Waves are influenced by wind, tides, and swell, and while the ocean floor also plays a role, it's minor at best.
Simply put, in deep water, the sea floor is unable to influence the formation of waves on the surface.
But this changes in shallow water, at which point the structure of the floor (known as "bathymetry", from the Greek word for "deep") determines the nature of the breaking waves.
The bathymetry impacts the wave through a process known as refraction.
What is Wave Refraction?
As waves travel over the sea floor at different speeds, they bend and contort, with the bend essentially turning toward its slowest point.
And because shallower water creates slower waves, they bend or "refract" toward that shallow water.
This is wave refraction, and it means that the shape of the waves above the surface can be heavily dependent on the topography before the surface.
The unique bathymetry of different surf spots creates equally unique breaking waves.
At Nazare, for instance, the waves are pushed into an A-frame, sending immense wave energy upwards to create monster wave crests and produce some of the sport's biggest ever surfs.
Focusing and Defocusing
There are two main factors to consider when talking about wave refraction: focusing and defocusing.
Focusing wave refraction can occur when the swell reaches a jutting coral reef with deep water on either side.
The wave bends inwards and concentrates all of the energy into a central spot, similar to what we see at Nazare, as well as the unforgiving breaks seen at Teahupoo in Tahiti.
Defocusing occurs when the wave rapidly approaches a shallow water depth, such as a bay. At this point, it begins to slow down.
The wave changes and starts to bend outwards.
Depending on the bathymetry, some locations may produce both types of wave refraction.