The salinity of ocean waters is measured by the amount of dissolved solids present in a unit weight of ocean water, usually expressed in parts per thousand by weight or grams per kilogram. The salts dissolved in the ocean waters have their origin on the continental landmasses. They can be carried into the oceans by rain, rivers, groundwater table, sea-waves, winds, and glaciers. However, some of the dissolved salts have their origin from the ocean bottom. The layers of the earth beneath the crust contain minerals in a molten state which can reach the crust either due to volcanic activity or due to their outgassing (continuous emission in the form of gasses) from the fissures present at the bottom of the ocean. In addition, the dead and decomposing organic matter also adds to the salinity of the oceans.