About ten percent of the world's deserts are composed of sand dunes, which are driven across the desert by the wind. Sand grains march across the desert floor under the influence of strong winds by a process known as saltation. The grains of sand become airborne for a moment, and upon landing they dislodge additional sand grains, which repeat the process. In this manner, sand dunes engulf everything in their path, including structures made by people, and pose a major problem in the construction and maintenance of highways and railroads that cross sandy areas of desert. Sand-dune migration near desert oases poses another serious problem, especially when encroaching on villages. Methods to mitigate damage to structures from sand dunes include building windbreaks and funneling sand out of the way. Without such measures, insruption of roads, airports, agricultural settlements, and towns could become a major problem in desert regions.
The direction, strength, and variability of the wind, the moisture content of the soil, the vegetation cover, the underlying topography, and the amount of movable soil exposed to the wind determine the size and form of sand dunes. Sand dunes generally have four basic shapes, determined by the topography of the land and patterns of wind flow. Linear dunes align in roughly the direction of strong prevailing winds.
Their length is substantially greater than their width, and they lie parallel to each other, sometimes with a wavy pattern. Crescent dunes, also called barchans, are symmetrically shaped, with horns pointing downwind. They travel across the desert at speeds of up to 50 feet a year. Parabolic dunes
form in areas where sparse vegetation anchors the side arms while the center is blown outward, causing sand in the middle to move forward. Star dunes form by shifting winds that pile up sand into central points that can rise 1,500 feet and more, with several arms radiating outward, looking much like giant pinwheels.
42. Which of the following is probably
most influential in determining the
shape of parabolic dunes?
まMoisture
まVegetation
まWind speed
まSoil composition
文中只有寫到造成的因素有那麼多並沒有地方是寫到most influential的呀,請問要從哪兒看呢?
