Atlas of geological phenomena in the world

Exogenic processes

Marine processes

Description

The landforms of coastal areas usually result from the ocean wave action. Some of the most spectacular erosional landforms are created by the wave action. The depositional landforms like beaches are preserved on the spots where the wave energy is reduced.

The wave action is the source of energy for coastal erosion and sediment transport. The frictional effect of winds moving over the ocean surface generates the energy of waves. The potential energy of the wave is converted into the kinetic energy as the wave breaks (where the ocean depth is less than half of the wavelength). The kinetic energy of the waves is large enough to work change the shape of the shoreline.

The erosion of rocky shorelines by the wave action produces a number of mechanical and chemical effects. Erosional landforms such as wave-cut notches, sea cliffs and even rare forms like caves, sea arches and sea stacks can emerge, depending on the geology of coastline, nature of wave actions and changes in the sea-level. Where a marine cliff lies within reach of the moving water, it is impacted with enormous force. Rock fragments of all sizes, from sand to cobbles, are carried by the surging water and thrust against the bedrock of the cliff. The impact breaks away new rock fragments, and the cliff is undercut at the base. In this way, the cliff erodes backwards, maintaining its form as it retreats.

Transport by the wave and current actions is necessary to move eroded rock particles to other places of deposition on the shoreline. Many different depositional landforms (e.g. spits, shoals, barrier and bayhead beaches, tombolos), can emerge along the shoreline. The factors that affect these processes are configuration of the original coastline, direction of sediment transport, nature of the waves, and steepness of the underwater slope. Although beaches and similar landforms are most often formed from particles from fine to coarse quartz sand, some beaches are built from rounded pebbles or cobbles. Still others are formed from fragments of volcanic rock or even shells.

We can distinguish seven major types of coastlines. Ria coasts, fiord coasts, Barrier-island coasts, delta coasts, volcano coast, coral-reef coasts and fault coasts.

Ria coasts and fiord coasts are typical examples of submergence coastlines. A ria coast is formed when the rise of sea level or crustal sinking brings the shoreline to the elevation of former slopes carved by streams. The ria coast has many off shore islands and estuaries. Fiord coast is similar to the ria coast, but the submerged valleys were carved by the glaciers. The valleys are deep with steep slopes.

The barrier island coasts are low-lying from which the land slopes gently beneath the sea. The plains were originally accumulated beneath the sea and then emerged above the sea level as a result of crustal uplift. The barrier islands are low ridges of sand built by waves and further increased in height by the growth of sand dunes.

Delta and volcano coasts – deltas are deposits of clay, silt and sand that were transported by a river and deposited in the body of standing water or ocean. The deposition is caused by rapid reduction in velocity of the stream. The deltas show a wide variety of outlines (compare the Nile and Mississippi deltas). The volcano coasts emerge where volcanic deposits flow into the ocean. Waves erode the coastline to the shape of low cliffs, beaches are usually narrow and steep.

Coral-reef coasts are unique in the addition of the affect of organisms – corals. Coral reefs live near the water surface and when they die, new generation grows on the top of the dead colonies. Coral reefs are remarkably flat on top. They are exposed at low tide and covered at high tide. There are three types of coral reefs – fringing reefs, barrier reefs and atolls.

Fault shores are formed by the steep straight cliff with original shoreline submerged under the sea level. By the opposite action the marine terraces are formed – the original shoreline is raised above the sea level and forms the marine terrace. The fluvial denudation acts to erode the terrace as soon as it is formed.

References:

[1] Goudie, A.S., 2004. Encyclopedia of Geomorphology, Routledge, London, 1156 s.

[2] Strahler A., Strahler, A., 2003. Introducing physical geography, Wiley, New York, 684 s.