PHENOMENA AT THE LAND–ATMOSPHERE INTERFACE

17.14 PHENOMENA AT THE LAND–ATMOSPHERE INTERFACE

The interface between the atmosphere and land is a boundary of intense environ- mental activity. The combined effects of air and water tend to cause significant changes to the land materials at this interface. The top layer of exposed land is especially susceptible to physical and chemical weathering. Here, air laden with oxidant oxygen contacts rock, originally formed under reducing conditions, causing

oxidation reactions to occur. Acid naturally present in rainwater as dissolved CO 2 or as pollutant sulfuric, sulfurous, nitric, or hydrochloric acid, can dissolve portions of some kinds of rocks. Organisms such as lichens, which consist of fungi and algae growing symbiotically on rock surfaces, drawing carbon dioxide, oxygen, or nitrogen from air, can grow on rock surfaces at the boundary of the atmosphere and geosphere, causing additional weathering to take place.

One of the most significant agents affecting exposed geospheric solids at the atmosphere/geosphere boundary is wind. Wind both erodes solids and acts as an agent to deposit solids on geospheric surfaces. The influence of wind is especially pronounced in dry areas. A major factor in wind erosion is wind abrasions in which solid particles of sand and rock carried by wind tend to wear away exposed rock and soil. Loose, unconsolidated sand and soil may be removed in large volumes by wind,

a process called deflation. The potential for wind to move matter is illustrated by the formation of large deposits of loess, consisting of finely divided soil carried by wind. Loess particles are typically several tens of micrometers in size, small enough to be carried great distances by wind. Especially common are loess deposits that originated with matter composed of rock ground to a fine flour by Ice Age glaciers. This material was first deposited in river valleys by flood waters issuing from melting glaciers, then blown some distance from the rivers by strong winds after drying out.

One of the more common geospheric features created by wind is a dune, con- sisting of a mound of debris, usually sand, dropped when wind slows down. As a dune builds, it forms an obstruction that slows wind even more, so that more sedi- ment is dropped and, in the presence of sediment-laden wind, dunes several meters or more high may form rapidly. In forming a dune, heavier, coarser particles settle first so that the matter in dunes is sorted according to size, just like sediments depos- ited by flowing streams. In areas in which winds are prevalently from one direction,

as is usually the case, dunes show a typical shape, as illustrated in Figure 17.11 . It is seen that the steeply sloping side, called the slip face, is downwind. Some of the environmental effects of dunes result from their tendency to migrate with the prevailing winds. Migration occurs as matter is blown by the wind up the gently sloping face of the dune and falls down the slip face. Migrating sand dunes have buried forest trees, and dust dunes in drought-stricken agricultural areas have filled road ditches, causing severely increased maintenance costs.