Back to Coastal Zones: Plate Tectonics and Coastal Classification

Back to Coastal Zones: Plate Tectonics and Coastal Classification

What are the characteristics of coastal zones along collision coasts and trailing margins?

The tectonic setting and tectonic history of a continental margin are probably the most important factors that control the character of a continental shelf and coast. You can think of the tectonic setting as a first-order control on the morphology of a coastal zone. There are other factors that certainly are important, such as sediment supply, but we will get to those in later sections.

A very useful and widely recognized classification scheme of a coastal zone is the tectonic coastal classification developed by two scientists. In 1971, Inman and Nordstrom published a scientific paper that linked different types of coastal zones to tectonic characteristics. They suggested that the most important factors in determining the coastal characteristics were:

1. location of the coast with respect to plate boundaries;
2. tectonic setting of the coastline on the opposite side of the continent;
3. time;
4. exposure of the coast to open ocean conditions.

1. Collision Coasts

   Collision coasts face a plate boundary. They occur at the point of subduction zones, on the edge of a plate. These are tectonically active coasts, with frequent earthquakes and volcanic activity. Features found on and near collision coasts include mountains of more than 3,000 meters; deep trenches offshore, into which sediment falls when it reaches the coast. Here the continental shelf is narrow and there are few depositional features to be found. The rivers that drain to collision coasts are relatively small and there are few of them.

   Examples of collision coasts are the west coasts of North and South America.

2. Trailing Edge Coasts

   Trailing edge coasts face a spreading center where new material is added to the oceanic plates (e.g., mid-Atlantic spreading center). Trailing edge coasts are near the middle of a tectonic plate and far from the plate boundary. The characteristics all trailing edge coasts have in common are lack of tectonic activity (no earthquakes or volcanism); There are three types of training edge coasts:

   1. Amero-trailing edge coasts

      Amero-trailing edge coasts occur in the middle of a tectonic plate and face a spreading center. The coast on the opposite side of the continent is a collision coast. Characteristics of Amero-trailing edge coasts include NO tectonic activity; a wide continental shelf and a wide, low coastal plain. There is abundant sediment deposition and therefore numerous depositional features. These features include marshes, barrier islands, spits, mangroves, and deltas. Examples of Amero-trailing edge coasts are the east coast of North America, the east coast of South America, and India.

   2. Afro-trailing edge coasts

      The distinguishing characteristics of Afro-trailing edge coasts are: they face a major spreading center, AND the opposite coast also faces a spreading center. The entire continent is located within a plate. They also have no tectonic activity. The continental shelf can vary in width from wide to narrow. The coast itself can feature cliffs, hills and low mountains, and coastal plains. Sections of an Afro-trailing edge coast can have many depositional features, while other sections have very little sediment deposition, so they are highly variable. Examples of Afro-trailing edge coasts are the east and west coasts of Africa, the coast of Greenland, and parts of coastal Australia.

   3. Neo-trailing edge coast

      This type of coast faces relatively young plate tectonic spreading centers, which means that there has not been enough time for them to develop. There may be some volcanism, and they may be seismically active with rugged, young topography adjacent to them. The continental shelf is very narrow here. There is very little deposition taking place on these coasts, as few rivers enter the ocean. A few pocket beaches form, but no other depositional features. They are backed by cliffs, hills and low mountains. Examples include the Gulf of California, where Baja rifted from mainland Mexico about 20 million years ago (relatively recent in geologic time).

3. Marginal Sea Coasts

   Coasts of this character occur along continental coasts facing an island arc. They are sheltered from the conditions of the open ocean by other land masses, such as island arcs, which were created by the collision of tectonic plates. They feature wide continental shelves and are backed by hilly or low-lying regions. There is no tectonic activity on these coasts. There is high sediment influx from rivers, resulting in depositional features such as deltas. Examples of marginal sea coasts include the coasts of mainland China on the South China Sea, the East China Sea, the Sea of Japan, and the Yellow sea. The Caribbean islands form an island arc that protects the coasts of mainland central and South America on the Caribbean Sea.