MadSci Network: Earth Sciences

Re: Why are there barrier islands on the easts coast and not the west of the US

Date: Thu Dec 20 13:58:29 2007
Posted By: Dave Clark, Staff, Chemical and Environmental Technologies, Battelle
Area of science: Earth Sciences
ID: 1197548814.Es

David, it turns out that there are indeed many barrier islands (and also 
spits, which are essentially the same thing) on all three coasts (East, 
West, Gulf), but that the conditions for barrier island formation are 
just better on the East coast in general.  Although no one knows exactly 
why some places form barrier islands and some don't, several key 
requirements are:

-Low gradient (gentle slope)off of the shelf off shore 
-Abundant sediment supply (large drainage areas with abundant rain)
-Low to moderate tidal range (to reduce the erosion of the barrier 
-Wide shelf off shore to provide ample sediment and a long distance to 
deep water
-Wide, low-relief coastal plain 
-Absence of submarine canyons (which would drain sediments to deep water) 
-Lack of seismic activity that can destroy islands
-Long wavelength swells that approach the beach at an angle so sediment 
can get moved along the coast to replenish the islands
-Coastal drowning(caused by rising ocean or sinking coast) that 
introduces more sediment below the water line

Where these conditions exist, barrier islands are more likely to form - 
but even then, there is no gaurantee.  It turns out that these conditions 
are far more common on the East and Gulf coasts than the West coast so 
barrier islands tend to be more common in the East and the Gulf.


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