Beaches are made as rocks, worn away from headlands, are ground down into shingle and sand. The sea then deposits these particles in a sheltered place, forming a beach.
Rocks or coral reefs located off the shore are worn down by moving waves. As these materials are worn down, they become small particles of sediment that are carried by the waves in a state of suspension. In the case of sediment from further inland, the particles are washed to the larger body of water, where they are swept up by the waves and into the same state of suspension. These suspended particles cause the moving water to have increased erosive ability, resulting in greater amounts of eroded particles in the water.
In some cases, fish and other marine animals contribute to the speed of erosion. This is particularly true in beaches that are located near coral reefs. Many of these animals rely on algae growing on the coral as a major dietary supplement. As they eat away the algae, they inadvertently cause the coral to break off into small pieces. Some pieces may even work their way through the digestive tracts of these animals, resulting in even smaller particles that are washed up into the waves.
Erosion is typically thought to decrease the size of certain landforms, however, this is not always the case. In fact, erosion actually works to increase the size and width of some beaches. This growth occurs as the waves deposit the aforementioned sediment onto the land. Additionally, beaches may experience growth in size near river deltas, where rivers carry eroded sediment to the ocean. This sediment is deposited along the beach before being carried off into the ocean.
The type of wave that reaches the coastline also plays a part in the formation of beaches. Constructive waves, which are those that allow the water to recede and the beach particles to stop moving between waves, result in compacted sediment. This firm beach surface prevents future erosion. Destructive waves, which are fast forming and do not allow the water to recede between waves, result in a near-constant state of sediment suspension in the water. Because the particles remain in the waves, rather than being deposited on the shoreline, the beach in these areas is more likely to suffer from future erosion. With destructive waves, the sediment is not given a chance to settle and become compacted.
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