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Geology - Deserts and Winds

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    Study Guide Geology – Deserts and Winds 1. Desert Features Deserts 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 moves across the land is very different from wetter regions. 1.1 Streams in Deserts Most deserts do not have rivers or streams that flow all year. The climate is too dry to support constant water flow. Instead, many desert streams flow only occasionally. These temporary streams are called ephemeral streams . They appear after sudden rainfall and dry up soon afterward. There are a few important exceptions. Some major rivers begin in high, wet mountains and carry enough water to cross desert regions. Examples include the Colorado River in the southwestern United States and the Nile River in Egypt. Many deserts also have interior drainage systems . In these systems, streams do not flow to the ocean. Instead, they empty into closed, landlocked basins. Water collects in these basins for a short time and then evaporates. As the water disappears, salts and other minerals are left behind, forming salt b eds and evaporite deposits. 1.2 Flash Floods Even though deserts are dry, rainfall can be intense when it does occur. Most desert rain comes from short but powerful thunderstorms. The ground in deserts is often hard and compact, so water cannot soak in easily. Narrow stream channels also cannot carry large amounts of water. As a result, rainwater rushes across the surface as sheetwash , causing sudden and dangerous flash floods . Because deserts have little vegetation, the flowing water easily erodes the soil and rock. This erosion can carve deep gullies and steep slopes. Sometimes, the water becomes so filled with sediment that it turns into a thick mudflow .

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    Study Guide Fast - moving floodwater can cut narrow, steep - walled channels with gravel bottoms. These features are known by different names around the world: • Arroyos or dry washes • Wadis in Arabia and North Africa • Dongas in South America • Nullahs in India 1.3 Basin and Range Topography Some deserts, especially in the American Southwest, have a Basin and Range landscape. This pattern consists of long, steep mountain ranges separated by wide valleys. These mountains create rain 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 an alluvial fan . If water flows farther into the valley, it may collect in the center and form a playa lake . Playa lakes are shallow, muddy, and rich in clay. They evaporate quickly, leaving behind a flat, hard surface cracked by drying. If the water contains dissolved salts, salt flats develop instead. Over time, neighboring alluvial fans may merge to form a broad, gently sloping surface called a bajada . Between the mountain front and the bajada lies the pediment , a gently sloping erosion surface covered by thick layers of sediment — sometimes up to 30 meters (100 feet). Occasionally, isolated rocky hills rise sharply above the pediment. These are called inselbergs , and they are remnants of the former mountain front. 1.4 Plateaus, Mesas, and Bu Ʃ es Some desert landscapes are shaped by strong, resistant rock layers such as sandstone, limestone, or volcanic lava. Large elevated areas made of these rocks are called plateaus . Plateaus usually have steep edges marked by cliffs and gullies. As weathering and erosion wear away the sides of a plateau, smaller flat - topped landforms may remain. These are known as mesas . With continued erosion, mesas shrink further and become narrow, tower - like features called buttes . This gradual change from plateau to mesa to butte is called parallel retreat .

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    Study Guide Although these landforms are most common in deserts, they are controlled more by rock structure than by climate alone. 2. The E ff ects of Wind Wind is one of the most powerful forces shaping desert landscapes. Because deserts have very little vegetation, the wind can move freely across the land and carry large amounts of sediment. Over time, this movement of sediment changes the surface of the desert in dramatic ways. 2.1 How Wind Moves Sediment In deserts, wind acts as both an erosion and transportation agent, especially when it blows across fine - grained material like sand, silt, and clay. Wind carries sediment in two main ways: • Bed load consists of heavier grains, usually sand. These grains move by saltation , meaning they hop and bounce along the ground. Most of the time, they rise less than 1 meter (about 3 feet) into the air. • Suspended load is made up of much finer particles, such as silt and clay. These particles can stay in the air for long periods and travel great distances with the wind. 2.2 Wind Velocity in Deserts Wind speed depends on differences in air pressure caused by heating and cooling. Deserts experience 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 more during the day. These large temperature differences can create powerful winds that sometimes reach speeds of up to 70 miles per hour. 2.3 Dust Storms When strong winds blow across areas with large amounts of fine sediment, dust storms can form. These storms can become so intense that they block out sunlight.

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