第六章 风的地质作用.pdf

1 第六章风的地质作用 Mushroom 概述 高压区的空气向低压 区流动而成风 能量P 1 2 V2 运动形式 层流和紊 流 作用方式 机械作用 干旱 半干旱地区影 响巨大 2 Wind is common in arid desert regions because 1 Air near the surface is heated and rises cooler air comes in to replace hot rising air and this movement of air results in winds 2 Arid regions have little or no soil moisture to hold rock and mineral fragments 6 1 Wind Erosion 风蚀作用风蚀作用 Little Power to Attack Solid Rock Most Effective in Moving Loose Material Wind can be effective agent of erosion anywhere that it is strong enough to act Wind can erode by deflation and abrasion Deflation 风蚀风蚀 Deflation is the lowering of the land surface due to removal of fine grained particles by the wind Deflation concentrates the coarser grained particles at the surface eventually resulting in a surface composed only of the coarser grained fragments that cannot be transported by the wind Such a surface is called desert pavement 沙漠 砾石滩 沙漠 砾石滩 Abrasion 磨蚀磨蚀 Ventifacts 风棱石风棱石 are any bedrock surface or stone that has been abraded or shaped by wind blown sediment in a process similar to sand blasting Yardangs 雅丹地貌雅丹地貌 are streamlined wind eroded ridges commonly found in deserts 3 覆盖着荒漠漆的戈壁滩 Yardang Yadang 6 2 Sediment Transportation by Wind Rolling Coarse Sand and gravel Wind transports sediment near the surface by saltation Sand Smaller particles Silt can become suspended in the wind and may travel for longer distances 4 Wind Transportion 风的搬运作用资料 内蒙古西部雅布赖山口的砂河 6 3 Wind Deposits Wind can deposit sediment when its velocity decreases to the point where the particles can no longer be transported This can happen when topographic barriers slow the wind velocity on the downwind side of the barrier As the air moves over the top of the barrier streamlines converge and the velocity increases 堆积型地貌之一 沙漠 风成沙波纹 新月形沙丘 排状沙堤 5 Desert 塔克拉玛干沙漠 Sand dunes 沙丘 Sand Dunes Sand dunes form when there is 1 a ready supply of sand 2 a steady wind and 3 some kind of obstacle such as vegetation rocks or fences to trap some of the sand Sand dunes are asymmetrical mounds with a gentle slope in the upwind direction and steep slope called a slip face on the downwind side Dunes migrate by erosion of sand by wind saltation on the gentle upwind slope and deposition and sliding on the slip face and thus are cross bedded deposits Dunes may cover large areas and reach heights up to 500m Types of sand dunes Barchan Dunes 新月形沙丘新月形沙丘 are crescent shaped dunes with the points of the crescents pointing in the downwind direction and a curved slip face on the downwind side of the dune They form in areas where there is a hard ground surface a moderate supply of sand and a constant wind direction Types of sand dunes Transverse dunes 横向沙丘横向沙丘 are large fields of dunes that resemble sand ripples on a large scale They consist of ridges of sand with a steep face in the downwind side and form in areas where there is abundant supply of sand and a constant wind direction Barchan dunes merge into transverse dunes if the supply of sand increases Types of sand dunes Linear Dunes 线形沙丘 are long straight dunes that form in areas with a limited sand supply and converging wind directions 6 Types of sand dunes Parabolic also called blowout Dunes 抛物线沙丘抛物线沙丘 are U shaped dunes with an open end facing upwind They are usually stabilized by vegetation and occur where there is abundant vegetation a constant wind direction and an abundant sand supply They are common in coastal areas Types of sand dunes Star Dunes 星状沙丘星状沙丘 are dunes with several arms and variable slip face directions that form in areas where there is abundant sand and variable wind directions 沙丘类型 A新月形 B横向沙丘 C弧形沙洲 D纵向沙丘 E抛物线形沙丘 F星形沙丘 斜层理的形成 风向的改变可 以形成交错层 理 斜层理的特点 地质历史上风成沉积 7 风成交错层理的形成 风成大型交错层理 Loess 黄土黄土 Silt sized Derived from Glacial Outwash in U S Found in U S E Europe China Loess in China is indirectly derived from glacial outwash which is carried into the desert lowlands by streams Wind transports loess from the deserts and the loess is in turn redeposited by the rivers Desert and Loess in China 黄土峰林 堆积型地貌 风成黄土 8 美国南达科达州的风成黄土 2006年4月17日北 京的早晨 日降 尘量 33万吨 6 4 Desertification Desertification occurs as a result of climatic changes such as changing positions of the continents or changes in ocean and air circulation patterns Human impacts such as overgrazing draining of land and lowering of the groundwater table can also contribute to desertification As vegetation dies out the soil is more easily eroded an