The small strips of land where the ocean meets the sea - coastlines - are some of the most dynamic places on Earth. Waves, tides, and currents are constantly reshaping the shoreline; most of the time this is done gradually and invisibly.

When waves break they carry sand and gravel onto the beach. A beach is the area in which sediment is deposited along the shore; they are composed of loose sediments that are deposited and moved by waves along the shoreline. If more sediment is deposited than is removed, a beach builds up. If more sediment is removed than is added, the beach erodes.


Longshore currents are one way sand is deposited on a beach. Longshore currents are caused by waves striking the beach at an angle; they do this because coastlines are not uniform, so some parts of a wave will strike the beach before others. The energy from the wave also causes some sand particles to be carried away from the beach. The net movement, however, is parallel to the coast. This causes beaches to migrate as sand is being eroded and deposited farther down the beach by the longshore current.


Rip currents are formed in low areas or breaks in sandbars. While longshore currents move parallel to the shore, rip currents move perpendicularly away from the shore & towards the ocean. They typically move away from the shore at speeds of 1-2 feet per second, but if there is a lot of wave energy they can travel at much faster speeds.



The type of sediment (sand, gravel, pebbles) a beach is made of depends on the amount of energy in the waves, the source of the sediment, and the topography of the coastline. Rocky beaches, for example, are formed on steeper shorelines composed mainly of cliffs and are made of coarse materials such as cobbles and pebbles rather than sand. The next 2 pages will allow you to explore the features and processes associated with both sandy and rocky beaches.


Sources: http://www.nature.nps.gov/geology/usgsnps/sea/sea.html and http://oceanservice.noaa.gov/education/kits/currents/03coastal3.html