Wave height in the open ocean can be over 10m, with the largest observed being 34m in the north Pacific in 1933. Surf formation on sloping coasts begins at water depths of about half wave length. The velocity and wavelength diminish, and the wave height increases. Tsunami are destructive seismic sea waves caused by earthquakes, volcanic eruptions, or submarine landslides. A tsunami watch is operating in the Pacific to signal coastal dwellers of potential trouble.
Tides are also important in ocean margin processes.
Tides are caused by Moon and Sun attraction on the Earths' surface. Two tidal bulges on alternate sides of the Earth, with the solid Earth rotating under them. Tidal effects are greatest when the configuration of the coastline includes narrow openings - such as estuaries, bays, straits, etc. Wave refraction is another important shore process, produced when a linear wave front encounters shallowing water on a path at an angle to the coastline. The portion of the wave front in contact with the coast slows, bending the front coastwards, and concentrating the erosional power on the headlands, while depositing beaches in the bays.