How do differences in density to cause ocean currents

The differences in the density of the water truly cause ocean currents and any other currents. Density may change since it is directly proportional to its temperature and salinity. For example, a cold water with dissolved salts is denser than a warm water with dissolved salts. That is mainly because the dissolved salts were more dissolved by the warm water because of its higher temperature. 

Answermost of the time it would be ground into fine sediment,and sometimes they are carried away from the original location;

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