8.2: Classification

What are coasts like generally, and how can they be classified? I won’t try to develop any formal or exhaustive classification here, but there are several important varieties I want you to be aware of and thinking about.

Figure 8-1 An attempt to show, in a simple diagram, the important relationships among the factors that govern the nature of coasts.

Look at the present shorelines of the world. What do you see?

Beaches. Beaches are perhaps the most common kind of coastline. It’s easy to define a beach: an accumulation of loose sediment at the shoreline, shaped by the action of shoaling and breaking waves. Sediment size ranges from fine sand to coarse gravel. Beaches range in extent from little pockets only meters to tens of meters long, to many hundreds of kilometers without a break.

Rocky or cliffy coasts. Along such coasts there’s little or no loose sediment above mean low tide, so the bedrock of the continent is exposed to slow marine erosion. (The bedrock is usually resistant, or else the forces of the sea would produce enough sediment to form a sedimentary coastal environment like a beach.) There are all gradations from rocky coasts to beachy coasts.

Tidal flats. These are usually found in protected embayments or along coasts where waves are not important. They typically show an interlacing network of tidal distributary channels, with coarse sediment in the channels, passing into broad areas of fine sediment away from the channels.

Estuaries. An estuary is a body of water with salinity lower than the open ocean, with restricted access to the ocean, where fresh water mixes with salt water. There are several varieties, the main kinds these days being drowned-river estuaries and barrier-system estuaries. They seem to be unrepresentatively common today, presumably because of the effects of the most recent Pleistocene glaciation and deglaciation. Estuaries are often closely associated with tidal flats.

Deltas. A delta is a body of sediment delivered to the coastline by a river, and built out into a body of water, a lake or the ocean. Deltas show a great variety of geometries, and they can exist in combination with many of the other kinds of coasts.

Reefs. A reef is a marine structure, built by organisms, that withstands the erosive action of waves. Reefs are common in low-latitude areas with perennially warm water and low supply of siliciclastic sediment from land. They are often found in combination with carbonate-sand beaches.

Muddy open shorelines. These are not as common, and lie well outside the coastal experiences of most of us (including me). They are found mainly in the tropics in areas where substantial fine siliciclastic sediment is supplied from land but no carbonate sediment is produced near the shoreline, by reason of either too-cold water or swamping by the siliciclastics. (Why are they more common in the tropics? Presumably because of the nature of terrestrial weathering.) Mangrove swamps, important along some low-latitude shorelines, should be included here as well.

Coasts can also be classified into three kinds on the basis of the effect of sea-level change:

Emergent. Sea level falling relative to the land. These exist today, but they are not common. They are found only in areas where tectonic uplift has more than offset the postglacial sea-level rise.

Submergent. Sea level rising relative to the land. These are very common now, because of the recent rise in sea level.

Stable: These are not common now, but they must have been throughout much of geologic time.

Keep in mind that this is an atypical time, because of the large changes in sea level worldwide since the disappearance of the last great Pleistocene continental ice sheets. Sea level rose to nearly its present level from minus 120–130 m between 20,000 yr BP (years before the present) and now, and most of that was between 18,000 yr BP and 6,000 yr BP!

Coasts can be usefully classified on the basis of their plate-tectonic setting into leading-edge coasts and tailing-edge coasts:

Trailing-edge coasts. These are tectonically undifferentiated, once sedimentation has become established well enough to mask the complexities of initial rifting; slow subsidence, relatively low sediment supply, wide shelves; the east coast of North America is a good example.

Leading-edge coasts. These show varied uplift and subsidence; topographically differentiated (basins and ridges both offshore and onshore); relatively high sediment supply, narrow shelves; the west coast of North America and the west coast of South America are two different examples.

Finally, coasts are naturally classified on the basis of hydrodynamic setting into four basic groups, obviously with intergradations among them:

Tide-dominated coasts. Large tidal range; day-to-day sediment movement by strong tidal currents overshadows the effects of important but infrequent storms.

Wave-dominated coasts. Large breaking waves from distant storms overshadow the effects of important but infrequent nearby storms.

Storm-dominated coasts. Waves and currents produced by large coastal storms overshadow other effects.

Current-dominated coasts. Throughgoing strong ocean currents impinging from the adjacent deep ocean dominate sediment movement and shaping of the coast. (These are the least common of the four kinds.)