Sea arches form when waves are deflected to the sides by a point on the headland. These waves erode a plane of weakness on both sides of the headland creating an opening. The openings on each side eventually meet in the middle with stronger rock joining overhead.
Sea arches do not last very long on a geological time scale. Most collapse within a few decades to a few centuries. When the center collapses, the stacks of rocks remaining on the sides are known as tombolos. The rock spanning the opening in the sea arch is called a keystone.
The coastal erosion that forms sea arches is responsible for a variety of other landforms. Caves, tunnels, columns and pillars are some of the landforms created by the action of waves on sections of rock that are more easily eroded than the rock around it. Erosion of rock in one area may result in accumulation of sediment in another area, creating a wide variety of structures.
Though other forms of erosion can create similar structures on dry land, the constant action of waves along the coastline greatly decreases the time required for these structures to form. The rapidity of coastal erosion often threatens property and results in the expenditure of billions of dollars each year to preserve coastlines.