Rocky coasts in the world

(mainly from Beaches and Coasts by R. A. Davis Jr. and D. M. Fitzgerald, Blackwell Publishing, Oxford, 2004)

Rocky coasts are estimated to represent 75% of the world's shorelines, but this includes beaches backed by rocks, with many different morphologies and several different dynamical processes in action. Nevertheless, there are many cases in which wave erosion is recongized as the main erosive process.

Many of the existing rocky coasts have been carved by glacial processes: Cape Cod, Massachussets, Long Island, New York, extensive areas along the Great Lakes and portion of the eastern shore of Nova Scotia. Furthermore fjiords of Scandinavia, Iceland, Chile and western Canada are other examples.

Tectonically active coasts often display rocky coasts with very limited sediment deposited by rivers (as along Peru and Chile or along the North America cordillera). The rugged appearance of this coasts are usually considered as an extension of the rugged mountains characterizing the nearby landscape. Anyhow, it is difficult to exclude that sea wave erosion do not play a role in their morphology.

Collision coasts tend to be rocky containing few depositional features. Because of their relative youth, neo-trailing edge coasts such as the Arabian coast along the Red Sea are also rugged and moslty rocky.

Furthermore, there are many sites throughout the world where rocky and rugged coasts are found in tectonically passive margins, such as South Africa, parts of Argentina and Brazil, eastern Canada, southern Australia and section of northwest Europe.

One can think that wave erosion can play a role in relatively low rocky coasts. Instead, the height of the cliff is not a general controindication for a sea erosive dynamics. Marine erosion of volcanic rocks has produced dramatic cliff (for instance in south coast of Hawaii).

Volcanic coasts occur where hot spot activity in the mantle has produced island chains (Hawaiian and Marshall islands) Outpourings of lava and welded tuff have also formed portions of rocky coasts along island arc in the Caribbean, and the norther and western Pacific.

Another variety of rocky coasts is formed from the shells of dead marine organisms. This type of coast is most common in Caribbean and Mediterranean seas. High rates of shell production may also occur in higher-latitude regions, such as the south coast of Australia and South Africa, where inputs of other types of land-derived sediment are absent. Many of these rocky coasts were created during Pleistocene, when sea level fell and onshore winds blew carbonate sand onshore, building dunes and beaches. Then this sediment was turned to rock by a process called lithification. Dunes converted in rocks (eolianites) can be seen for instance througout Bermuda and along the Yucatan coast of Mexico.

Rocky coasts are often rugged, and have been taken as an introductory archetype of fractal morphology in nature (B.B.Mandelbrot, How long is the coast of Britain? Science 155, 636 (1967)).

For a very recent fractal analysis of the world coastline see here.