616 SEDIMENTARY PROCESSES/Aeolian Processes and bedrock outcrops to form ventifacts (Figure 5C) Smoothing and polishing, faceting, fluting, and grooving of rock surfaces occur in areas of high-intensity sand transport and affect a variety of lithologies, including granite, marble, and basalt Dust Storms Dust storms are defined as events in which visibility is reduced to km or less as a result of blowing dust (Figure 6) Interest in air quality and in the impact of aerosols on atmospheric properties has promoted intensive study of dust storms and their occurrence in space using meteorological records and satellite images The Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images and the Total Ozone Monitoring Sensor(TOMS) have been used to identify major dust source areas and to track the dispersion of particulate matter (Figure 7) Major dust source areas include the Bodele´ Depression in Tchad, the Aral Sea area, south-east Iran, and the loess plateau of China These areas are characterized by seasonally strong winds and areas of fine-grained sediments (including those of playa lakes or previously deposited aeolian materials) The interannual frequency and magnitude of dust storms are strongly linked to climatic variability, so that in many areas dust emissions are inversely correlated to rainfall Although dust storm intensity in some areas is affected by human impacts, including water diversion for irrigated agriculture and poor land use practices, the largest sources of dust emissions in the Sahara are in uninhabited regions Aeolian Deposits Aeolian deposits include sand seas and dune fields, deposits of silt (loess), and fine-grained material that forms a significant component of desert margin and other soils Many marine deposits, especially those downwind of major desert areas, also include significant proportions of wind-transported material Silt and Clay Size Terrestrial deposits of wind-transported silt-sized quartz particles, termed loess, cover as much as 10% of Earth’s land surface Loess deposits are widespread in areas of northern China, central Russia, the Ukraine, Kazakhstan, central Europe, Argentina, Alaska, and the central United States Much of the material was thought to be derived from silt particles produced by glacial grinding and supplied to aeolian processes by glacial outwash (‘glacial loess’), but aeolian abrasion and other processes, including frost shattering, salt weathering, and fluvial comminution, are important, especially in the formation of ‘desert loess’ At least some loess (e.g., in Colorado and Nebraska) has its origins in weathered bedrock (e.g., siltstones, volcaniclastic deposits) Loess deposits and their interstratified buried palaeosols provide a long record of Quaternary climate change (Figure 8) In these sequences, the rate of loess deposition varies Figure Reduced visibility from major dust storm in West Texas, 15 December 2003 Compare with clear day (inset) Photographs by J Lee, Texas Tech University