A water wave is a mechanism for transferring energy across a body of water, not a mass movement of the water itself. While the wave form travels great distances, individual water molecules primarily move in a circular or orbital motion, returning almost to their original position once the wave passes. This orbital motion causes a floating object to bob up and down and slightly forward and back, rather than being carried along by the wave.
Wind as the Main Engine of Surface Waves
Wind is the most common driver of surface waves. The process begins when air molecules moving across the water surface create friction, or shear stress, which ruffles the flat surface. These initial tiny disturbances, known as capillary waves, provide the roughness necessary for the wind to grip the water more effectively. As the wind pushes on the leeward side of these ripples, it transfers kinetic energy, allowing them to grow into larger gravity waves where gravity, not surface tension, is the main restoring force.
The size of a wind-generated wave is governed by three factors: wind speed, which must be fast enough to create and sustain the ripples; duration, the length of time the wind blows continuously; and fetch, the uninterrupted distance of open water over which the wind blows in a consistent direction.
Once waves travel away from the area where they were generated, they organize themselves into groups called swells. Swells are characterized by their smooth, uniform shape and can travel for thousands of miles across an ocean basin with minimal energy loss.
As a swell approaches the shore, the ocean floor interferes with the circular motion of the water molecules near the bottom. This friction causes the wave to slow down and the wavelength to decrease, forcing the wave to grow taller and steeper. When the wave’s steepness reaches a point where the crest moves faster than the base, it becomes unstable and plunges forward, creating a breaking wave.
The Role of Gravity in Creating Tides
Tides are the largest and most predictable waves on Earth, caused by the differential gravitational pull of celestial bodies. The Moon is the primary influence because the tidal force depends on the inverse cube of the distance, making proximity more significant than mass. The Sun’s gravity contributes, but its effect is about half that of the Moon’s.
The Moon’s gravitational attraction pulls the water toward it, creating a bulge on the side of Earth facing the Moon. A second, simultaneous bulge forms on the opposite side because the Moon’s gravity pulls the solid Earth away from the water on the far side. These two bulges, or high tides, move around the planet as the Earth rotates beneath them.
This mechanism creates a wave that affects the entire depth of the ocean, unlike wind-driven waves that only impact the surface layers. When the Sun, Moon, and Earth align during a new or full moon, their gravitational forces combine to create the largest tidal range, known as spring tides.
Seismic Events and Massive Water Displacement
The tsunami is the most powerful and destructive type of water wave, created by a sudden, massive vertical displacement of the entire water column. Tsunamis are not generated by wind or gravitational forces. The most common cause is a large, shallow underwater earthquake, typically occurring in subduction zones where one tectonic plate is forced beneath another.
During a subduction zone earthquake, the seafloor is abruptly uplifted or subsided over a vast area. This rapid movement acts like a giant paddle, pushing the overlying water and initiating a tsunami wave. Other events, such as large submarine landslides or volcanic eruptions, can also cause the necessary volume of water to be suddenly displaced.
In the deep ocean, tsunamis travel at speeds over 500 miles per hour, but their wave height is often less than a meter, making them virtually undetectable to ships. They possess an extremely long wavelength, allowing them to carry energy across entire ocean basins. As the tsunami enters shallower coastal waters, friction with the seafloor causes the wave to slow dramatically. The immense volume of water then piles up, leading to a destructive increase in wave height experienced at the shore.