Read:

• Inman, D.L., and Nordstrom. C.E., 1971

Inman and Nordstrom eloquently incorporate the effects of tectonic environment on the morphology of first and second order coastal features. They define first order features as tectonically controlled and having length dimensions along the coast of approximately 1000 km, width dimensions perpendicular to the coast of around 100 km, and a relief as much as 10 km. Included in these spatial dimensions would be the continental margin, the coastal plain and any coastal mountains. Second order features, such as bays, deltas, dune fields, and submarine canyons, have approximate length-width-height dimensions of 100 km by 10 km by 1 km and are produced by glaciers, rivers, wind and other erosional or depositional agents that modify first order features. Third order features are those beach components formed from the modification of second order features along the shore zone by waves and tides. Inman and Nordstrom's classification is based on the large-scale, first and second order features of coasts and is summarized below.

Tectonic classification of coasts

KEY Collision Coasts Continental Island arc Trailing Edge Coasts Neo-trailing edge coasts Afro-trailing edge coasts Amero-trailing edge coasts   Marginal Sea Coasts Figure 1. Tectonic classification of coasts (Redrawn from Inman and Nordstrom, 1971)

Collision coasts

A collision coast typically lies along a convergent plate boundary. However, an exception is made for coastal California. Although presently located along a transform boundary the coast of California retain features created during its collision with the Farallon plate which terminated approximately 20 million years ago.

Collision coasts are divided into two categories:

• Continental collision coast: A continental margin that is located along collision boundary, such as the west coast of South America (fig. 2).
• Island arc collision coast: A coast located along the collision margin of an island arc, such as the islands in the western Pacific.

  Characteristics of collision coasts:

  • Rocks are folded and raised along coastline forming mountains in excess of 10,000 ft.
  • Coasts are often associated with and affected by earthquake and volcanic activity.
  • Coasts are relatively straight with narrow continental shelves and steep continental slopes.
  • The high mountains adjacent to the coast are potentially a great source of sediment, but the narrow shelf results in submarine canyon heads that lie close to shore. Consequently most of the sediment deposited along the coast is eventually drained off through canyons. Second order features such as large deltas and barrier island chains are typically lacking on these narrow coasts.

Figure 2. Cross section through a continental collision coast and Amero-trailing edge coasts, such as that viewed across South America. Note that the westerly divide deflects most continental drainage and consequently sediment transport towards the eastern margin of the continent. The western shelf is narrow and steep while th eastern shelf is broad and shallow. After Inman and Nortstrum, 1971.

Trailing edge coasts

This is a plate-imbedded coast that faces a spreading center. There are three subcategories of trailing edge coasts:

• Neo-trailing edge coast: Coastline developed along the margin of a newly rifting land mass.
  • Characteristics:
    • Immature coast, similar in many ways to collision coasts
    • volcanism and seismic activity are commonly present, adding to the region's instability.
    • Topography adjacent to the coast is typically rugged and includes cliffs, mountains, and/or plateaus.
    • Coastlines lack continental shelves because there has not been enough time for depositional features to form.
• Amero-trailing edge coast: Trailing edge coast where the opposite side of the continent is a collision coast. This arrangement directs the drainage and flow of sediment from the collision side to the trailing edge coast (fig. 2).
  • Characteristics:
    • The discharge of large drainage systems, high sediment supplies and maturity of these coasts results in the formation of wide depositional continental shelves (i.e. East Coast of U.S. has sediment thickness' of 10 km or more in some places)
    • Coastline is backed by a wide, relatively flat coastal plain.
    • Abundance of second order depositional features such as barrier islands, deltas, marshes, mangrove swamps, and tidal flats. Large deltas and barrier island complexes may be missing in glaciated regions.
    • Deep incision by submarine canyons is restricted to the continental slope.
• Afro-trailing edge coast: Trailing edge coast where the opposite side of the continent is also a trailing edge. (Examples: Greenland and Africa)
  • Characteristics:
    • Many characteristics are similar to amero-trailing edge coasts. However streams typically drain from the interior of the continent. Drainage basins are slightly smaller and sediment supply is less resulting in narrower margins. Deltas and barrier islands are common secondary features.
    • Coasts lines are commonly backed by plateaus

Marginal sea coast

• Those coasts that are in semi-protected environments. Examples: Gulf of Mexico, north slope of Alaska, coasts along eastern Asia.
  • Types of marginal sea coasts
    • sheltered by surrounding land masses
    • sheltered by island arcs
    • sheltered by pack ice
    • Characteristics:
      • wide continental shelves
      • coasts are baked by hilly or low-lying regions
      • usually tectonically inactive
      • low wave energy

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Summary: Effects of tectonic setting

• Tectonic setting controls the size, relief and orientation of drainage basins as well as sediment supply and direction of transport.
• These factors coupled with the age of the coast determine the width of the continental shelf.
• Wave energy is greatly dissipated across a wide continental shelf. Collision coasts with narrow margins would have higher waves that a sheltered marginal sea coast or a trailing edge coast with a broad shelf.
• All other factors (ocean basin size, depth, etc.) being equal the broader the continental shelf the greater the tidal range.

• Works best for first order features. Many second and higher order features, such as reefs, mangrove swamps, and sediment composition are controlled by climate.
• The tectonic environment is sum areas, such as the Mediterranean, is unclear.
• Because of the time it takes for a coast to adjust to a new setting some coasts may reflect recent and not the present tectonic setting. For example the west coast of California is no longer a collision coast senso stricto, but retains most of the features created by the subduction of the Farallon Plate.
• Second order features may completely mask or dominate the first order coastal features. In such instances more traditional descriptions should also be used, such as glaciated coast, reef coast, deltaic coast, etc.

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Lindley Hanson/email /Gls214

Department of Geological Sciences, Salem State College, Salem, MA