Your assignment is to listen to the recording and read this webpage as you listen. You are also supposed to copy the notes at the end of each recording. When you finish copying the notes, go back over the reading material and study the pictures and charts so that you understand what you have read. You may also want to listen to the recording again. This page has been modified since I made the mp3.

TIDES   Listen to the MP3

Although the wind plays a major role in most ocean motion, it is not the predominant cause of tides, the slow, periodic rise and fall of ocean waters.

The water level of the ocean rises and falls throughout the day. Early in the day, ocean water rises and covers part of the beach. Later in the day, the ocean level falls. The beach is exposed. These regular changes in ocean water levels are called tides. A low water level is called low tide. A high water level is called high tide.

A tide is a rise or fall of the ocean’s surface caused mostly by the gravitational pull of the moon. The gravitational pull of the sun also affects a tide, but not as much as that of the moon.

Tides are caused by two factors: the gravitational pull of the Moon and Sun and the inertia of water in the oceans on Earth. The combination of gravity and inertia are responsible for the creation of two major tidal bulges on the Earth. Gravity creates a bulge of water on the side of the Earth facing the Moon and inertia creates a bulge on the side of the Earth facing away from the Moon. The sun also affects the size and position of the two tidal bulges.

The Sun, Moon, and Earth are three extremely large objects separated by very great distances. Despite the large distances between them, each object affects the others. Earth is kept in orbit around the sun by the gravitational forces between them. The Moon is kept in orbit around the Earth by the gravitational forces between them. These forces are mutual, meaning each object attracts and is attracted to the other. However, the gravitational pull of the Sun is almost 200 times greater than the Moon, but since the Moon is almost 400 times closer to the Earth than the Sun, the Moon’s gravitational pull is more than twice that of the Sun.

Click on the link below to see the tides in motion;

A Great Tides Demonstration website- pause the audio before going to this website.

Most coastal areas, with some exceptions along the coast of Alaska and the Gulf of Mexico , experience two high tides and two low tides every lunar day. We are familiar with the 24-hour solar day, which is the time that it takes for a specific site on the Earth to rotate from an exact point under the sun to the same point under the sun. A lunar day is the time it takes for a specific site on the Earth to rotate from an exact point under the moon to the same point under the moon. Unlike a solar day, however, a lunar day is 24 hours and 50 minutes. The lunar day is 50 minutes longer than a solar day because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. It takes the Earth an extra 50 minutes to catch up to the moon. Since the Earth rotates through two tidal bulges every lunar day, coastal areas experience two high and two low tides every 24 hours and 50 minutes. In other words, high tides occur 12 hours and 25 minutes apart. It takes six hours and 12 minutes for the water at the shore to go from high to low, or from low to high. Tide levels vary greatly throughout the world, from an average of 5 – 10 feet vertical change each day to a maximum of 56 feet in the Bay of Fundy , which separates New Brunswick from Nova Scotia .

Four times a month, the Sun’s effect on tides is noticeable. Twice a month, the Sun, Moon, and Earth align to produce very high and very low tides called Spring Tides. The name has nothing to do with the season. Spring is from a Saxon word meaning “to swell”. Spring Tides occur during the New Moon and Full Moon. When the Sun and Moon are at right angles to each other, the gravity of each pulls the ocean water in different directions. In this position, there is little difference between the depth at high and low tide. These tides are called Neap Tides. Neap is from a Saxon word meaning “scarce or lacking”. Neap Tides occur twice a month, during the first and third quarter phases of the Moon. Spring and Neap Tides occur twice every 29.5 days (or twice each month).

1.  tides-the regular change in the level of Earth’s oceans.

2. flood tide-the incoming, or rising tide

3. ebb tide-outgoing, or falling tide

4. Moon’s gravity causes most tides.

5. changing tides happen about every 6 hours and  12 minutes.

6. there are about 4 tides every 24 hours and 50 minutes, two high tides and two low tides.

7. Spring tide is an extra high and extra low tide caused by the alignment of the Moon and the Sun.

8. Full Moon and New Moon cause the Spring tides.

9.  Neap tide is smaller tidal change than normal caused by the Moon and Sun being at a 90 degree angle, a right angle.

10. 1st quarter Moon and 3rd quarter Moon cause Neap tides.

11. intertidal zone-the land that is exposed (the beach) during low tide but is covered during high tide.

Tidal Geography

It is worth noting that other factors can affect tides. The shape of bays and estuaries can magnify the intensity of tides. Funnel-shaped bays can dramatically alter tidal magnitude. The Bay of Fundy is a classic example of this effect. It has the highest tides in the world. Narrow inlets and shallow water tend to dissipate incoming tides. Examples include inland bays such as Laguna Madre , Texas , and Pamlico Sound , North Carolina . In estuaries with strong tidal rivers, such as the Delaware River and Columbia River , powerful seasonal river flows in the spring can alter or mask the incoming tide. Local wind and weather patterns can also affect tides.

We have talked about tides and waves, but what is a “tidal wave”? Tidal waves are actually seismic sea waves or tsunamis. Students may want to read more about this phenomenon.

Some of the largest and most destructive ocean waves are not formed by wind. They are caused by movements of the Earth’s crust, such as earthquakes on the ocean floor. Seismic Sea Waves or Tsunamis are set in motion by earthquakes, underwater volcanic eruptions, or underwater landslides. Tsunamis can speed through the ocean at nearly 500 miles per hour and can rise as high as a 20-story building with a wavelength as much as 124 miles. When the wall slams onto the shore it causes mass destruction. The largest Tsunami on record occurred in 1971 off Ishigaki Island in Japan . It was an amazing 278 feet high. The most recent Tsunami occurred on December 26, 2004 in the Indian Ocean . An underwater earthquake measuring 9.0 on the Richter scale caused it. It devastated eleven countries and killed hundreds of thousands of people.

STOP! Copy your notes.

Your team members should be able to answer the following questions.

1. What are tides?

2. What causes tides?

3. How many tides occur each day?

4. Name four tides that occur, tell when they occur, and
how they occur.

5. Which of these tides causes the greatest intertidal zone?

6. How often do spring tides occur?

Glossary of Terms

Neap tide: The smallest rise and fall in tides that occur when the sun and the moon are at right angles to the Earth.

Spring tide: The greatest rise and fall in tides that occur when the sun and the moon are in line with the Earth.

Tsunami: The Japanese word for seismic sea wave. A series of traveling ocean waves of extremely long length generated by disturbances associated primarily with earthquakes occurring below or near the ocean floor. Underwater volcanic eruptions and landslides can also generate tsunamis.