NCERT Class 7 Geography — Chapter 1: Water

The correct option is

The speed of the wind blowing across the surface.

Explanation

Waves are oscillatory movements that result in the rise and fall of the water surface. They represent the transfer of energy, rather than the movement of water mass itself, across the ocean. This energy is primarily derived from the friction between the wind and the surface water.

Detailed Analysis:

• The depth of the ocean. is incorrect: The depth of the ocean influences the speed and shape of waves as they approach shallow waters (shoaling), but it is not the primary generator of wave height in the open ocean.
• The speed of the wind blowing across the surface. is correct: The height of a wave is directly determined by three principal factors:
  1. Wind Speed: Higher wind speeds transfer more energy to the water, creating larger waves.
  2. Wind Duration: The length of time the wind blows allows energy to accumulate.
  3. Fetch: The uninterrupted distance over which the wind blows across the water surface.
• The temperature of the water. is incorrect: Water temperature affects the density of seawater and influences ocean currents (thermohaline circulation) but does not directly determine the height of surface waves.
• The salinity of the water. is incorrect: Salinity affects water density and buoyancy but is not a factor in the generation of wind-driven waves.

Key Takeaway:

The height and energy of ocean waves depend primarily on the speed of the wind, the duration for which it blows, and the fetch (the distance over open water).