PrintTectonics, Sediment Supply, and Morphologies Along Rocky Coasts
Along active collisional margins, tectonic processes have uplifted and deformed rocks to create rugged landscapes with very little sediment input because of immature drainage basins along such geologically youthful landscapes. The presence of recently uplifted mountainous areas can also act as a barrier and prevent river systems from flowing, and thus carrying sediment, to the coast. Moreover, as you learned in Module 2, the relatively narrow and steep continental shelves that are characteristic of active tectonic margins do not help to dampen waves that move onshore from the open oceans. The result is that along active tectonic margins there is little delivery of sediment that could accumulate, and that sediment that does make it to the coast can be easily eroded and transported away by the high wave energy. Globally, there are many different types of morphologies along rocky coasts because of a wide range of rock types, styles of tectonic deformation, hydrographic regime, and styles of weathering.
Some of the most spectacular features of rocky coasts include sea arches and sea stacks, which are produced by the constant erosion of waves. A sea arch develops when a headland protruding into the ocean causes waves to refract around it. This refraction of waves concentrates their energy in specific locations along the headland, causing particularly rapid erosion if weakness such as faults and fractures are present in the rocks. In other cases, the waves may simply begin to erode into rock that is less resistant to erosion than the surrounding rock. Either way, the erosion leads to the development of small caves that may eventually meet below a promontory, leaving an arch above. The erosion continues and, for this reason, sea arches are very ephemeral coastal features. When they ultimately collapse, the remnants of the arch are called sea stacks.
For more information on sea arch formation and sea stacks, check out the following videos: