Tides Class 7 Geography Notes Maharashtra State Board
Both the photographs show the same place. If you stay near the coast for some time, you will realize that the sea water is sometimes very close to the coast while at other times, it is far away from the coast. We call these movements of the sea water ‘tides’. Barring a few exceptions, all the coasts on the earth experience tides. High and low tides are natural phenomena. Let us try to understand the scientific reason behind these natural events.
Tides are movements of seawater occurring daily and regularly. The level of seawater changes after a specific period. After every 12 hours and 25 minutes, a cycle of high tide and low tide is completed.
This regularly occurring event appears to be quite simple and natural, however, it is directly related to the sun, the moon, and the Earth and the gravitational and centrifugal forces that interact between them.
Keep a small stone or a chalk piece on your notebook and move the notebook from left to right with some force.
Take water in a small container that has a handle. See what happens if the container is whirled around with force. Fill the mixer jars with water and switch on the mixer. Observe. (Do this under the supervision of parents.) Observe a fan and a slingshot while they are moving. Take half a glass of water. Slowly move the glass in one direction, in a circular fashion. Observe what happens to the water.
Observe what happens if you whirl a keyring around a finger.
In all the above activities, the effect of the centrifugal force is visible. The centrifugal force acts in the direction opposite to the gravitational force. Centrifugal means going away from the center. You must have experienced it too. At the local fairs if you sit in a Ferris Wheel, your seat spins and leans outward away from the wheel. This also is an effect of centrifugal force.
Centrifugal and Gravitational Force:
Due to its rotation, the earth gets a type of power or force. This force works away from the center. It is called centrifugal force. Due to this force, any object on the Earth would be thrown into space. However, the gravitational force is working towards the center of the earth at the same time. This force is many times greater than the centrifugal force. Hence any object on the surface of earth remains at the place where it exists.
Tides:
The following factors are responsible for the occurrence of tides.
- The gravitational pull of the moon and the sun as well as that of the earth.
- Revolution of the earth around the sun and the indirect revolution of the moon around the sun.
- The centrifugal force is generated due to the rotation of the earth.
The moon is closer to the Earth than the sun, hence its gravitational force becomes more effective than that of the sun. Tides occur due to the relative positions of the moon, the sun, and the earth. A place on the earth located at the opposite point of the place experiencing high or low tide also experiences high or low tide at the same time. This is a result of the centrifugal force. See the figure and try to understand the locations of high and low tides on the Earth. When there is a high tide at 0° meridian, the 180° meridian also experiences high tide. The meridians that are at right angles to those having high tides will experience low tides at the same time.
Types of Tides:
As the timing of the tide varies every day, the range of a tide also keeps on changing. Generally, the highest high tide occurs on full moon and new moon days whereas on the days of the first and the third quarter, the high tide is at its minimum. Spring tides and neap tides are the two types of tides.
Spring Tide:
On new moon and full moon days, the gravitational pull of the sun and the moon act in the same direction. Due to this, the total pull increases. Hence the tide on these days is much higher than the average high tide. This is known as spring tide. As the bulge at the high tide is greater, the water at low tide recedes much more. The water level during spring tide is higher than the average high tides and lower than the average low.
Neap Tide:
While revolving around the earth, the moon makes a right angle concerning the earth and the sun, twice a month. This position occurs on the first and the third quarter of each month. On both these days, the forces of both the sun and the moon operate at right angles on the Earth. At the places where the sun causes high tide, the gravitational pull of the moon which is at right angles also acts on the sea water. Due to such conditions, the water level rise is less than usual at the time of high tide.
Similarly, the fall in water level is less than usual at the time of low tide. This is because the attraction of the sun and the moon are not complementary but at right angles to each other. Such tides are called neap tides. Neap tides are a little lower than the average high tides and a little higher than the average low tides.
Intertidal Range
The difference in the water level of the high tide and the low tide is called tidal range. In open seas, this range is about 30 cm. However, towards the coastal areas, it is increasing. Along the coast of Peninsular India, the range is around 100 to 150 cm for most of the part. The highest tidal range in the world is observed at the Bay of Fundy along the Atlantic Coast of North America. Here the tidal range is about 1600 cm. In India, the highest tidal range is in the Gulf of Khambhat. It is about 1100 cm.
Effects of Tides:
- With the high tide, fish move into the creeks and this helps fishing activity.
- The tides clear the waste and hence the coasts become clean.
- Ports do not get filled with sediments.
- Ships can move up to the ports during high tide.
- During high tide, seawater can be stored in salt pans from which salt can be obtained.
- The tidal force can be used to generate electricity.
- A lack of an understanding of the timings of high and low tides may cause accidents for swimmers entering the sea.
- The tides help in maintaining the mangroves and the coastal biodiversity.
Timings of the Tides Change Daily:
The tides are continuously occurring phenomena. After reaching the maximum water level during high tide, the low tide sets in. Similarly, after reaching the lowest water level during low tide, the high tide sets in. Remember that in the following discussion, the highest limit of the high tides is mentioned. You will understand why the timings of the high and the low tides change daily.
- There will be high tide at point ‘P’ as it is opposite to the moon.
- As point ‘F’ is opposite (at an antipodal location) to point ‘P’, it will also experience high tide at the same time.
- It will take 24 hours for point ‘P’ to come back to its original position (360°). Point ‘P’ will be at point ‘F’ after 12 hours (180°).
- The same change will also occur concerning point ‘F’.
- Point ‘F’ will not experience the same high tide, because in the meanwhile the moon also would have moved a little ahead (approximately 6° 15′). Hence it will take 25 minutes more for point ‘F’ to come to the position opposite to the moon.
- After 12 hours 25 minutes, ‘F’ will arrive opposite the moon and it will experience high tide. Point ‘P’ opposite to ‘F’ will also experience high tide.
- Later again, after 12 hours and 25 minutes point ‘P1’ will experience high tide as it comes in front of the moon. There will be high tide at F1 too as shown in the figure. There is high tide and low tide twice a day (24 hours) generally. The time difference between two high tides is 12 hours and 25 minutes.
Waves:
If you blow air over hot milk or tea, while drinking, ripples are generated on the milk or the tea. In the same way, because of the force of the wind, water appears to be moving. The sea water gets pushed by the wind and ripples are generated on the water’s surface. These are called waves. The seawater moves up and down or slightly forward and backward due to the waves. The waves bring the energy contained in them to the coast. They break into the shallow waters near the coast. Large or small waves are formed continuously at the surface of the sea. The generation of waves is a natural and regularly occurring phenomenon too.
Structure of the Waves:
The seawater gets pushed up and down because of the wind. The raised-up portion of a wave is called the crest and the depressed one is called the trough. If a strong wind is blowing in one direction, large waves are generated. The vertical distance between a crest and the following trough is called the amplitude of the wave whereas the distance between two successive crests or troughs is called wave length. The wavelength, its amplitude, and its velocity depend on the velocity of the wind.
Velocity of Waves:
If we stand at the coast, we feel the waves are coming towards the coast. If a floating object is thrown at a distance in the sea, we find the object moving up and down at the same place. It does not come to the coast. It means the water forming the wave also does not come to the coast. Remember that the water in the wave does not move, what moves is the energy.
The main reason for wave generation is wind, but at times, due to earthquakes or volcanic eruptions occurring below the floor of the sea, waves get generated. Such waves assume a great height in the shallow waters near the coast. These waves are very destructive. They cause huge loss of property and life. These waves are called tsunamis. In 2004, tsunami waves were generated because of the earthquake that took place near Sumatra island of Indonesia. They devastated the east coast of India as well as Sri Lanka. The waves cause erosion along the headlands and create beaches in the protected locations in the bays. If an earthquake occurs in the ocean, the coastal areas face a risk of tsunamis. In such cases, it is better to go away from the coasts or towards higher altitudes. This way, loss of life can be avoided.
While moving along the sea coast or playing in the water we must take adequate care regarding the timings of the tides otherwise it can lead to serious accidents. For that, we must know the timings of the tides. For this, you must know the phase of the moon on the given day. Three-fourths of the phase of the moon is the time of the high tide. For example, suppose you are at the coast on the fourth phase of the moon, then the fourth phase is three. It means the high tide will occur at 3 am and 3 pm, and approximately 6 hours from it, that is, 9 am and 9 pm will be the low tide. There may be slight differences according to local conditions. We must collect information from local people and learn from them about the structure of the coast, its slope, rocky areas as well as the streams near the coast before entering the sea waters for enjoyment.
Well-organized Maharashtra State Board Class 7 Geography Notes Tides can aid in exam preparation and quick revision.