Elements of Marine Ecology
Conditions which determine the types of materials forming the sea floor include:
(a) the geology, including the proximity of land and the geographical
and geological features of the coastline such as rock formations and the
outflows of rivers or glaciers;
(b) the speed of the bottom current;
(c) the depth;
(d) the types of suspended matter in the overlying water, including the pelagic organisms; and
(e) the type of benthic population.
Sediments cover the sea floor except where the bottom current is strong enough to sweep away particles, or where the gradient is too steep for them to lodge. The scouring action of tidal currents may expose rocks beneath shallow water, and in deep water uncovered rock occurs on steep sides of submarine peaks or trenches.
In sediments, the type of deposit varies with the speed of the bottom current and the size and density of suspended particles. The faster the water moves, the coarser is the texture of the substrate because finely divided material is more easily held in suspension than larger particles of the same density. Stokes' equation for the settling velocity, W, applies fairly well to small particles in seawater:
g = gravitational acceleration
D = density of particles
d = density of liquid
µ = dynamic viscosity of liquid
r = radius of spherical particles.
In many areas, parts of the sea-bed over the continental shelf are kept
in motion by strong tidal currents. Once the settling velocity of particles
in the sediment is exceeded, the transport of sediment increases rapidly
with increase of current speed.
Marine sediments are classified into two main groups, terrigenous and pelagic deposits.
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