DENSITY CURRENT

any current in either a liquid or a gas that is kept in motion by the force of gravity acting on small differences in density. A density difference can exist between two fluids or between different parts of the same fluid because of a difference in temperature, salinity, or concentration of suspended sediment. Density currents in nature are exemplified by those currents that flow along the bottom of oceans or lakes. Such subaqueous currents occur because the water entering an ocean or a lake is colder, saltier, or contains more suspended sediment and, thus, is denser than the surrounding waters. As a consequence, it sinks and flows along the bottom under the effect of gravity. The difference in density, moreover, slows down the mixing of the subaqueous current with the overlying waters, enabling it to maintain itself for a relatively long distance. Density currents are of considerable practical importance. For example, the deposition of sediment from turbidity currentsi.e., density currents in which the density difference is caused by suspended sedimentin lakes may result in a rapid decrease of reservoir capacity. Equally significant, the industrial discharge of large amounts of polluted or heated water may generate density currents that have adverse effects on neighbouring human or animal communities. Because of such considerations, many experimental studies on the properties of density currents have been undertaken. Small turbidity currents have been investigated in the laboratory and have been observed directly in lakes. Indirect evidence strongly suggests that large-scale turbidity currents occur in ocean basins. Many researchers believe that a current of this type is caused by the slumping of sediments that have accumulated at the head of a submarine canyon. Slumping of large masses of sediment produces a very dense sedimentwater mixture that eventually flows down the canyon to spread out across the seafloor and deposit a layer of sand in the deep water. Repeated deposition results in the formation of submarine fans, structures that closely resemble the alluvial fans that occur at the mouth of many river canyons. Sedimentary rocks that are thought to have originated from ancient turbidity currents are called turbidites and are common in the geological record.