A beach is a landform where wind, waves, and water wear away rock and sediment. Beaches are also home to marine animals like crabs and sea anemones. The composition and size of a beach depend on its location and the geological history of its region.
Sand, shells, and other materials make up the majority of beaches. They may be terrigenous (from eroded rock) or biogenic (formed from the breakage and accumulation of the skeletal fragments of organisms).
Beach erosion is the main process that creates and changes beaches. As sand is eroded and carried away by waves, it forms a sloping, low-tide terrace that is inclined toward the ocean. The terrace surface can be covered with a variety of deposits, including beach clays, lava flows, sand, and pebbles.
The shape of a beach depends on its geology and the way it is protected from erosion by cliffs or other landforms. In addition, beach erosion is influenced by the type of wave that hits the coast. Waves are classified as constructive or destructive, depending on how much energy they have.
Constructive waves tend to have stronger swashes than backwashes, so they carry more material to the beach. In contrast, destructive waves have stronger backwashes and can rapidly remove a significant amount of beach material from the shore.
Beaches are constantly changing in size, shape, and composition. In fact, beaches can change in just a few weeks or days during major storms. They also serve as buffers that protect nearshore coastal land from erosion by dissipating the energy of waves that would otherwise impact these terrains.
In addition to beach erosion, other natural processes influence the formation and growth of beaches. In many regions, rocks are weathered (broken down into smaller pieces by wind or water) and then carried by rivers or streams to the coast. This material is then deposited by waves on the coastline as they break.
The color of a beach also depends on its local geology. For example, black basalt from volcanic eruptions can provide the sediment supply for black beaches. In contrast, coral and white quarts from the ocean provide the sediment supply for white beaches.
Besides the natural forces that shape beaches, human activity can substantially influence beach erosion and deposition patterns. For example, the emplacement of jetties to stabilize a river delta can cause an adjacent coastal sand spit to retreat at rates up to 65 m/yr.