Vertical density currents

Bradley, W.

Bradley notes that the individual calcite particles in varves cannot have settled out of a lake at rates governed by Stokes law. Rather, he postulates that the particles combine at the surface of a lake to produce a vertical density current. He describes the results of experiments that demonstrate this behaviour and applies it to the interpretation of lake sediments. He concludes with a section on broader geological applications. 'One would expect vertical density currents to form when volcanic ash falls into a lake or other body of relatively quiet water. The rapidity of transport offered by the density currents apparently accounts for the sharply defined lower and, especially, upper boundaries of layers of volcanic ash in sediments. Only very rapid deposition can account for such features, particularly if the particles are very fine. Shards and grains larger than 0.14 millimeter in diameter settle so rapidly that no special process is required to account for the fact that they make sharply defined layers. What is true of sharply defined individual layers of tuff would, of course, be equally true of the clearly defined layers and laminae within thinly banded water-laid tuffs. Had these fine ash particles not been transported rapidly to the bottom they surely would have been mixed with particles of organic matter, clay, silt, and the like. Fine particles of sediment brought into a lake by great dust storms by warm muddy water spreading into the surface water should also result in the formation of vertical density currents and in consequent relatively rapid transport of the particles to the bottom. Because air is a fluid, one would expect also that vertical density currents would form and flow downward from the clouds of volcanic ash thrown from explosive volcanoes. Many years ago Grout, and later Wager and Deer, proposed that the evidence of flow in certain igneous rocks could be accounted for by in initially vertical density currents in the magma, which were set in motion by growth of crystals and the cooling the magma. Grout showed that the growth of crystals would have much more effective than the cooling of magma because the crystals have much greater specific gravities than the melts from which they grow. This proposed process is analogous to growth of calcite crystals in the surface waters of lakes as carbon dioxide is abstracted from the water either through photosynthesis or through warming of the water.'

Vertical density currents

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