Coast and Beach
Observing Erosion And Deposition
Earth is constantly changing. Mountains are built up by tectonic forces, weathered, and eroded away. The erosional debris is deposited in the sea. In most places these changes occur so slowly they are barely noticeable, but at the beach, they are often observable.
Most features of the beach environment are temporary, steady state features. To illustrate this, consider an excavation in soil, where groundwater is flowing in, and being pumped out by mechanical pumps. The level of the water in the hole is maintained because it is being pumped out just as fast as it is coming in. It is in a steady state, but changing either rate will promptly change the level of the water. A casual observer may fail to notice the pumps, and erroneously conclude that the water in the hole is stationary. Similarly, a casual observer may think that the sand on the beach is stationary, instead of in a steady state. The size and shape of a spit, which is a body of sand stretching out from a point, parallel to the shore, is similar to the level of the water in this example. To stay the same, the rate at which sand is being added to the spit must be exactly balanced by the rate at which it is being removed. Failure to recognize this has often led to serious degradation of the coastal environment.
Sea level is the point from which elevation is measured. A minor change in elevation high on a mountain is undetectable without sophisticated surveying equipment. The environment at 4,320 ft (1,316.7 m) above sea level is not much different from that at 4,310 ft (1,313.6 m). The same 10-ft (3-m) change in the elevation of a beach would expose formerly submerged land, or inundate formerly exposed land, making it easy to notice. Not only is the environment different, but also the dominant geologic processes are different: Erosion occurs above sea level, deposition occurs below sea level. As a result, coasts where the land is rising relative to sea level (emergent coasts) are usually very different from those where the land is sinking relative to sea level (submergent coasts).