Waves break when the back of the wave moves faster than the front of the wave, causing it to spill over. The shape of a breaking wave is dictated by the shape of the ocean floor below it, with gentle slopes causing a gentle spill on a cresting wave.
When waves break over a steeply sloped ocean floor, the waves typically break suddenly, which has a dramatic visual effect. Waves are created by the collision and combination of swells of moving sea water. Larger swells typically travel in the same approximate direction as any prevailing winds that originally stimulated their movement.
The speed of moving swells generally slows as the depth of water below them begins to decrease. This causes waves to come closer together, making the front part slower and more vertical. The back end moves slightly faster, creating a rounded slope visual effect. As swells travel across the surface of the water, the energy of the waves and ocean floor below them pushes more water upwards before they break.
Breaking, or cresting, waves can potentially travel over thousands of miles before reaching the shore. Big waves are generally enjoyed by surfers. However, waves carry a large amount of momentum and power, making them dangerous.