In addition to sea-ice, icebergs which are of land origin occur at sea.
In the polar areas the melting of sea-ice and of ice formed by precipitation lowers the density of the seawater and causes a difference of level which sets up streaming movements towards the equator.
During the rapid formation of ice the still unfrozen brine is often imprisoned between the little plates of frozen water; hence without some special treatment sea-ice is not suitable as a source of drinking water.
Such old sea-ice when prevented from escaping forms the palaeocrystic sea of Nares; but, as a rule, it is carried southward in the .East Greenland and Labrador currents, and melted in the warmer seas of lower latitudes.
Even in the Mediterranean sea-ice is formed annually in the northern part of the Black Sea, and more rarely in the Gulf of Salonica and at the head of the Adriatic off Triest.