Desert Dust: Characteristics and rates of deposition in central Arizona

Great quantities of dust are transported to central Arizona by strong summer winds blowing mainly from southern and eastern Arizona and northern Mexico. Dust collected periodically from April 22, 1972, to July 21, 1973, from the cedar shingle roof of a house in Tempe, Arizona, in the lower Sonoran Desert of the southwest United States, indicates a rate of deposition of 54 g · m 2/yr, a rate equivalent to 5,400 t · ha/10,000 yr. Most of the dust is deposited in association with summertime dust storms (haboobs), which on the average occur 3.5 times per year in the area. Approximately 75% of the dust particles have diameters between 0.05 and 0.005 mm. Chemical study and examination of heavy minerals indicate that the dust is mostly derived from igneous and metamorphic rocks. Three percent of the dust is CaCO 3, 5% is iron oxides, and 0.7% is MnO. It can be calculated that 162 t of CaCO 3is deposited per hectare in 10,000 years. The abundance of CaCO 3in the dust strongly supports the suggestion that the source of CaCO 3in caliche is from windblown dust. It has recently been demonstrated that the iron-manganese coating that forms desert varnish on rocks and stones in arid regions is at least 70% clay minerals, and that the source of these fine-grain particles is desert dust. Our studies indicate 270 t/ha of iron oxides and 3.8 t/ha of MnO are deposited every 10,000 years in the area. There is sufficient iron-manganese deposited annually in southern Arizona by windblown dust to form the desert varnish at the rate presently hypothesized. The process of blowing dust is a geologic hazard and affects man mainly in the form of air pollution. Manifestations of this air pollution are loss of visibility to motorists during dust storms, thereby causing accidents, many of them fatal, and a sometimes fatal illness known as “valley fever.”