Chapter 13 Tides| Class 11 Geography Notes

The periodical rise and fall of the sea level, once or twice a day, mainly due to the attraction of the sun and the moon, is called a tide. Movement of water caused by meteorological effects (winds and atmospheric pressure changes) are called surges.

Let us learn more about tides in context of class 12 Geography!

Tides

The periodic rise and fall of sea levels, occurring once or twice a day, primarily due to the gravitational attraction of the sun and the moon, is known as a tide. Unlike tides, which follow a regular pattern, movements of water caused by meteorological effects such as winds and atmospheric pressure changes are termed surges. The study of tides is intricate due to significant spatial and temporal variations in their frequency, magnitude, and height.

Tides are primarily caused by the gravitational pull of the moon, with a lesser contribution from the sun’s gravitational pull. Additionally, centrifugal force, which acts to counterbalance gravity, plays a role in tide formation. The combined effects of gravitational attraction and centrifugal force result in the creation of two major tidal bulges on the Earth’s surface. One bulge occurs on the side of the Earth facing the moon, where the gravitational pull is stronger, while the other bulge occurs on the opposite side due to the dominant centrifugal force.

The difference between the gravitational attraction of the moon and the centrifugal force is termed the ‘tide-generating’ force. This force causes a bulge towards the moon on the side of the Earth nearest to it, and a bulge away from the moon on the opposite side. Horizontal tide-generating forces are more significant than vertical forces in creating tidal bulges on the Earth’s surface.

Tidal bulges tend to be higher on wide continental shelves and lower when they reach mid-oceanic islands. The shape of bays and estuaries along coastlines can amplify tidal intensity, with funnel-shaped bays particularly affecting tidal magnitudes. When tides are channeled between islands or into bays and estuaries, they are referred to as tidal currents.

Types of Tides

Tides exhibit variations in their frequency, direction, and movement across different locations and over time. They can be classified into different types based on how often they occur within a day or 24-hour period, as well as based on their height.

Tides Based on Frequency:

1. Semi-diurnal Tide: This is the most common tidal pattern, characterized by two high tides and two low tides each day. Successive high or low tides are generally of similar heights.
2. Diurnal Tide: In this type, there is only one high tide and one low tide during each day, with successive high and low tides being approximately of the same height.
3. Mixed Tide: These tides exhibit variations in height and occur along the west coast of North America and many Pacific Ocean islands.

Tides Based on the Positions of the Sun, Moon, and Earth: The height of high tides varies significantly based on the positions of the sun and moon relative to the earth. Two types of tides fall under this category:

1. Spring Tides: Occur when the sun, moon, and earth are aligned in a straight line, resulting in higher tidal heights. Spring tides happen twice a month, during the full moon and new moon periods.
2. Neap Tides: These occur approximately seven days after spring tides when the sun and moon are at right angles to each other, partially canceling out each other’s gravitational forces. Neap tides have lower tidal ranges compared to spring tides.

Additionally, there are monthly variations in tidal ranges due to the moon’s orbit around the earth. When the moon is closest to the earth (perigee), unusually high and low tides occur, while when it is farthest (apogee), tidal ranges are less than average. Similar variations occur annually based on the earth’s position relative to the sun, with tidal ranges being greater during perihelion (closest to the sun) and lesser during aphelion (farthest from the sun).

The time when the water level is falling between high tide and low tide is called the ebb.

Importance of Tides

Given that tides result from the precise positions of the earth, moon, and sun, which are accurately known, they can be forecasted well ahead of time. This advance prediction aids navigators and fishermen in planning their activities effectively. Tidal currents play a crucial role in navigation, while tidal heights hold particular significance for harbors situated near rivers and estuaries with shallow bars at their entrances, which obstruct ships and boats from accessing the harbor. Tides also play a beneficial role in dredging sediments and purging polluted water from river estuaries. Moreover, tides serve as a source for generating electrical power, as seen in countries like Canada, France, Russia, and China. Notably, there is an ongoing 3 MW tidal power project at Durgaduani in the Sunderbans region of West Bengal.

Tides- FAQs

What causes tides?

Tides are primarily caused by the gravitational pull of the moon and, to a lesser extent, the sun on Earth’s oceans. The gravitational attraction between these celestial bodies creates a bulge of water on the side of Earth facing the moon and a corresponding bulge on the opposite side, leading to the rise and fall of sea levels, known as tides.

How often do tides occur?

Tides occur approximately twice a day, with two high tides and two low tides in each tidal cycle. The interval between consecutive high or low tides is approximately 12 hours and 25 minutes.

What are spring tides and neap tides?

Spring tides occur when the gravitational forces of the sun and moon align, producing higher than usual tidal ranges. They happen twice a month, around the times of the new moon and full moon. Neap tides occur when the gravitational forces of the sun and moon are perpendicular to each other, resulting in lower than usual tidal ranges. They also occur twice a month, approximately one week after spring tides.