Name
CHAPTER 25
Class
Date
Movements of the Ocean
3 Tides
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How does the gravitational pull of the moon cause
tides?
• How are spring tides and neap tides similar and
different?
• How do tidal oscillations affect tidal patterns?
• How does the coastline affect tidal currents?
What Causes Tides?
If you stand on a beach for a long time, you can see the
water level slowly change. This change in the water level is
called the tide. High tide is when the water level is at its
highest. Low tide is when the water level is at its lowest.
Gravity causes most tides. As the moon revolves around
Earth, the force of gravity between the moon and Earth
pulls on the ocean water. The moon’s gravitational force is
strongest on the part of Earth that is closest to the moon.
Therefore, the water on that side bulges outward, toward
the moon. That side of Earth has a high tide.
At the same time, the ocean water on the far side of
Earth is not pulled as much toward the moon. Therefore, it
bulges away from the moon. It forms a high tide, too.
As ocean water flows toward the area of high tides, the
water level in other areas drops. Therefore, low tides form
halfway between the two high tides, as shown below.
READING TOOLBOX
Organize As you read this
section, underline the main
ideas and important terms.
When you finish reading the
section, create a concept
map about tides using
the ideas and terms you
underlined.
READING CHECK
1. Explain Why are there
two tidal bulges?
Low tide
High
tide
High
tide
Moon
North Pole
Critical Thinking
2. Predict If the moon were
on the opposite side of Earth,
where would the tidal bulges
be?
Low tide
Because of Earth’s rotation, most places on Earth have two high tides and two low
tides every day.
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Movements of the Ocean
Name
SECTION 3
Class
Date
Tides continued
How Does Earth’s Rotation Affect Tides?
Math Skills
3. Calculate One day, the
high tides on one coastline
are at 9:56 A.M. and
10:24 P.M. About what
time will the high tides be
the next day?
Earth rotates on its axis once every 24 h. All areas
of the ocean pass under the moon every 24 h 50 min.
Therefore, the times of high tide and low tide in an area
are about 50 min later each day.
Because there are two tidal bulges, most places have
two high tides and two low tides each day. A tidal range
is the difference between the water level at high tide and
at low tide. The tidal range is different in different places.
How Does the Sun Affect Tides?
Critical Thinking
4. Describe When do the
lowest low tides occur?
The moon’s gravitational pull is the main cause of
tides. However, the sun’s gravitational pull can also
change the height of the tides.
The pictures below show the positions of the sun,
Earth, and moon during spring tides and neap tides.
Spring tides happen when the daily tidal range is larger
than normal. Neap tides happen when the daily tidal
range is lower than normal.
Sun
Sun
New Moon
Full Moon
Moon
LOOKING CLOSER
5. Identify During which
moon phases do spring tides
happen?
Spring tides happen when the sun, the moon, and Earth are aligned. The sun’s
gravitational pull and the moon’s gravitational pull combine. This stronger pull
makes high tides higher and low tides lower.
Sun
Sun
Third-quarter Moon
First-quarter Moon
Neap tides happen when the sun, moon, and Earth form a 90° angle. The sun’s
gravitational pull acts against the moon’s gravitational pull. The weaker pull makes
high tides lower and low tides higher.
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Movements of the Ocean
Name
SECTION 3
Class
Date
Tides continued
How Are Tides in Different Areas Different?
Tides are different in different parts of the ocean.
The size, shape, depth, and location of a body of water
affect the tides there. For example, the eastern coastline
of North America has two high tides and two low tides
every day. However, along the Gulf of Mexico, only one
high tide and one low tide occur each day.
Tidal oscillations also affect tidal patterns. Tidal
oscillations are slow, rocking motions of ocean water.
They happen as tidal bulges move through the oceans.
Tidal oscillations are not obvious in the open ocean
or near straight coastlines. However, in some seas, tidal
oscillations can reduce the effects of tidal bulges. As a
result, these seas have very small tidal ranges. In other
bodies of water, tidal oscillations can strengthen the
effects of tidal bulges. As a result, these seas have very
large tidal ranges.
READING CHECK
6. Contrast How are tides in
the Gulf of Mexico different
from tides along the eastern
coast of North America?
LOOKING CLOSER
7. Infer How would these
pictures be different if tidal
oscillations did not exist?
The Bay of Fundy in Canada has a tidal range of 15 m. This large tidal range is
caused by tidal oscillations.
What Are Tidal Currents?
While the tide is rising, ocean water moves onto the
shore. When the tide falls, ocean water moves away from
the shore. These movements are called tidal currents.
When a tidal current flows toward the coast, it is called
flood tide. When the tidal current flows back toward the
ocean, it is called ebb tide. The time period between flood
tide and ebb tide is called slack water.
When a river flows into the ocean, a tidal bore can
sometimes form. A tidal bore is a surge of water that
rushes upstream. Some tidal bores are large waves that
move very quickly until they lose energy.
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Movements of the Ocean
Name
Class
Date
Section 3 Review
SECTION VOCABULARY
tidal current the movement of water toward and
away from the coast as a result of the rise and
fall of the tides
tidal oscillation the slow, rocking motion of
ocean water that occurs as the tidal bulges
move around the ocean basins
tidal range the difference in levels of ocean
water at high tide and low tide
tide the periodic rise and fall of the water level in
the oceans and other large bodies of water
1. Describe How can tidal oscillations affect tidal ranges?
2. Explain How does the moon cause tidal bulges to form?
3. Infer If you were in a boat on the open ocean, would you be able to see a tidal
bore? Why or why not?
4. Compare Complete the Venn diagram below to compare spring tides and neap
tides.
Spring tides
Neap tides
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Holt McDougal Earth Science
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Movements of the Ocean