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Introduction to Geology |
Chapter 11 - Deserts |
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| 1. Interpret the distribution of deserts on the Earth's surface. 2. Explain how precipitation and weathering affect surface topography of arid regions. 3. Examine desert landscape features. 4. Describe how wind can erode and transport materials. 5. Review the types of deposits created from wind-blown sediment. 6. Examine deserts and semiarid regions of North America. |
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| 1. Interpret the distribution of deserts on the Earth's surface. Dry regions comprise approximate what percent of Earth's land area? About 30 percent. desert—an arid, often sandy region of little rainfall, extreme temperatures, and with sparse vegetation or no vegetation at all. steppe—a large semiarid area of flat unforested grassland, such as the Great Plains region of the United States and Canada. tundra—a region of permanent cold that is largely or entirely devoid of life. Factors influencing the location of deserts and arid lands: * Global wind patterns control regional weather patterns. * Rising winds (low pressure) cool, causing precipitation; sinking winds (high pressure) warm and become unsaturated and dry. * Arid environments occur in both warm and cool regions. * Seasonal weather patterns also influence arid weather patterns (summer drought and winter monsoons). * Mountain ranges block atmospheric flow, forcing precipitation on the upwind side, and rainshadow conditions downwind. * Drought may create temporary desert conditions. Regional and global climate changes can expand or shrink desert regions. rainshadow—the downwind side of a mountain range (or high volcano) that partially blocks the the flow of moist air, forcing precipitation on the prevailing windward side, and creates more arid conditions on the downwind side. * Microclimate regions: south-facing slopes are dryer than north facing slopes in the northern hemisphere.
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| 2. Explain how precipitation and weathering affect surface topography of arid regions. * Bedrock composition and topography influence development of desert landforms. * Less weathering occurs in deserts because there is a lack of moisture and because there is reduced organic acids from plants. * Infrequent storms cause rapid erosion and prevent the establishment of plants and soil formation. * Intense rainfall causes flash floods and reduces infiltration. * Desert environmental conditions allow salts, particularly calcium carbonate, to accumulate and reduce porosity and permeability in surficial sediments. badlands—uncultivatable land with typically rugged relief, a heavily eroded appearance, and bares little or no vegetation. arroyo—a watercourse (water-carved gully, channel) in an arid region. Arroyos are typically dry (ephemeral) but are prone to flash floods after rare seasonal thunderstorms. caliche—a hardened zone in soils and surficial deposits found in semiarid regions where of calcium carbonate and possibly other carbonates, clay minerals, or crystalline salts such as sodium chloride or sodium nitrate impregnated the pore spaces in the sediment or soil.
pediment—a gently inclined erosional surface carved into bedrock, typically covered with stream gravel that has developed at the foot of mountains. It develops when running water erodes most of the mass of the mountain down to a base-level consistent with surrounding alluvial fans in an arid or semiarid region. inselberg—an isolated rocky hill or mountain rising above a plain-like landscape in a typically hot, dry region. grus—grainy sand and fine gravel (sediments) derived from the weathering of granitic rocks, typically in arid or semiarid regions. desert pavement—a gravel layer that forms on the surface of many desert landscapes where the wind has removed finer materials (sand, silt, dust).
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| 3. Examine desert landscape features and Depositional environments Formation of alluvial fans and bajada alluvial fan— An outspread, gently sloping mass of sediment deposited by a stream where it issues out of the mouth of a narrow canyon draining from and upland area. Viewed from above, an alluvial fan typically has the shape of an open fan with the apex being at the mouth of the canyon. Alluvial fans are common in arid to semi-arid regions, but can be covered with forests in the California Coast Ranges. Alluvial fans may merge together to form an apron-like slope along the base of a mountain front. bajada—an alluvial plain along the base of a mountain front formed from the accumulation and coalescing of of alluvial fans.
Salt Lakes, Salt Pans, and Playas oasis—a fertile or green spot in a desert or wasteland, made so by the presence of water. playa—an ephemeral (intermittently flooded) lake bed that occupies the lowest part of a internally drained, isolated valley in an arid or semiarid region. salt pan—a shallow basin, usually in a desert region, containing salt, gypsum, sodium carbonate, or other evaporite minerals that was deposited from an evaporated salt lake (playa) setting.
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| 4. How does wind erode and transport materials? Wind in arid regions lifts and carries away dust-sized particles. Sand grains are carried in the "bed load" moving by rolling and bouncing along the surface (a process called saltation). Pebbles, cobbles, and boulders accumulate on the land surface forming desert pavements. Over time, exposed rock can take on an wind abraded appearance. abrasion—the process of wearing down or rubbing away by means of friction, typically by wind-blown dust or sand. deflation—the removal of particles of rock, sand, soil or dust, by the wind, often leaving a rocky crust (a desert pavement) on a desert landscape. ventifact—a stone on the surface in a desert environment that is unusually shaped, abraded by wind-blown sediment. Markings on ventifacts typically reflect seasonal prevailing wind patterns. dune—an accumulation of wind-blown sand and/or silt found in association with deserts or sandy beach settings where there is a constant supply of silt to sand grain-sized sediments. star dune—a dune with three or four arms radiating from its usually higher center so that it resembles a star in shape. Star dunes form when winds blow from three or four directions, or when the wind direction shifts frequently. loess—a tan, buff to gray windblown deposit of fine-grained, loamy, calcareous silt or clay; fine-grained deposits typically derived from glacial outwash plains or dust derived from arid regions. Loess deposits blanket large regions of the northern Great Plains and the Midwest.
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5. Review the types of deposits created from wind-blown sediment.
Other desert landscape features desert varnish—a dark, hard film of oxides formed on exposed rock surfaces in arid regions.
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6. Examine deserts and semiarid regions of North America. Chihuanhuan Desert—North America's largest desert, considered a "rainshadow" desert on the east side of the Sierra Madre (the mountain range of central Mexico. Extends from near Mexico City northward into west Texas and New Mexico. Sonoran Desert—a desert with a unique ecosystem that covers large parts of the Southwestern United States in Arizona and California, and Northwest Mexico in Sonora and Baja California, best know for its Saguaro and Organ Pipe Cactus, generally a frost free region. Colorado Desert—western part of the Sonoran Desert, located in low elevations along the lower Colorado River and southern California encompassing the Salton Sea region. Mojave Desert—a high desert region located in southern California and parts of southern Nevada, southwestern Utah and northwestern Arizona. It is a region with "Basin and Range" topography. Great Plains—high, semi-arid grasslands (steppe) region in the rainshadow east of the Rocky Mountains extending from central Canada to central Texas. Colorado Plateau—high, semi-arid region between the Great Basin and the Rocky Mountains in the 4Corners region (Colorado, Utah, Arizona, and New Mexico. Much of the region is covered with pinyon-pine and juniper shrub forests.
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Desert Landforms and Surface Processes in the Mojave National Preserve and Vicinity National Parks in Desert Environments Death Valley National Park Mojave National Preserve Joshua Tree National Park Saguaro National Park Lake Mead National Recreation Area Big Bend National Park Badlands National Park Parks of the Colorado Plateau Arches National Park Canyonlands National Park Natural Bridges National Monument Capital Reef National Park Bryce Canyon National Park Cedar Breaks National Monument Grand Canyon National Park Glen Canyon National Recreation Area |
Quiz Questions |