Back to Coastal Zones: Plate Tectonics and Coastal Classification

What are the characteristics of coastal zones along passive margins or 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 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 a 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.

In total, the Inman and Nordstrom (1971) classification recognized 3 primary types of plate tectonic settings with several subclasses within two of these.

1) Collisional Coasts

Recall that a collisional tectonic setting is one where two plates, regardless of composition, are moving toward one another.

A. Continental collision coasts

Coasts of this tectonic setting overlie and are adjacent to crustal bedrock that has been and is actively undergoing deformation as two plates collide. The coastal zone borders mountains that are in excess of approximately 3,000 meters. There may be nearby volcanoes and earthquake activity because of the active collision. Additionally, the coastal area may be undergoing very slow uplift because of the plate collision. In general, these coastal areas are relatively straight and have narrow continental shelves. Nearby uplifted mountainous areas can provide a ready supply of sediment to the coastal zone through river systems but because of the narrow continental shelf there are often submarine canyons that can provide a conduit for this sediment to move through and eventually be deposited in deep ocean water far from the coastal zone.

B. Island arc collision coasts

Island arc coasts are located where two oceanic crust plate edges are colliding. Similar to continental collision coasts, they are tectonically active with volcanoes and earthquakes, and are actively being uplifted. Deep ocean trenches created by the subduction of oceanic crust under oceanic crust border the narrow continental shelves that lie seaward of these types of coasts.

2) Trailing Edge Coasts

Recall that a passive or trailing edge continental margin is one that is tectonically inactive and where the coninental margin is facing a spreading center.

A. 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.

B. Amero-trailing edge coast

This type of coast faces a spreading center, but the opposite side of the continent is a collisional coast. They are typified by wide continental shelves with a thick sequence of sediment on the continental shelf. Wide, relatively flat coastal plains are typical of the landward side of these coasts. Also, a wide range of deltas, barrier islands, marshes and tidal flats are common features in Amero-trailing edge coasts. In summary, these areas are tectonically inactive.

C. Afro-trailing edge coast

Coastal zones of this category face a spreading center, and the opposite side of the continent also faces a spreading center. They typically have moderate to wide continental shelves, and the coast is backed by hills or lowlands. They are tectonically inactive and have a smaller sediment supply to the coast then do Amero-trailing edge coasts.

3) Marginal Sea Coasts

Coasts of this character are sheltered from the conditions of the open ocean by other land masses, such as island arcs created by the collision of tectonic plates. They are fronted by wide continental shelves and backed by hilly or low-lying regions frequently present in tectonically inactive areas.

For more information on plate tectonics, check out the video below which provides lots of background noise but an overview of plate tectonics and coastal zone characteristics.

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