COASTLINES
Waves can:
a. ERODE rock from high energy environments (fast moving water).
b. TRANSPORT sediment, both from coastal erosion, and sediment supplied to the
coast by rivers.
c. DEPOSIT sediment in low energy coastal environments (slow moving water).
Wave Erosion.
Wind-generated oscillatory waves break when they approach shore. The resulting
landward-directed surge of water can cause erosion, particularly on steep
coastlines because all the wave energy is concentrated into a narrow intertidal
zone.
This usually results in a wave-cut notch at the base of cliffs, which periodically collapse, maintaining the steepness of the cliff.
The result of continuous wave erosion at the base of cliffed coastlines is the formation of a wave-cut platform - similar to parallel retreat & pediment formation in deserts.
If sea-level falls, the wave-cut platform becomes a marine terrace.
Where the slope of the coast is gentle, the wave energy is dissipated over a wider intertidal zone, creating lower energy conditions and allowing sediment to accumulate, forming a wide beach. Erosion still occurs in the form of alongshore sediment transport.
The alongshore-moving sediment accumulates at bends in coastlines forming various depositional landforms (e.g. spits, baymouth bars, barrier islands). Many spits, baymouth bars and barrier islands create sheltered coastal wetlands where salt marshes develop. The shelter from wave-action allows mud to accumulate (usually from streams and rivers) and salt-tolerant vegetation grows on the mud.
Classification of North American Coastlines
In North America, coasts are very varied, but they can be classified according to large scale tectonic setting and smaller scale local processes. A. TECTONIC SETTING: Convergent coasts, passive margin coasts, marginal seas (protected from open ocean by island arcs - dominated by rivers and deltas). In the U.S., the west coast is convergent; the east coast and Gulf Coast is a passive margin.
1. Convergent coastlines: West Coast. Characteristics = erosion>deposition; relatively straight; mountainous; sea cliffs; rocky coasts; beaches in sheltered bays; marine terraces; narrow continental shelves; active volcanism; earthquakes.
2. Passive margin coasts: East Coast/Gulf Coast. Characteristics = deposition>erosion; low-lying; wide coastal plain + continental shelf; commonly submerged; common spits, barrier islands, beaches, marshes.
B. LOCAL PROCESSES: Local effects are superimposed onto the larger scale tectonic features. These processes are subdivided into PRIMARY COASTS - dominated by terrestrial processes; and SECONDARY COASTS - dominated by marine processes.
1. Primary Coasts:
a). Stream erosion coasts: areas dominated by well-developed stream networks
which have been partly submerged by rising sea-level. Stream valleys become
estuaries often with a dendritic pattern; hills become islands e.g. Chesapeake
Bay, Delaware Bay, Pamlico Sound (N. Carolina).
b). Stream deposition coasts: at the mouths of large rivers, deltas may dominate the coastline e.g. Mississippi Delta (dominates the entire coast of Mississippi and Louisiana).
c). Glacial erosion & deposition coasts: due to their ruggedness and great relief, submerged glacially-eroded valleys usually dominate coastlines, whether in the east or west. Fjords tend to be straight and very deep (glacially scoured). E.g. Finger Lakes region of New York State, Southern New England. Glacial terminal moraines (deposits) form linear islands.
2. Secondary Coasts:
a). Wave eroded coasts: characterized by cliffs, stacks, arches, wave-cut platforms; headlands are worn back and bays filled in, smoothing the coastline in the long-term. e.g. California, Oregon, Washington.
b). Marine deposition coasts: characterized by beaches, barriers, marshes: in areas of wider coastal plain submerged by post-glacial sea-level rise. E.g. South Carolina, Georgia, Florida, Alabama, Texas.
Example Questions:
1. Describe the effects of waves on a). steep coastlines, and b). gently-sloping coastlines.
2. Explain the difference between primary coasts and secondary coasts, giving 3 examples of primary coasts and 2 examples of secondary coasts.