Notes Class 7 Chapter 3 Geography Maharashtra Board

Notes For All Chapters – Geography Class 7

Tides

1. Introduction to Tides

• Tides are the regular rise and fall of seawater along coastlines.
• They occur due to the gravitational pull of the moon and sun and the centrifugal force caused by Earth’s rotation.
• A complete high tide and low tide cycle occurs every 12 hours and 25 minutes.
• Tides occur along almost all coasts of the world.

2. Forces Responsible for Tides

(A) Gravitational Force

• The moon’s gravitational pull attracts the seawater, causing it to rise, resulting in a high tide.
• Since the moon is closer to Earth than the sun, its gravitational force is stronger than the sun’s.

(B) Centrifugal Force

• The Earth rotates, creating a force that pushes objects away from its center.
• This force creates a second high tide on the opposite side of Earth.

(C) Earth’s Rotation and Revolution

• The rotation of the Earth (spinning on its axis) changes the location of tides.
• The revolution of Earth around the sun and the moon’s revolution around Earth also affect tides.

3. Types of Tides

Tides can be classified based on their occurrence and intensity.

(A) Based on Occurrence

High Tide

• When seawater rises and moves inland.
• Occurs when the moon’s gravitational pull is strongest.

Low Tide

• When seawater recedes, exposing more land.
• Occurs when the gravitational pull is weaker.

(B) Based on Intensity

Spring Tide (Strongest Tide)

• Occurs on full moon and new moon days.
• The gravitational pull of the sun and moon combine, increasing the total pull.
• Results in higher high tides and lower low tides.

Neap Tide (Weakest Tide)

• Occurs during the first and third quarter phases of the moon.
• The gravitational pull of the sun and moon act at right angles, reducing the total effect.
• Results in lower high tides and higher low tides.

4. Tidal Range

• Tidal range is the difference in sea level between high tide and low tide.
• The tidal range varies depending on location:
  • Open seas – Around 30 cm.
  • Coastal areas – Around 100 to 150 cm.
  • Bay of Fundy (Canada) – 1600 cm (highest in the world).
  • Gulf of Khambhat (India) – 1100 cm (highest in India).

5. Effects of Tides

Tides have both positive and negative effects on nature and human activities.

(A) Positive Effects

1. Fishing – Fish move into creeks during high tide, helping fishermen.
2. Coastal Cleaning – Tides wash away waste and keep the coast clean.
3. Port Maintenance – Ports do not fill up with sediments due to tidal movement.
4. Navigation – Ships use high tides to move closer to ports.
5. Salt Production – Salt pans store seawater during high tide, and salt is collected after evaporation.
6. Tidal Energy – Tidal power plants use tides to generate electricity.
7. Coastal Biodiversity – Tides help sustain mangrove forests and marine ecosystems.

(B) Negative Effects

1. Flooding – High tides can flood low-lying coastal areas.
2. Accidents – Swimmers and travelers can get caught in unexpected high tides.
3. Coastal Erosion – Strong tides erode coastal areas over time.

6. Daily Changes in Tide Timings

• Tides occur twice a day at most places.
• The time difference between two high tides is 12 hours and 25 minutes.
• Example: If high tide occurs at 7 AM, the next high tide will be at 7:25 PM.

7. Waves and Their Structure

• Waves are the movement of seawater caused mainly by wind.
• Parts of a wave:
  1. Crest – The highest point of a wave.
  2. Trough – The lowest point of a wave.
  3. Amplitude – The height of a wave from the trough to the crest.
  4. Wavelength – The distance between two consecutive crests or troughs.

(A) Velocity of Waves

• Waves appear to move towards the shore, but the actual water does not move forward, only energy is transferred.

(B) Causes of Waves

• Wind – Main cause of waves.
• Earthquakes and Volcanoes – Sudden movements under the sea can create giant waves called tsunamis.

(C) Tsunamis

• Tsunamis are large, destructive waves caused by underwater earthquakes or volcanic eruptions.
• 2004 Tsunami – A massive tsunami hit India and Sri Lanka due to an earthquake near Sumatra, Indonesia.

8. Practical Applications of Tides

9. Important Questions and Answers

(A) Give Geographical Reasons

1. Why are tides influenced more by the moon than the sun?
   • The moon is closer to Earth, so its gravitational pull is stronger than the sun’s.
2. Why do some coastal areas turn into lagoons or marshlands?
   • During high tide, seawater enters low-lying areas, and during low tide, it recedes, forming lagoons.
3. Why do places on opposite meridians experience tides at the same time?
   • Due to centrifugal force, high tide occurs on both sides of Earth simultaneously.

(B) Answer in Brief

1. If high tide occurs at 7 AM, when will the next tides occur?
   • Next high tide: 7:25 PM.
   • Next low tides: 1:12 PM and 1:37 AM.
2. If Mumbai (73°E) has high tide at 1 PM, which other meridian will have high tide?
   • The opposite meridian (253°E) will also have high tide.
3. What causes waves in the sea?
   • Wind, earthquakes, and volcanic eruptions generate waves.

(C) Differentiate Between