The prominant first order influence, tectonic setting, controls the gross morphology of a coast, when viewed at a scale of hundreds of kilometers. Tectonically active coasts are commonly straight and backed by mountains. On the otherhand passive margin coasts are broadly irregular and backed by a wide coastal plain. However, when viewed on a scale of a kilometer or less shorelines along an active margin may exhibit a greater variation.

Factors that interact to control the evolution of smaller scale features within a particular tectonic setting. Many of these factors are directly or indirectly related to tectonic environment. Some factors are interdependent.

• Biological Activity
  • In warmer climates mangrove coasts can be developed in low energy or sheltered
  • Coral reef development is temperature dependent
  • Away from large terrigenous sources, a higher proportion of beach sediments will be comprised of carbonate materials as water temperature increases
• Weathering
  • rates of weathering and hence sediment supply are controlled by temperature and precipitation
• Storms and wind patterns
  • The Gulf Coast is influenced by tropical cyclones whereas the Northeast is affected by extratropical cyclones. Cyclones are low pressure systems which have counterclockwise circulation (in northern hemisphere).
  • The overall global wind patterns cause the prevailing westerly winds to affect most of the US. Consequently, the West Coast is a windward shore and the East Coast is a leeward shore. This results in a higher wave energy along the West Coast compared to that of the East Coast. This is also affected by the width of the continental margin in each of these regions.
  • West coast monsoonal climate results in high waves and related beach erosion during the winter followed by swell waves and related deposition in the Summer.
• Production of sea ice
  • Some coasts may be sheltered from high waves by sea ice
  • In cold climates ice push ridges may be an important process in the development of beach morphology.

The geologic history of a coast and adjacent regions my play a major role in local and regional coastal features.

• Relative sea level changes (isostatic and eustatic): Relative sea-level rise may result in a deeply embayed coast (ria coast) such as northern N.E. and transgressive barrier island coast such as in southern New England and to the south.
• Glaciation:
  • Glaciers deepened and enlarged valleys and stripped off sediment. In New England much of the sediment which was eroded from interior regions was deposited locally along coastal regions and/or on the continental shelf. In Maine most of the sediment was stripped of coastal regions and deposited on the continental shelf whereas to the south (i.e. coastal MA) large volumes of sediment were deposited in coastal regions by the ice margin.
  • Examples: Long Island, Martha's Vineyard, Nantucket, Cape Cod and the Elizabeth Islands are composed of glacial sediments. Most of the harbor Islands and coastal features in the Boston area are modified drumlins.
  • Marginal meltwater flowed well beyond the glacial margin depositing sediment into non-glaciated coastal regions. For example, sediment composing some Gulf coast barrier islands was derived from erosion of large Pleistocene deltas.
    
  • Fjords: Glacially scoured coastal valleys which have been further submerged by eustatic sea level rise accompanying the melting of ice sheet.
• Past Tectonic Environment
• Coastal regions may still be affected by mountains formed by an earlier tectonic event.
  • For instance the East coast of North America was a collision boundary. This results in diversion of continental drainage to the SSW to the Gulf of Mexico.
  • California's coast reflects its recent history as a collision coast.

The influence of waves and tides. More

• Wave processes
  • Waves tend to move sediment along the coast which results in shore beaches, spit formation and lateral barrier island migration and/or elongation.
  • Waves attack headlands and transport sediment into embayments forming pocket beaches.
  • The ultimate effect of waves is to straighten a shoreline so that the distribution of energy along its length is equalized.
• Tidal processes:
  • Tidal current transport sediment perpendicular to the shore.
  • Causes sediment to be transported in seaward and landward directions.
  • Tidal Range is controlled by shelf width, coastal configuration, size and dimension of amphidromic system (ocean basin) and the position of the coast within the system.
  • According to Hayes (1975) the hydrographic regime controls:
    • type of barrier island
    • number and size of tidal inlets (the openings between barrier islands which are maintained by tidal currents)
    • extent of salt marshes (flat intertidal, poorly drained,grass covered land which is vegetated with Spartina grasses)
    • extent of tidal flats (flat intertidal, non-vegetated plains.
    • he occurrence and morphology of tidal deltas (the sand shoals which are found landward and seaward of the tidal inlets)
    • extent and morphology of river deltas
    • type of back barrier environment--whether this region is comprised of bays and lagoons or march and tidal creek systems.

Sediment supply affects the distribution and persistance of deposional features such as deltas and barrier islands, wide sandy beaches. Sediment supply is a dependent variable influenced by tectonic setting, climate, geologic history, erodibility of materials in drainage area. According to Ollier (1984) 90% of all sediment supplied to coasts worldwide is done so by rivers and only 5% from cliff erosion (in Woodroffe, 2003). Biogeneous and hydrogeneous sediments supply the rest.

• Tectonic setting
  • Along Afro-trailing edge coasts less sediment is delivered to the coast when compared to the amount deposited along Amero-trailing edge coasts.
    • This is because Amero-trailing edge coasts have large mountain systems on the opposite side of the continent from which sediment can be supplied. Also the regional drainage is directed toward a single direction. This is not true of Afro-trailing edges because the opposite side of the continent is also a trailing edge.
• Climate (see above discussion)
  • The rate and type (chemical vs. mechanical) of weathering influence the relative amounts of dissolved load, suspended load, and bedload load.
  • Regions with highly variable climates (i.e. monsoons of India) typically yield large quantities of sediment.
• Erodibility of materials in the drainage area
  • Examples:
    New England rivers drain the resistant crystalline rock of the Northern Appalachains. In addition, repeated glaciations removed nearly all saprolite and pre-Quaternary coastal plain sediments. Sediment yield is dependent on the erosion of glacial deposits and large beaches, such as those on Cape Cod, are restricted to areas where Pleistocene glacial fluvial deposits are eroded along the shore.
  • The high sediment load of the Huang Ho (Yellow River) in northern China is acquired as it carves through thick (up to 100 ft.) deposits of wind blown silt (loess)
  • Rivers with the highest sediment loads, such as the Ganges and Bramaputra, drain active collision boundaries where gradients are steep and where highly erodible sedimentary rocks are abundant.

 

Lindley Hanson/email /Gls214

Department of Geological Sciences, Salem State College, Salem, MA
last updated 5/19/03