Hydrosphere
Multiple Choice Questions (MCQ)
- The primary force responsible for the occurrence of tides and ebbs is:
(a) The Earth’s rotation
(b) Atmospheric pressure
(c) Gravitational pull of the Moon and the Sun
(d) Coriolis effect
Answer: (c) Gravitational pull of the Moon and the Sun - When the Sun, Earth, and Moon are aligned in a straight line, the resulting tides are called:
(a) Neap tides
(b) Diurnal tides
(c) Spring tides
(d) Mixed tides
Answer: (c) Spring tides - Most coastal regions experience how many high tides and low tides within a lunar day?
(a) One high tide and one low tide
(b) Two high tides and one low tide
(c) One high tide and two low tides
(d) Two high tides and two low tides
Answer: (d) Two high tides and two low tides - The approximate time interval between two consecutive high tides in a semi-diurnal tidal pattern is:
(a) 24 hours
(b) 12 hours
(c) 24 hours and 50 minutes
(d) 12 hours and 25 minutes
Answer: (d) 12 hours and 25 minutes - Which geographical feature can amplify the tidal range, leading to significantly higher high tides and lower low tides?
(a) Open ocean islands
(b) Wide continental shelves
(c) Narrow bays and estuaries
(d) Deep ocean basins
Answer: (c) Narrow bays and estuaries
Short Answer Questions (SAQ)
- Explain the difference between a direct tidal bulge and an indirect tidal bulge.
Answer: A direct tidal bulge occurs on the side of the Earth facing the Moon because the Moon’s gravitational pull is strongest there, pulling the water outwards. An indirect tidal bulge occurs on the opposite side of the Earth because the solid Earth is pulled more strongly towards the Moon than the water on that side, effectively leaving the water to bulge outwards away from the Earth’s center. - What are neap tides, and under what astronomical alignment do they occur?
Answer: Neap tides are tides with a smaller tidal range, meaning lower high tides and higher low tides. They occur when the Sun and Moon are at right angles with respect to the Earth (during the first and third quarter moon phases), causing their gravitational forces to partially counteract each other. - What is a lunar day, and why is it important in understanding tidal timing?
Answer: A lunar day is the time it takes for a specific point on Earth to rotate and come directly under the same position relative to the Moon again. It is approximately 24 hours and 50 minutes, which is about 50 minutes longer than a solar day because the Moon also moves in its orbit around the Earth. This extra 50 minutes causes a delay in the timing of successive high tides (and low tides) each day.
Long Answer Questions (LAQ)
- Describe the roles of both the Moon and the Sun in the origin of tides and ebbs. Explain why the Moon has a greater influence on tidal range than the Sun.
Answer: The rhythmic rise and fall of sea levels, known as tides and ebbs, are primarily caused by the gravitational forces exerted by both the Moon and the Sun on the Earth’s oceans.
The Moon’s Role: The Moon’s gravitational pull is the dominant factor in tide generation. Due to its closer proximity to the Earth, the Moon’s gravitational influence is about twice as strong as that of the Sun in terms of tide generation. This differential gravitational pull across the Earth creates two main tidal bulges. On the side facing the Moon (the sublunar point), the water is pulled more strongly towards the Moon, creating a direct tidal bulge and resulting in a high tide. On the opposite side of the Earth (the antipodal point), an indirect tidal bulge also occurs. Here, the solid Earth is pulled more strongly towards the Moon than the water, effectively causing the Earth to be pulled away from the water, leaving the water to bulge outwards, resulting in another high tide. Between these two bulges, water flows away, leading to low tides. As the Earth rotates, coastal areas pass through these bulges and troughs, experiencing the cycle of high and low tides.
The Sun’s Role: Although the Sun is much farther away than the Moon, its immense mass also exerts a significant gravitational pull on the Earth and its oceans, contributing to solar tides. These solar tides are about half the magnitude of the lunar tides. The Sun’s gravitational influence becomes particularly important when it aligns with the Moon relative to the Earth.
Why the Moon’s Influence is Greater: Despite the Sun’s much larger mass, the Moon has a greater influence on tidal range primarily due to its closer proximity to the Earth. Gravity’s strength is inversely proportional to the square of the distance between two objects. Since the Moon is significantly closer to the Earth than the Sun, the difference in gravitational pull across the Earth (the differential gravitational force that causes tidal bulges) is much greater for the Moon than for the Sun. This differential force is the key factor in generating tides. While the Sun’s gravity does contribute to tides, the Moon’s closer position results in a differential gravitational force that is about twice as strong, leading to lunar tides being the dominant force in determining tidal range. The interplay of these lunar and solar tides leads to the variations in tidal range observed as spring and neap tides. - Discuss the different types of tidal patterns observed globally (semi-diurnal, diurnal, and mixed tides), explaining the primary characteristics and the factors that contribute to their occurrence in specific geographical locations.
Answer: The timing and characteristics of tides vary significantly across the globe, resulting in three main types of tidal patterns: semi-diurnal, diurnal, and mixed tides.
Semi-diurnal Tides: This is the most common tidal pattern, characterized by two high tides and two low tides occurring approximately every lunar day. The time interval between successive high tides (or low tides) is about 12 hours and 25 minutes. The heights of the two high tides are usually about the same, and the heights of the two low tides are also similar. This pattern arises primarily due to the Earth rotating through the two tidal bulges created by the Moon’s gravitational pull. As a coastal location passes under each bulge, it experiences a high tide, and as it passes between the bulges, it experiences a low tide. The consistency of these two bulges as the Earth rotates leads to the regular pattern of two highs and two lows per lunar day. Most of the Atlantic coasts of North America and Europe experience semi-diurnal tides.
Diurnal Tides: In contrast to semi-diurnal tides, diurnal tides are characterized by only one high tide and one low tide occurring per lunar day. The time difference between successive high tides (or low tides) is approximately 24 hours and 50 minutes, matching the length of the lunar day. This type of tidal pattern is less common and is often found in enclosed or semi-enclosed basins such as the Gulf of Mexico and parts of Southeast Asia. The occurrence of diurnal tides is largely attributed to the complex interaction of tidal waves with the specific bathymetry (underwater topography) and coastal geography of these basins. These geographical features can filter out one of the semi-diurnal tidal cycles, allowing only one dominant tidal wave to propagate and influence the coastal water levels.
Mixed Tides: Many coastal areas exhibit mixed tides, which are characterized by two high tides and two low tides each lunar day, but with significant differences in the heights of the two high tides (higher high water and lower high water) and/or the two low tides (higher low water and lower low water). The time intervals between these highs and lows can also be irregular and vary throughout the lunar month. Mixed tides are common along the Pacific coasts of North America and parts of Asia. Their occurrence is often influenced by local coastal geography and bathymetry, which can differentially affect the two semi-diurnal tidal bulges as they propagate through the ocean basins and onto the continental shelves. The interaction of different tidal constituents (the individual tidal frequencies that make up the overall tide) also contributes to the complexity of mixed tides. The shape of the coastline, the depth of the water, and the presence of features like bays and estuaries can modify the propagation and amplification of these different tidal constituents, leading to the observed inequalities in high and low water heights and irregular timing.
In summary, while the fundamental cause of tides is the gravitational influence of the Moon and the Sun, the specific pattern observed at a particular coastal location is heavily influenced by the interplay of the Earth’s rotation and the local geographical characteristics, particularly the bathymetry and coastal configuration, which can modify the propagation and behavior of tidal waves.