lot about tides, but there can be high tides or low tides. I think the weather causes tides.
Jackson: Low tide is when the water is pulled back. That’s usually caused by when there is a new moon.
Meghan: If it’s winter the tide would be high, just with the weather I guess, because it’s colder and then the
summer it would be a lower tide because it’s warmer outside.

What Would You Do?

What concepts do students understand at this point? What are the incorrect ideas that students are still retaining
even after a couple of lessons about tides?

Which misconceptions would you want to address? How would you do this?

24 The Ocean System

 Ocean Currents,
Tides, and Waves

S
tudents typically are not clear how currents, tides, or waves differ. They associate information–often
incorrectly–about depth, the moon, and gravity and their effects on currents, tides, and waves. When
asked what the difference among the three is, the confidence and clarity of students’ responses waiver.
Occasionally, tectonic activity is included in students’ associations of these physical processes (i.e., confusion of the
difference between a tidal wave, or tsunami, and a tide). When asked what causes currents, students often refer to
the wind as a driving factor, or they may claim currents are caused by moon’s gravitational pull (confusing currents
and tides). Students do not generally think about tides, currents, or waves specifically as global or local processes.

Common Student Ideas Scientific Concepts

Currents Currents move in one direction A specific current has a standard direction
of flow, but changing winds and interaction
with other currents can affect its motion. See
pages 17–19.

Tides Tides are a local process in which water goes Tides are caused primarily by the
up and down at different times in the day. moon’s gravitational pull on the waters of
Something pushes the water ashore. Some Earth. (Simanek 2009) See page 22.
students may see that gravity is involved but
identify the wrong external factor. High tide
brings larger waves.

Waves Waves are caused by boats or ships on Wind transfers its energy to the water as
the water or storms and strong winds. it travels across the surface of the ocean.
Waves move in one direction—onshore This kinetic energy of wind motion is
to the beach. translated into wave motion. Strong winds
that accompany a storm can create large
waves. Waves move in the direction of
the wind creating them. Wind blowing
to the shore creates waves that travel
approximately toward the shore and vice
versa. See page 23.

Ask Your Students

Explain how a drop of water on the California coast would travel to Japan. What are other places it
could travel?

Where does the water go during low tide? Why does it go there?

Where do waves start? What determines the direction a wave will travel?

The Ocean System 25

 Explaining Waves

S
tudents in coastal areas may have daily or weekly experiences
with currents, tides, and waves. They may drive by a coast and see
waves coming to shore. They hear about times that low tide and
high tide are expected to occur. They may go fishing or surfing or hang out
at a beach with friends or family. When they are at the beach, they know
about areas to avoid swimming near because of rip currents or undertows.
Even students living in inland communities have experiences with coasts,
either through trips to a beach or what they see in movies. Students may
have a difficult time understanding the different mechanisms that drive rip
currents and undertows (see Student Thinking: Ocean Currents, Tides, and Students: Grades 5 and 7
Waves, on page 25, for more information). While waves are relatively easier Location: California
to understand compared to currents and tides, students still struggle with (in coastal communities)
identifying wind as the driving mechanism. Goal of the Video: The goal of
watching this video is to hear
Classroom Context students describe what causes
waves and compare these ideas
The interview clips shown in this video were taken during the spring of the
to a scientific explanation.
school year after both sets of students learned about the ocean. The first
part of the video shows fifth-grade students describing mechanisms that
cause waves. The second half of the video shows seventh-grade students answering the same question. Think
about the different types of responses you hear from students in the same grade as well as differences between
grade levels. How do these compare to a scientific explanation?

Video Analysis
In this video, fifth and seventh graders were asked the same question: What causes waves? A scientific
explanation of waves focuses on wind moving across the surface of the water. Dominant wind patterns
determine the direction of the waves. Tsunamis are a very specific kind of wave initiated by ocean floor activity,
such as earthquakes on the ocean floor. But most waves are wind-driven. As you listen to the fifth-grade and
seventh-grade students describe waves, think about how their answers match or do not match the scientific
description of waves. For example, some students point to gravitational pull or Earth’s rotation as driving waves.
These students seem to be confusing mechanisms that cause tides with mechanisms that cause waves. Other
students seem to have more developed ideas but could still improve their understanding to be more in line with a
scientific explanation. As you listen, compare each student’s answer to the scientific description and think about
how to help each student improve his or her understanding.

Reflect
What patterns do you see in student ideas about waves?
What ideas seem to be common across students? Which misconceptions would you choose to address?
How would you do this in your own classroom?

26 The Ocean System

References
Ballantyne, R. “Young Students’ Conceptions of the Marine Environment and Their Role in the Development of Aquaria
Exhibits.” GeoJournal, 60.2 (2004): 159–63. Print.

Earle, Sylvia. The World Is Blue: How Our Fate and the Ocean’s Are One. Washington, DC: National Geographic Society, 2009.
Print.

Simanek, D. E. “Tidal Misconceptions.” “ Lock Haven University. 2009. Web. http://www.lhup.edu/~dsimanek/scenario/tides.htm.

May 25, 2010.

Teaching Resources
Fuller, K. Cartesian Diver Lab Activity, 2006. http://learn-science.20m.com/diver-lab1.html

California Education and Environmental Initiative resources: http://www.calepa.ca.gov/Education/EEI/default.htm

COSEE-West, with the College of Exploration online workshops: http://www.usc.edu/org/cosee-west/resources.html#oos0809

Drain the Ocean. DVD. National Geographic Society, 2009. Media.

National Geographic interactive on ocean expeditions: http://ocean.nationalgeographic.com/ocean/explore/expedition-tracker/

National Geographic Ocean Education materials: http://www.nationalgeographic.com/geography-action/oceans.html

National Geographic Society. Ocean: An Illustrated Atlas. Washington, D.C.: National Geographic Society, 2008. Print.

NASASciFiles’s Channel: The Case of the Ocean Odyssey: http://www.youtube.com/user/NASASciFiles#p/c/65770374240A3EB9

NOAA animation for upwelling and thermohaline circulation: http://www.learningdemo.com/noaa/lesson08.html

NOAA tutorial on tides: http://oceanservice.noaa.gov/education/tutorial_tides/tides01_intro.html

The Ocean System 27