Geology - Deserts and Winds - Quick Guides | Geology

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Study GuideGeology–Deserts and Winds1. Desert FeaturesDeserts may look simple at first glance, but they contain many unique landforms created by water,wind, and erosion. Because deserts receive very little rainfall, the way water movesacross the land isvery different from wetter regions.1.1 Streams in DesertsMost deserts do not have rivers or streams that flow all year. The climate is too dry to supportconstant water flow. Instead, many desert streams flow only occasionally.These temporary streams are calledephemeral streams. They appear after sudden rainfall and dryup soon afterward.There are a few important exceptions. Some major rivers begin in high, wet mountains and carryenough water to cross desert regions. Examples include the Colorado River in the southwesternUnited States and the Nile River in Egypt.Many deserts also haveinterior drainage systems. In these systems, streams do not flow to theocean. Instead, they empty into closed, landlocked basins. Water collects in these basins for a shorttime and then evaporates. As the water disappears, salts and other minerals are left behind, formingsalt beds and evaporite deposits.1.2 Flash FloodsEven though deserts are dry, rainfall can be intense when it does occur. Most desert rain comes fromshort but powerful thunderstorms.The ground in deserts is often hard and compact, so water cannot soak in easily. Narrow streamchannels also cannot carry large amounts of water. As a result, rainwater rushes across the surfaceassheetwash, causing sudden and dangerousflash floods.Because deserts have little vegetation, the flowing water easily erodes the soil and rock. This erosioncan carve deep gullies and steep slopes. Sometimes, the water becomes so filled with sediment thatit turns into a thickmudflow.

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Study GuideFast-moving floodwater can cut narrow, steep-walled channels with gravel bottoms. These featuresare known by different names around the world:•Arroyosordry washes•Wadisin Arabia and North Africa•Dongasin South America•Nullahsin India1.3 Basin and RangeTopographySome deserts, especially in the American Southwest, have aBasin and Rangelandscape. Thispattern consists of long, steep mountain ranges separated by wide valleys.These mountains createrain shadows, blocking moisture and helping maintain desert conditions.When rain falls on the mountains, it carries rock fragments and sediment downhill.At the mouth of a canyon, this sediment spreads out to form analluvial fan. If water flows farther intothe valley, it may collect in the center and form aplaya lake. Playa lakes are shallow, muddy, andrich in clay. They evaporate quickly, leaving behind a flat, hard surface cracked by drying. If the watercontains dissolved salts,salt flatsdevelop instead.Over time, neighboring alluvial fans may merge to form a broad, gently sloping surface called abajada. Between the mountain front and the bajada lies thepediment, a gently sloping erosionsurface covered by thick layers of sediment—sometimes up to 30 meters (100 feet).Occasionally, isolated rocky hills rise sharply above the pediment. These are calledinselbergs, andthey are remnants of the former mountain front.1.4 Plateaus, Mesas, and BuƩesSome desert landscapes are shaped by strong, resistant rock layers such as sandstone, limestone, orvolcanic lava. Large elevated areas made of these rocks are calledplateaus. Plateaus usually havesteep edges marked by cliffs and gullies.As weathering and erosion wear away the sides of a plateau, smaller flat-topped landforms mayremain. These are known asmesas.With continued erosion, mesas shrink further and become narrow, tower-like features calledbuttes.This gradual change from plateau to mesa to butte is calledparallel retreat.

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Study GuideAlthough these landforms are most common in deserts, they are controlled more by rock structurethan by climate alone.2. The Effects of WindWind is one of the most powerful forces shaping desert landscapes. Because deserts have very littlevegetation, the wind can move freely across the land and carry large amounts ofsediment. Over time,this movement of sediment changes the surface of the desert in dramatic ways.2.1 How Wind Moves SedimentIn deserts, wind acts as both an erosion and transportation agent, especially when it blows acrossfine-grained material like sand, silt, and clay.Wind carries sediment in two main ways:•Bed loadconsists of heavier grains, usually sand. These grains move bysaltation, meaningthey hop and bounce along the ground. Most of the time, they rise less than 1 meter (about 3feet)into the air.•Suspended loadis made up of much finer particles, such as silt and clay. These particlescan stay in the air for long periods and travel great distances with the wind.2.2 Wind Velocity in DesertsWind speed depends on differences in air pressure caused by heating and cooling. Desertsexperience extreme temperature changes, which strongly affect wind movement.Temperatures in deserts may drop to about 7°C (45°F) at night and rise to 43°C (110°F) or moreduring the day. These large temperature differences can create powerful winds that sometimes reachspeeds of up to 70 miles per hour.2.3 Dust StormsWhen strong winds blow across areas with large amounts of fine sediment,dust stormscan form.These storms can become so intense that they block out sunlight.

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