Causes of Coastal Variability

Causes of Coastal Variability 

Coastal variability depends upon following factors.

Plate Tectonic Setting 

The tectonic setting of a coast plays a role in determining whether the coast has steep-sided mountain slopes or a broad plain that borders the sea. Along an active margin, compression squeezes the crust and pushes it up, creating mountains like the Andes along the western coast of South America. Along a passive margin, the cooling and sinking of the lithosphere may create a broad coastal plain, a flatland that merges with the continental shelf, as exists along the Gulf Coast and south-eastern Atlantic coast of the United States. 

Not all passive margins have coastal plains. The coastal areas of some passive margins were uplifted during the rifting event that preceded establishment of the passive margin. For example, highlands formed during rifting border the Red Sea and portions of the Brazilian and southern African coasts. Highlands also rise along the east coast of Australia.

Relative Sea-Level Changes 

Because of sea-level drop during the ice age, there was more dry land.

Sea level, relative to the land surface, changes during geologic time. Some changes develop due to vertical movement of the land. These may reflect plate-tectonic processes or the addition or removal of a load (such as a glacier) on the crust. Local changes in sea level may reflect human activity when people pump out groundwater or oil, for example, the pores between grains in the sediment beneath the ground collapse, and the land surface sinks. Some relative sea-level changes, however, are due to a global rise or fall of the ocean surface. Such eustatic sea-level changes may reflect changes in the volume of mid-ocean ridges. An increase in the number or width of ridges, for example, displaces water and causes sea level to rise. Eustatic sea-level changes may also reflect changes in the volume of glaciers, for glaciers store water on land (figure above). As glaciers grow, sea level falls, and as glaciers shrink, sea level rises. 

Features of emergent coastlines (relative sea level is falling) and submergent coastlines (relative sea level is rising).

Geologists refer to coasts where the land is rising or rose relative to sea level as emergent coasts. At emergent coasts, steep slopes typically border the shore. A series of step-like terraces form along some emergent coasts (figure above a). These terraces reflect episodic changes in relative sea level and/or ground uplift. Those coasts at which the land sinks relative to sea level become submergent coasts (figure above b). At submergent coasts, landforms include estuaries and fjords that  developed when the rising sea flooded coastal valleys. 

Sediment Supply and Climate 

The quantity and character of sediment supplied to a shore affects its character. That is, coastlines where the sea washes sediment away faster than it can be supplied (erosional coasts) recede landward and may become rocky, whereas coastlines that receive more sediment than erodes away (accretionary coasts) grow seaward and develop broad beaches. 

Climate also affects the character of a coast. Shores that enjoy generally calm weather erode less rapidly than those constantly subjected to ravaging storms. A sediment supply large enough to generate an accretionary coast in a calm environment may be insufficient to prevent the development of an erosional coast in a stormy environment. The climate also affects biological activity along coasts. For example, in the warm water of tropical climates, mangrove swamps flourish along the shore, and coral reefs form offshore. The reefs may build into a broad carbonate platform such as appears in the Bahamas today. In cooler climates, salt marshes develop, whereas in arctic regions, the coast may be a stark environment of lichen-covered rock and barren sediment.

Source: Essentials of Geology; book by Stephen Marshak

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