CoastsThe characteristics of a coast depend on its geologic structure, initial topographic configuration, and shoreline process that shape it. There are several ways and varying scales one can classify these complex systems. Submerged coasts form when river mouths are flooded due to rising sea level or subsidence of land. A drop in sea level or rise in the land surface creates an emergent coast. Depositional coasts have abundant depositional features like deltas, bars, spits and reefs where new land has been built. Coral coasts are formed by biological rather than physical processes. Submerged CoastsRia coasts
are
formed by the submergence of river valleys emptying into the ocean.
"Ria" is a Spanish term for coasts with prominent headlands and
embayments typical of these coasts. Wave action turns the smooth
valley sides into receding cliffs with
sand spits and
tombolos
common.
Figure 21.17 Kenai Fjord National Park, Alaska Fjord (fiord ) coasts are form when glacial troughs are flooded due to a rise in sea level. Fiord coasts are deeply indented, with steep-walled valleys. Sandy beaches are rare as sediment eroded from valley walls collects on the floor. Fjords are common in Scandinavia, British Columbia, Alaska, and Patagonia. Emergent CoastsEmergent coasts are a result of forces acting to raise the land surface or drop sea level. The incredible weight of massive ice sheets during the the Pleistocene depressed the continental surfaces beneath them. The continents began to rebound as the ice melted and released the overlying pressure. The rising surface lifted the shoreline above sea level forming glacial uplift coasts along continental margins.
Figure 21. 18 Wave-cut
terraces on San Clemente Island, California.
Raised shorelines and erosional features like wave cut terraces are also found along tectonic coasts where endogenic forces have uplifted the surface. Such coasts are common along the mountain and island arcs of the Pacific Ocean. Depositional CoastsBarrier island coasts are those paralleled by deposits of sand separated from land by a lagoon. There is some controversy over how barrier island coasts form. They begin as offshore bars of submerged sediment that migrate landward unless stabilized. Barrier islands are often cut by tidal inlets, openings that allow water to move landward and seaward with the tides. Barrier islands border the Atlantic coastal plain of North America.
Delta coasts are those formed by the deposition of sediment at the mouth of a river that enters the ocean. Deposition is caused by the rapid decrease in water velocity as it enters the ocean. Sand and silt are the first to deposit while the mixing of fresh and salt water cause clay particles to bind together forming larger particles that settle to the bottom.
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Figure 21.19 Birdfoot or Balize delta of the
Mississippi River