A-Level Geography: Evolution of Arid Landforms in Different Regions of the World | A-Level 地理：世界不同地区干旱地貌的演变

📚 A-Level Geography: Evolution of Arid Landforms in Different Regions of the World | A-Level 地理：世界不同地区干旱地貌的演变

Arid landforms cover approximately one-third of the Earth’s land surface, yet they are not static features. They evolve over millions of years through an interplay of tectonic activity, climate shifts, and the slow but relentless work of wind, water, and weathering. This article examines the characteristic landforms of hot and cold deserts, explains the processes that create them, and compares their evolution in the Sahara, the Namib, and Death Valley.

干旱地貌覆盖地球陆地表面约三分之一，然而它们并非静止不变。在数百万年的地质时间里，它们通过构造运动、气候变化以及风、水与风化作用缓慢而持续的改造而不断演化。本文考察炎热与寒冷荒漠的典型地貌，解释塑造它们的过程，并比较其在撒哈拉、纳米布和死亡谷的演变差异。

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1. Defining Arid Environments | 干旱环境的定义

Arid environments are defined primarily by moisture deficiency. Most classifications use an aridity index, commonly the ratio of annual precipitation (P) to potential evapotranspiration (PET).

干旱环境主要由水分亏缺来定义。大多数分类采用干旱指数，通常为年降水量（P）与潜在蒸散量（PET）之比。

Aridity Index = P ÷ PET

Where P ÷ PET is less than 0.20, the climate is classified as arid; values between 0.20 and 0.50 represent semi-arid regimes. Hot deserts, such as the Sahara, typically receive less than 250 mm of rainfall per year, while some coastal deserts such as the Namib receive under 50 mm.

当P ÷ PET小于0.20时，气候被归类为干旱；介于0.20至0.50之间则属于半干旱。撒哈拉等热带荒漠年降水量通常不足250毫米，而纳米布等沿海荒漠年降水量甚至不足50毫米。

• High diurnal temperature ranges (up to 30 °C) promote mechanical weathering.

  巨大的昼夜温差（可达30 °C）促进机械风化。

• Sparse vegetation leaves the surface exposed to wind and runoff.

  植被稀疏使地表直接暴露于风力和径流作用之下。

• Rainfall is irregular and often arrives as intense convectional storms.

  降水极不稳定，常以强烈的对流性暴雨形式出现。

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2. Weathering Processes in Drylands | 干旱区的风化作用

Weathering is the breakdown of rock in situ. In arid environments, mechanical (physical) weathering dominates because both water and organic acids are scarce, while chemical weathering is limited but not absent.

风化作用是岩石在原地发生的崩解。在干旱环境中，由于水和有机酸稀缺，机械（物理）风化占主导地位，而化学风化虽有限但并非不存在。

Insolation weathering results from repeated thermal expansion and contraction of rock minerals during extreme day–night temperature changes. Differential expansion causes outer layers to peel away, a process sometimes called onion-skin weathering or exfoliation.

日晒风化源于极端昼夜温差引起的岩石矿物重复热胀冷缩。差异膨胀导致外层岩石片状剥落，这一过程有时被称为洋葱皮风化或剥离作用。

Salt weathering (haloclasty) occurs when saline groundwater or salt-laden spray evaporates inside rock pores. Crystallisation and hydration exert pressure up to several megapascals, disintegrating rock into angular debris. This process is especially effective in the salt pans of Death Valley and the coastal Namib.

盐类风化（盐裂作用）发生在含盐地下水或含盐飞沫在岩石孔隙中蒸发时。结晶与水化作用产生高达数兆帕的压力，使岩石崩解为棱角状碎屑。该过程在死亡谷盐沼和纳米布沿海尤为显著。

Chemical weathering, although limited, occurs through solution of limestone, hydration of silicates, and oxidation of iron-rich minerals. The Tassili n’Ajjer plateau in the Sahara displays karst-like solution features inherited from wetter periods.

化学风化虽然有限，但通过石灰岩的溶蚀、硅酸盐的水化以及富铁矿物的氧化而进行。撒哈拉的塔西利-恩-阿杰尔高原呈现出继承自较湿润时期的类喀斯特溶蚀地貌。

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3. Aeolian Erosion Landforms | 风蚀地貌

Wind is the most distinctive agent of erosion in deserts. Two processes operate: deflation, the lifting and removal of loose fine-grained particles, and abrasion, the sand-blasting of surfaces by wind-transported particles.

风是荒漠中最具特色的侵蚀营力。两种过程同时作用：风蚀吹蚀（defolation），即松散细颗粒被吹扬移走；磨蚀，即风运颗粒对地表进行砂粒轰击。

• Ventifacts are stones with flat, polished facets cut by abrasion. Their orientation records the prevailing wind direction.

  风棱石是被磨蚀出光滑平面的砾石，其朝向记录了盛行风向。

• Yardangs are streamlined ridges carved into soft rock, aligned parallel to the dominant wind. They can reach several metres high and hundreds of metres long, as in Iran’s Lut Desert.

  风蚀脊（雅丹）是软岩被风沿主风向刨蚀而成的流线型脊岗，高度可达数米、长度可达数百米，见于伊朗卢特荒漠。

• Zeugen are tabular blocks where a resistant cap rock protects softer underlying layers. Once the cap is breached, abrasion undercuts the softer rock to form mushroom-shaped pedestals.

  风蚀桌（石桌）是由坚硬盖层保护下部软弱岩层形成的桌状块体。一旦盖层被突破，风蚀便掏蚀下部软岩，形成蘑菇状石柱。

Deflation lowers the land surface over time. In unconsolidated sediments, it creates shallow depressions called blowouts; when the water table is reached, these may evolve into oases or playas.

吹蚀随时间推移而降低地表。在松散沉积物中，它形成浅洼地风蚀洼地；当洼地抵达地下水面时，可能演化为绿洲或干盐湖。

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4. Aeolian Depositional Landforms | 风积地貌

Wind deposition produces the most iconic desert features: dunes and sand seas. Dune form depends on sediment supply, wind direction variability, and vegetation cover.

风积作用塑造了最具标志性的荒漠地貌：沙丘与沙海。沙丘形态取决于物源供应、风向多变性和植被覆盖度。

• Barchans: crescent-shaped dunes with horns pointing downwind, formed on hard, flat ground with limited sand supply.

  新月形沙丘：脊形沙丘，两翼指向下风向，形成于坚硬平坦、沙源有限的地面上。

• Transverse dunes: long ridges perpendicular to the prevailing wind, formed where sand is abundant.

  横向沙丘：垂直于盛行风的长条沙脊，形成于沙源充足之处。

• Linear dunes (seif): long, straight or sinuous ridges parallel to the resultant wind direction, common in the Namib and Sahara.

  纵向沙丘（赛夫沙丘）：平行于合成风向的长直或波状沙脊，常见于纳米布和撒哈拉。

• Star dunes: multi-armed forms built where winds blow from many directions, such as in the Gran Erg Oriental.

  星状沙丘：在多向风汇合处形成的多臂状沙丘，如东方大沙海所见。

• Parabolic dunes: U-shaped with vegetation-anchored arms; they form in semi-arid fringes.

  抛物线沙丘：U形、两臂被植被固定的沙丘，形成于半干旱边缘地带。

Sand seas, known as ergs, cover up to 200,000 km² in the Sahara. Downwind of major drylands, wind-blown silt accumulates as loess, producing fertile but erodible soils in China’s Tengger region and the North African margins.

沙海即沙漠（ergs），在撒哈拉覆盖面积可达20万平方公里。在主要干旱区的下风向，风扬粉砂堆积为黄土，在中国腾格里周边和北非边缘形成肥沃但易侵蚀的土壤。

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5. Fluvial Landforms in Deserts | 荒漠中的流水地貌

Although deserts are dry, running water is paradoxically the most geomorphologically powerful agent in most of them. Ephemeral streams called wadis carry sediment only after rare torrential storms.

尽管荒漠干旱，但流水在大多数荒漠中恰是最具地貌威力的营力。称为旱谷（wadi）的间歇性河流仅在一次罕见的暴雨后才输移沉积物。

Flash floods have peak discharges many times greater than the long-term mean, enabling transport of boulders and rapid channel incision. When floodwaters exit mountain fronts onto flat lowlands, their velocity drops abruptly and sediment is deposited as alluvial fans. Adjacent fans merge into broad piedmont plains called bahadas.

暴洪的洪峰流量可达多年平均值的数倍以上，因而能搬运巨石并迅速下切河道。当洪水流出山口进入平坦低地时，流速骤降，泥沙以冲积扇形式堆积。相邻冲积扇联合形成宽阔的山前倾斜平原巴哈达（bahada）。

In interior drainage basins, water collects temporarily in playas (or chotts). Evaporation leaves evaporite crusts of halite and gypsum, which are later reworked by wind into salt-rich dust. Around the playa rim, groundwater capillary action sustains salt weathering.

在内流盆地中，水暂时汇集形成干盐湖（playa）。蒸发留下石盐和石膏蒸发壳，之后又被风改造为富含盐分的粉尘。在干盐湖边缘，地下水毛细作用持续驱动盐类风化。

Long-term river incision, combined with slope retreat, leaves isolated residual hills called inselbergs, developed on resistant granite or quartzite. Pediments – gently sloping bedrock surfaces – fringe these inselbergs and record the parallel retreat of steep slopes.

河流长期下切，加上坡地后退，在坚硬的花岗岩或石英岩上留下孤立的残丘岛山（inselberg）。山麓侵蚀面——平缓倾斜的基岩面——环绕岛山发育，记录了陡坡的平行后退过程。

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6. Desert Surface Evolution: Hamada, Reg and Erg | 荒漠地表演化：石漠、砾漠与沙漠

As weathering and erosion act over geological time, desert surfaces evolve through a predictable sequence of materials and forms.

随着风化和侵蚀在地质时间尺度上持续作用，荒漠地表会沿一种可预测的物质与形态序列演化。

Surface Type 地表类型	Description 描述	Evolutionary Stage 演化阶段
Hamada 石漠	Barren bedrock plateau stripped of all loose sediment 细粒沉积物被完全剥蚀的裸露基岩高原	Early–middle: deflation outpaces supply 早—中期：吹蚀快于补给
Reg / Serir 砾漠	Gravel plains with a lag pavement of wind-polished stones 由风磨砾石滞留层覆盖的砾石平原	Middle: deflation removes fines, pavement armours surface 中期：吹蚀移走细粒，砾石层保护地表
Erg 沙漠	Sand sea with mobile dunes 由活动沙丘组成的沙海	Late or local: sand accumulates where wind energy falls 晚期或局部：风力减弱处堆积成沙

Desert pavement forms a key stabilising layer. A stone-rich surface protects underlying fines from further deflation, while silt and clay accumulate beneath as a vesicular horizon. Over millennia, the pavement becomes darker due to manganese and iron oxide coatings known as desert varnish.

荒漠砾石层构成关键稳定层。富砾石表层保护下部细粒物质免受进一步吹蚀，而下层粉砂黏土则积聚形成气泡状层理。历经千年后，砾石表面因锰氧化物和铁氧化物膜而变暗，称为荒漠漆皮。

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7. Case Study 1: The Sahara Desert | 案例一：撒哈拉沙漠

The Sahara, covering about 9.4 million km², is the world’s largest hot desert. Its landforms include vast hamadas (Tassili n’Ajjer), limestone plateaus, and two great ergs – the Grand Erg Occidental and Grand Erg Oriental, each of which covers roughly 100,000 km².

撒哈拉面积约940万平方公里，是全球最大的热带荒漠。其地貌包括广阔的石漠（塔西利-恩-阿杰尔）、石灰岩高原，以及两大沙海——西方大沙海和东方大沙海，各覆盖约10万平方公里。

The evolution of the Sahara is a story of climate oscillation. During the African Humid Period (about 10,000–5,000 years BP), orbital forcing – changes in Earth’s axial precession – intensified the African monsoon. Lake Chad expanded to become Mega-Lake Chad, covering 350,000 km², and savanna vegetation supported large mammals and human pastoralists, as shown by the Tassili rock art.

撒哈拉的演化是一部气候震荡史。在非洲湿润期（约距今10,000至5,000年），轨道强迫——地球轴进动的变化——增强了非洲季风。乍得湖扩张为巨型乍得湖，面积达35万平方公里，热带稀树草原植被支撑了大型哺乳动物和人类游牧群体，塔西利岩画即是证据。

When orbital conditions reversed, the monsoon weakened, vegetation died back, and wind erosion reworked the exposed soils. The fossil dunes and river channels now buried beneath the Grand Erg record this rapid desertification around 5,000 years ago. Modern processes, especially deflation and salt weathering, continue to lower the hamada surfaces by roughly 1 cm per 1,000 years.

当轨道条件逆转时，季风减弱，植被枯死，风力侵蚀重新改造裸露土壤。如今埋藏在东方大沙海下的古沙丘和古河道，记录了约5,000年前发生的快速荒漠化。现代过程，尤其是吹蚀和盐类风化，正以约每千年1厘米的速率继续降低石漠表面。

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8. Case Study 2: The Namib Desert | 案例二：纳米布沙漠

The Namib, along the Atlantic coast of southwestern Africa, is often called the world’s oldest desert: arid conditions have persisted for more than 55 million years. Its age is attributed to the combination of the cold Benguela upwelling current and the rain shadow of the Namibian escarpment.

纳米布沙漠沿非洲西南大西洋海岸延伸，常被称为世界上最古老的荒漠：干旱条件已持续了逾5,500万年。其古老性归因于本格拉冷上升流和纳米比亚内陆悬崖雨影效应的共同作用。

The cold Benguela current cools the air, preventing convection and rainfall, and generates dense sea fogs that are the desert’s main moisture source. This fog sustains unique vegetation, including the endemic Welwitschia mirabilis, and drives hygroscopic salt weathering in the coastal strip.

本格拉冷洋流冷却空气，抑制对流和降水，并生成浓密海雾，成为该荒漠的主要水分来源。这种雾气不仅维系了包括特有植物百岁兰（Welwitschia mirabilis）在内的独特植被，也在沿海地带驱动吸湿性盐类风化。

In the northern Namib, the Naukluft sand sea contains some of the tallest linear dunes in the world, reaching 300 m. The dune fields are fed by sand transported northward from the Orange River mouth by longshore currents and then blown inland. Star dunes dominate where topographic obstacles create multi-directional winds near Sesriem and Sossusvlei.

在纳米布北部，瑙克卢夫特沙海拥有世界最高的纵向沙丘，最高达300米。沙丘沙源来自奥兰治河口的泥沙，经沿岸流北移后再被风向内陆搬运。在塞斯瑞姆和苏丝斯黎附近，地形障碍造成多向风，星状沙丘由此占据主导。

The Namib’s long evolution is also tectonic: continued uplift of southern Africa during the Miocene increased interior dryness and encouraged accelerated down-cutting of wadis, leaving broad pediments and deep canyon-like gorges at the desert margin.

纳米布的长期演化还与构造活动相关：中新世以来南非高原持续隆升，加剧了内陆干旱，并加速旱谷下切，在荒漠边缘留下了宽阔的山麓侵蚀面与深切的峡谷状沟壑。

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9. Case Study 3: Death Valley, USA | 案例三：美国死亡谷

Death Valley, in the Basin and Range province of California, is a tectonic graben bounded by normal faults and still deepening today. At its lowest point, Badwater Basin lies 86 m below sea level, making it the hottest and driest place in North America, with a record air temperature of 56.7 °C.

死亡谷位于加利福尼亚州的盆地与山脉区，是正断层围限的地堑，至今仍在加深。其最低点恶水盆地低于海平面86米，是北美最热最干之地，记录到56.7 °C的气温极值。

The valley floor is ringed by steep alluvial fans built from materials stripped from the Panamint Range and the Amargosa Range. During rare flash floods, debris flows deposit coarse breccia at the fan apex, while finer sediment reaches the playa. The fans are so young and active that their surfaces lack mature desert pavement.

谷底周围环绕着陡峭的冲积扇，物质来自帕纳明特山与阿马戈萨山的剥蚀。偶发暴洪时，泥石流将粗粒角砾岩堆积在扇顶，较细沉积物则到达干盐湖。这些冲积扇年轻而活跃，表面尚未形成成熟的荒漠砾石层。

Badwater Basin is a playa that repeatedly floods with brine and evaporates, depositing halite, gypsum, and borax. Salt polygons and tepee structures develop as crystallisation expands and warps the crust. Notably, traces of former deep lakes – shorelines of Pleistocene Lake Manly – can be seen at elevations up to 90 m above the playa floor, where the lake reached depths of 180 m about 185,000 and 10,000 years ago.

恶水盆地是干盐湖，反复被盐水淹没并蒸发，沉积石盐、石膏和硼砂。盐结晶膨胀使地壳翘曲，形成盐多边形和帐篷状构造。尤为重要的是，古深湖的遗迹——更新世曼利湖的古湖岸线——可见于高出湖底90米处；约在18.5万年前和1万年前，该湖深度曾达180米。

Death Valley thus illustrates how arid landforms evolve through the interaction of tectonics and Quaternary climate oscillation: active faulting creates accommodation space, while glacial–interglacial pluvials repeatedly drown and drain the basin, altering the position of playa, fan, and dune environments.

死亡谷因此展示了干旱地貌如何在构造与第四纪气候振荡的相互作用下演化：活动断裂创造沉降空间，而冰期—间冰期多雨阶段反复使盆地淹水和干涸

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