Geog 132 Exam 3 Study Guide
Geog 132 Exam 3 Study Guide GEOG 132
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GEOGRAPHY 132 EXAM 3 STUDY GUIDE Glacial Processes and Landforms Glaciers are large amounts of ice that rest on land or float in water. They are formed when snow continually accumulates on top of itself and compacts, and they generally move due to gravity and carve out the landscape, taking sediment with them and also leaving it behind Glaciers form in areas where there is permanent snow, such as very high latitudes and elevations. 11% of earth’s surface is currently covered by glaciers Alpine glaciers are smaller glaciers that form in mountains, and continental glaciers are large ice sheets that cover entire continents The first step in glacier formation is snow falls and accumulates. After snow survives the summer melting season, it becomes firn, which is more granular and higher density due to pressure from burial. After many years of snow piling on top and compacting it further, firn becomes glacial ice Glaciers are open systems with inputs being snow and outputs being ice, meltwater, and water vapor. Glaciers are advancing when inputs exceed outputs and retreating when outputs exceed inputs Accumulation occurs at the top of the glacier where new snow falls and mass is added. Ablation occurs at the bottom of the glacier where ice is lost due to melting, wind erosion, calving, etc. The equilibrium line, or firn line, is the lower boundary of the accumulation zone where it transitions to being the ablation zone Ice on the surface of the glacier is brittle and cracks easily when subjected to different speeds of movement or pressure; the cracks formed are called crevasses. Ice deep within the glacier is plastic and deforms easily Most glacial movement is internal. Basal slip is generally slower movement at the bottom of the glacier. Regelation is when the ice at the bottom of the glacier melts and refreezes. The top layer often moves faster than the bottom layer, which creates stress and forms crevasses. Most glaciers move less than 1 or 2 km per year. Occasionally, there are glacial surges, where glaciers move a lot in one day, sometimes up to 30 meters Alpine glaciers form erosional landforms. Cirques are semicircular depressions carved into the landscape, and horns are mountain peaks formed where cirques have been carved on all sides. Aretes are sharp ridges, possibly formed between two cirques, and a col is an arête that has been eroded. Tarns are lakes formed within cirques, and when there is a set of them, they are called paternoster lakes. Hanging valleys are side valleys higher up than the main valley, and U-shaped valleys are valleys that have been carved into a U shape, with steep sides and a flat bottom Fjords are u-shaped valleys carved out below sea level that subsequently fill with water and form inlets Glacial drift is the general term for all glacial deposits. Stratified drift is deposited material that is sorted by size, deposited by meltwater, and is mostly rounded. Glacial till is directly deposited by ice and is neither sorted nor stratified, so thus is less rounded Moraines are deposited till material. Lateral moraines are deposits at the side of the glacier. Medial moraines form where lateral moraines from two glaciers join when the glaciers come together, and they are less lasting than lateral moraines. Terminal moraines are at the very front end of the glacier, at its farthest extent. Recessional moraines form when a glacier forms a terminal moraine, then recedes and stagnates, forming another moraine behind it Glacial erratics are very large materials, such as boulders, transported by glaciers and dropped off in landscapes outside of where they would typically be found Continental glaciers today are found in Greenland and Antarctica, but previously, there were many Outwash plains form in front of terminal moraines where the meltwater flows out. Materials are sorted by size, and many streams and lakes form. Till plains form behind terminal moraines, and contain material that is unsorted because ice was on top of it Common features found in till plains where retreating glaciers were present are eskers, which are curving ridges of coarse sand and gravel formed along the channels of meltwater streams under glaciers, and kettle lakes, which are formed when a large block of ice is left behind to depress the land and melt as the rest of the glacier retreats. They are steep-sided and often clumped together. There are also kames, which are often poorly-sorted hills deposited directly by water, many times in crevasses. Features found with advancing glaciers are roche moutonnees, which are eroded, asymmetrical areas of exposed bedrock, and drumlins, which are accumulations of deposited till that has been streamlined in the direction of ice movement. Roche moutonnees are smooth on the side the glacier is coming from and cracked on the side where it is going, and as drumlins often occur in multitudes, a group of drumlins is called a drumlin field/swarm/basket of eggs Periglacial landscapes are areas near alpine or continental glaciers, and make up about 20% of earth’s surface. They have cold-climate processes, landforms, and topographic features, and are in climates with near- permanent ice cover, such as tundra, polar, and subarctic areas Permafrost develops when rock or soil temperatures are below freezing for at least 2 years, and permafrost layers can go as deep as 1000 meters but are usually around 400 meters. Discontinuous permafrost zones have some regions that do not stay frozen year-round, and are thus more susceptible to climate change than continuous zones, which do stay frozen year-round. The active zone of soil in permafrost areas is found between the surface and permafrost layer, is seasonally frozen, and is where trees grow in discontinuous areas. Permafrost areas have poor drainage, and thus have a bumpy appearance due to freeze-thaw action Ground ice is frozen groundwater. Cryoturbation is the freeze-thaw action that occurs in these areas. An ice wedge is formed when water seeps into a crack and freezes and it keeps happening in the same spot until a wedge is formed. Patterned ground is a congregation of stones/gravels/soils in an area that takes a long time to form. Flatter slops will have more polygon shapes, while areas with steeper slopes tend to be striped. Gelifluction is the slow flow of soil in permafrost areas When people build houses in periglacial landscapes and they are not insulated properly, permafrost will melt and the ground will subside, causing the house to collapse. For this reason, houses need to be properly insulated, and utilities and everything else must be stored above ground to prevent interference with the permafrost Pleistocene Glaciation and Paleoclimate The Pleistocene epoch began 2.6 million years ago and ended 11,700 years ago, and had at least 18 glacial-interglacial cycles. It was a period of marked instability in climate “Ice age” is a term given to cold periods in geologic time, and within an ice age, there are cold (glacial) periods and warm (interglacial) periods. The last glacial maximum (LGM) is the period most recent in earth’s history when ice sheets were at their greatest coverage, and the LGM was about 18000 years ago A stadial is a small period of cold during a larger interglacial period, and an interstadial is a small period of warm during a larger glacial period Marine isotope stages are derived from soil cores in the deep ocean, and they display the amount of isotopes of certain elements present at times throughout history, and the isotope presence varies based on global climate, so this tells us when the earth was experiencing certain types of climates Proxy evidence such as ice cores, ocean sediments, and glacial geomorphology is used to reconstruct past climates, but they are very general as proxy evidence is not very specific. Ice cores provide annual data, so they can be used to construct climates in more detail and can have individual years examined to learn things During the LGM, sea level was ~100 meters lower than it is today, sea temperatures were on average 1.4-1.7 degrees C lower than they are today, and air temperatures were as much as 12 degrees C lower than they are today. Large ice sheets covered most of northern Eurasia and North America, and were as much as 3000 meters thick over some parts of Canada. The Laurentide Ice sheet, which was over North America, extended to the Ohio River and carved the Great Lakes, but no ice from this ice sheet remains today; Greenland, however, did not melt The lower sea levels allowed the Beringia land bridge to exist, so people were able to travel by land to North America and populate the Americas Giant glacial lakes formed in the western US during deglaciation when a ton of water was being released from the glaciers, namely Agassiz, Chicago, and Ojibway. There were also paleolakes, such as Lake Bonneville, which is now Great Salt Lake. It existed from 30000 to 12000 years ago, and glacial melt was not its only source of water. It is thought to be the result of increased rainfall, making it a pluvial lake One potential cause of the Ice Ages is orbital forcing, or Milankovitch Cycles. These cycles account for variations in the amount of solar energy coming to earth, which causes the ice ages. Obliquity, which is the tilt of the earth’ axis, varies on a 40000 year cycle. Precession, which is the wobbling of the earth around its axis, varies on a 20000 year cycle. Eccentricity, which is the shape of the earth’s orbit, varies on a 100000 year cycle Deglaciation occurs faster than glaciation because warming is relatively rapid, and all ice from the LGM was gone by ~7000 years ago; there have, however, been several notable warm and cold periods since the LGM, such as Younger Dryas, Medieval Warm Period, and Little Ice Age. Currently, the Larsen A & B ice shelves are deglaciating Fluvial Processes and Landforms The largest river in the world by length is the Nile, and the largest river by discharge is the Amazon Fluvial denotes anything having to do with stream-related processes and hydrology is the study of the movement and distribution of water Material is picked up and eroded by the running water in streams as it gathers and carries particles, transported by the running water until it is no longer able to be carries, and then deposited as alluvium when the stream can no longer carry it Base level is the lowest level to which streams can erode their valleys, and sea level is ultimate base level. A local or temporary base level can be created by something like a lake, reservoir, or layer of resistant rock A drainage basin is the area of land that drains water into a stream of any size, and drainage basins are separated from each other by areas of higher ground called drainage divides, which make up the outer limits of the watershed. The watershed is the actual area that receives water within a given drainage basin When water moves downward across land in a thin film, it is called sheetflow or overland flow. This water then accumulates in small grooves called rills, which are eroded by the water until they grow into gullies. Gullies are separated by higher land areas called interfluves, and eventually, when gullies come together and grow in size, they form a stream Continental divides are drainage divides that separate very large drainage basins from each other; one example is the Rocky Mountains Exotic streams are streams that start in wet areas and flow into dry areas, such as the Nile and Colorado Rivers. Discharge decreases with distance because of evaporation, which is the opposite of what typically occurs in streams Internal drainage occurs in drainage basins with streams that don’t reach the sea, often ending in inland lakes with no outlet, resulting in these lakes being extremely salty because water is evaporated but the salts from within it have nowhere to go. Examples are the Dead Sea, Aral Sea, and Great Salt Lake Drainage basins are open systems, and that is important because it means changing anything within a drainage basin can and will impact the rest of the system. Regulation is difficult, however, because they often include more than one state or nation Drainage density is calculated by taking the total length of all streams within the drainage basin and divided by the area of the drainage basin. It is a measure of the importance of fluvial erosion. Drainage pattern is the spatial arrangement of channels There are 7 drainage patterns. The first is dendritic, which is the most efficient because it has the fewest number of channels and is also the most common; it is shaped like a tree. Next is trellis, which is common in dipping and folding topography. Ridges define drainage basins, and streams are found in valleys. When one stream erodes the interfluve of another and drains into another branch, it is called stream capture or stream piracy. Third is radial, which is a pattern in which streams flow outward off a central peak. Fourth is parallel, which is similar to dendritic, but is found in areas with steep slopes. The 5 pattern, rectangular, is typical of jointed and faulted areas, where water flows in faults and joints and streams often meet at right angles. The 6 pattern is annular, which is found in structured domes when rock strata dictate streams that flow in concentric circles. The 7 and final pattern is deranged, which has no real pattern of stream valleys and is often the result of glaciation Superimposed streams are streams that flow in areas where they typically should not, and they often exist because they were there before the topography of the landscape was what it currently is Water at base level has potential energy, but it becomes kinetic energy when the water is above base level and begins flowing down Discharge is the amount of water being moved by a stream at a given moment, and is calculated by Q (discharge) = width x depth x velocity Streams can vary in width, depth, velocity, and sediment load The slope near the headwaters of stream is steeper, and as the stream approaches its mouth, it flattens out. Over time, the stream will erode the rock of the channel and become less steep Hydraulic action is the work done by flowing water in the stream, and abrasion is the mechanical erosion of the stream with moving rock particles as they scrape against other materials Competence denotes a stream’s ability to move materials of a specific size, and is a function of the velocity and energy available within the stream to suspend materials. Capacity denotes the amount of particles the stream is able to transport In the solution mechanism of sediment transport, the stream carries dissolved particles which have been chemically weathered. With suspension, the water carries fine-grained clastic particles in the water, which is known as the suspended load, and these particles are deposited when the stream’s velocity is nearly 0. Saltation and traction move coarse materials too large to be carried along the bottom of the stream in what is known as the bedload. Saltation is when the materials move in a hopping motion, and traction is when they are dragged along When sediment load exceeds capacity, aggradation (build-up and deposition) occurs. When capacity exceeds sediment load, degradation (erosion) occurs Stream systems seek out equilibrium between sediment load, slope, and particle sizes The 3 types of stream channels are straight, braided, and meandering. Straight channels are formed when transport capacity exceeds sediment load and are common in the upper reaches of streams in the mountains. Braided streams occur when sediment load is greater than stream capacity, and sediment accumulated in channels and chokes the stream. Due to this, multiple channels form to move through the sediment instead of just one, and the drainage pattern is deranged. Braided channels are common in glacial outwash plains. Meandering streams occur when sediment load and capacity are about equal, and are sinuous, winding channels On the outside of a meander (curve), water moves faster, causing the area of land to be eroded and forming a steep bank called a cut or undercut bank. On the inside of the meander, water moves more slowly, causing alluvium to be deposited and forming a point bar Meandering streams are always moving, and eventually, the outsides of two meanders will meet and cut off part of the stream, changing the route. When this occurs, the cut off portion of the stream is called an oxbow lake, and when it dries up, it is called a meander scar Floodplains are flat, low-lying areas next to rivers that are prone to frequent flooding and contain rich farmland. Natural levees can form on the sides of some streams when the stream floods and larger particles are left behind on land. Backswamps form outside of natural levees when rivers overflow during spring melt and deposit sediments behind the levees in alluvial plains. Yazoo tributaries are streams flowing parallel to the main channel that are unable to join back in the main channel because they are blocked by natural levees, etc. The stream gradient is the degree of inclination (slope) of the stream. Nickpoints are points where the profile of the stream is interrupted and waterfalls form. Niagara Falls is an example of this Alluvial terraces are topographic steps that form in a valley landscape and extend above the fluvial system. They are generally paired at similar elevations on opposite sides of the valley, except for with meandering rivers, because they cut lower each time and thus will have different elevations and unpaired terraces. Floodplains can be abandoned due to uplift, climate change (e.g. more rain), and forced channel straightening Floodplains make excellent agricultural lands because they are periodically flooded and have nutrients, silts, and clays deposited on them, which increases the water holding capacity, cation exchange capacity, and organic content. For this reason, people often want to settle in floodplains It’s a terrible idea to live in floodplains because they flood all the time, which means that farms will flood, and damage will be caused to people, property, and crops/livestock during the inevitable event of a flood. People like to settle near rivers for fertile soil, transportation, and industrial applications, but this greatly increases risk, because if a flood occurs where waste is being stored, the area will become a contaminant sink. In addition, human settlement hardens the environment, which prevents water from being able to sink into the ground and makes flash floods more likely A flood is an event in which the water level overflows the riverbank, and floods are rated based on the expected time interval between them. For example, a 10 year flood is one expected to occur once every ten years or has a 10% chance of happening every year Deltas are level or nearly level floodplains located at the mouths of rivers. They form when streams reach base level and slow down, depositing sediment around the mouth of the river. The stream splits into smaller channels called distributaries and flows out that way, and deltas are composed of alluvium. Conditions favoring delta formation are large discharge and sediment load, shallow water offshore, a sheltered coastline, and a low tidal range Braided streams create braided deltas, and arcuate deltas are arc-shaped. No delta forms when ocean currents remove materials; the Amazon is an example of this because it shoots its sediments too far out into the ocean and they just get swept away. Estuarine deltas form where freshwater is discharged into seawater, and bird-foot deltas are combinations of many smaller deltas with lots of distributaries, such as the Mississippi River Alluvial fans are deltas that don’t reach the ocean and instead drain toward playas, which are large salt flats Physiography of North America Physiography refers to the form and physical characteristics of the landscape There are 10 physiographic regions in the US, which are broken up into 25 provinces The Alaska-Yukon region contains several significant mountain ranges and the Yukon valley, and a mixture of forest, tundra, and wetland. It contains low vegetation, low energy systems, and glaciation The Arctic Coastal Plain is a low area dominated by tundra and wetlands on the north coast of Alaska. It is flat and low, very cold, at the coast, and contains tundra vegetation. The Arctic National Wildlife Refuge is also here The Canadian Shield is an exposed craton. Glacial ice covered this region during the Pleistocene, has a super rocky landscape, and deranged drainage is common here The Interior Plains is a large area of gentle topography between the Appalachians and the Rockies, and is underlain by deep deposits of gravel and nonconsolidated material. Within the Interior Plains is the High Plains Province, which has a higher elevation. It exists due to t=being in the rain shadow of the Rockies, and has mollisols, grassland vegetation, and tall grass, making up the western portion of the Great Plains The Interior Highlands contain the Ozark Plateaus and Ouachita Mountains, and this is a mountainous area composed of rock folded in the collision of Laurasia and Gondwana The Atlantic Coastal Plain wraps around the Gulf of Mexico and the coastline of the Atlantic Ocean, and is made of marine sediments that now gently slope toward the coast. It is a relatively flat area with little relief and a passive coast, and it extends through the Mississippi Valley to southern Illinois, and contains the Mississippi Embayment, which is the former coast of a large bay that used to exist. The border between the Coastal Plain and Appalachian Region is called a fall line, which is sort of a cliff where rapids form in all the rivers because flow changes, and many cities exist on the fall line, such as Philadelphia and Baltimore The Appalachian region is a fold and thrust belt uplifted during the collision of Laurasia and Gondwana 225 million years ago and consists of old, severely eroded mountains. A common feature on these mountains is a bald, which is when the mountain is at a latitude to support tree growth, but elevation is too high to allow for trees to grow while also being too low for alpine features to form The Rocky Mountain region transitions abruptly from the Great Plains, contains the continental divide, and has elevations greater than 14000 feet, which is much higher than the Appalachians, with tundra environments existing at high elevations. The Rockies contain intrusive and extrusive features of igneous, metamorphic, and sedimentary rock, and the current Rockies uplifted around 69 million years ago in the Laramide Orogeny. There was extensive glaciation in the Rockies, and out of the 3 parts (northern, central, and southern), only the Southern Rockies were never glaciated The Intermontane region is an arid to semiarid area west of the Rockies with 3 notable sub regions. The first is the Columbia Plateau, which is a high nd desert atop 2-3km thick basaltic lava. 2 is the Basin and Range province, which has a tensional horst and graben landscape and a series of north-south mountain ranges and valleys, giving the area lots of relief. 3 is the Colorado Plateau, which is composed of uplifted but almost perfectly horizontal sedimentary rocks and has very dramatic geologic features, such as canyons The Pacific Mountain System is a series of mountain ranges, such as the Sierra Nevadas and Cascades, extending from Mexico to Alaska. The converging plate boundary along the Pacific coast makes it an active coast, so there are no coastal plains. The mountain ranges all hav different origins: the Sierra Nevadas are uplifted granite, the Cascades are volcanoes, and the mountains of Alaska and Canada are accreted terranes Physiography of Tennessee Tennessee contains 9 provinces. From west to east, they are as follows: Mississippi Floodplain, Coastal Plain, Western Valley, Western Highland Rim, Central Basin, Eastern Highland Rim, Cumberland Plateau, Ridge and Valley, and Blue Ridge The Mississippi Floodplain is ~15 miles wide and is between the bluffs and the Mississippi River. It is dominated my fluvial features from the Mississippi, a meandering river, and contains important agriculture due to a regular inflow of alluvium The Coastal Plain is the Mississippi Embayment portion of the Atlantic Coastal Plain and extends from the bluffs east of the river to the western valley of the Tennessee River, about 80-100 miles wide. It has flat to gently rolling topography and large deposits of loess, which is mobilized fine particulate matter from glacial periods. Loess makes it a really good area for agriculture, and it is several feet thick in most places, and over 100 feet thick in the bluffs The Western Valley of the Tennessee River is a 12 mile wide valley below the Coastal Plain and Western Highland Rim. The valley was eroded by the river during the last ice age, making it geologically young, and it was carved because the lower sea level during the time lowered the base level, so the channel was scoured. Sea level has since risen again, so a floodplain was created, and now it is a depositional site The Western Highland Rim contains rolling terrain highly dissected by rivers, and elevations range from 700 to 100 feet. It is underlain by limestone, giving this area a lot of karst features. It also contains the Wells Creek Cryptoexplosive Structure, which is a circular depression near Cumberland City that may have been caused by the impact of a meteor The Central Basin is part of a larger structure called the Nashville Dome, and was formed due to differential erosion of the dome. Different rocks exposed within the basin have led to different soils being formed there, and these soils contain a lot of limestone, so they are poor and not suited to agriculture The Eastern Highland Rim is 15-20 miles wide, relatively flat due to horizontal rock strata, and sits about 1000 feet above sea level. Lots of rivers cut through the landscape, and it is underlain by limestone, so there are many karst features The Cumberland Plateau is a high tableland with an average elevation of ~2000 feet, though in the north it can be as high as 3400 feet. The plateau is capped by a thick layer of sandstone, which has produced many waterfalls and gorges, such as Falls Creek Falls (highest waterfall in the eastern US), and has also weathered into arches and natural bridges. Sequatchie Valley is an eroded anticline in the southern portion of the plateau and is between 1000 and 1500 feet below the top of the plateau The Valley and Ridge province contains numerous parallel valleys and ridges that run from northeast to southwest. They were formed from differential erosion, which caused soft rock layers to be valleys and hard rock layers to be ridges. Streams meet at right angles, so there is a trellis drainage pattern, and there is lots of limestone, giving the landscape karst features The Blue Ridge province is composed of high mountains in extreme eastern Tennessee, which are sometimes called the Unaka Mountains or the Great Smokies. Elevations range from 3000 to 6500 feet, and mountains include Clingman’s Dome, Mount LeConte, and Mount Guyot. They are very old mountains, folded and deformed, composed of sedimentary and metamorphic rock, and highly eroded. Coves are formed where soft rock in some areas was eroded. Karst features are common Aeolian Processes and Arid Landscapes Wind erodes, transports, and deposits material just like water does, but there are two main differences between the 2. While wind erosion is more prevalent in deserts and on coastlines, it occurs everywhere, unlike water. In addition, the amount of materials wind is able to carry is small compared to the capacity of water Grain size is very important in wind erosion. Grains of intermediate size are transported most easily because large sizes are too heavy to be carried and small sizes stick together too much and are too aerodynamic The 2 types of erosion performed by wind are deflation and abrasion. Deflation occurs when sand and other materials are actually picked up by the wind and carried out of the area. Abrasion occurs when materials being carried in the wind scrape against rocks and wear them down as the wind blows by. Most abrasive action occurs fairly close to the ground because the wind can only lift sand so high When all the small, loose sediments are blown out of an area via deflation and only larger, gravel-sized particles are left behind, it leaves a tougher layer called desert pavement behind. Since the materials in a desert pavement layer are too large to be carried, this actually lessens wind erosion in the area. Deflation may also remove enough material to form a blowout depression Ventifacts are rocks that have been abraded by the wind and are now grooved, polished, etc. Yardangs are rocks that have been sculpted by the wind Material is transported much like in water, except for that there is no restriction on where the particles can go and there is also no dissolved load. Small particles move farther than large particles, and the 3 types of movement are surface creep, saltation, and suspension. Surface creep is the movement of larger particles as they are dragged along the ground because they are too heavy to be lifted. Saltation is the bounding of slightly smaller particles that the wind can somewhat pick up but are still too heavy to be really carried. Suspension is the actual carrying of fine sands along in the wind. In a dust storm, sands will be saltated and dusts will be suspended When the system runs out of energy to continue carrying materials, they will be deposited. Ripples are the smallest depositional feature shaped by saltating grains, and they run transverse to the direction of wind. They are very small, and because of this, also very short-lived Dunes are wind-sculpted accumulations of sand. They have a windward side, which is gently sloping on the side the wind is coming from, and a leeward side, which is a steep slip face on the wide toward which the wind is blowing. Over time, dunes migrate in the direction the wind is blowing, which causes problems for human activities. The different types of dunes are based on wind speed and direction and abundance of sediment. There are 3 main types: crescentic, linear, and star. Crescentic dunes include barchan and parabolic dunes. Barchan dunes are crescent-shaped and open in the direction the wind is blowing, and they are formed in areas with limited sediment supply. Despite this, they can still be quite large, and in areas where there is slightly more sediment, several barchans can join together to form a barchanoid ridge. Parabolic dunes form where there is anchoring vegetation, and open up in the direction from which the wind is coming. Linear dune types include transverse and longitudinal dunes. Transverse dunes form in areas with abundant sediment and are mostly parallel to each other. Longitudinal dunes form in abundant sediment areas and are created by wind coming strongly together at angles to each other and pushing sand together. Star domes form when strong wind pushes sand together from all sides. The other types of dunes are reversing, which are produced when strong winds come from complete opposite directions, and dome. Loess is very fine-grained windblown sediment. It is mostly made up of what was left behind when Pleistocene glaciers melted because they left behind rock flour, which is very finely ground rock, and because it is so fine, it can be transported very large distances. It is very rich in nutrients, so loess deposits are good for agriculture Dry climates make up ~26% of earth’s surface, and ~35% of all land areas on earth tend toward dryness. The distribution of dry climates is controlled by 3 things: subtropical high pressure zones, rain shadows on the leeward sides of mountain ranges, and areas being at a great distance from moisture-bearing air masses Precipitation in deserts is rare, so when it does occur, flash floods often occur because the ground is hardened due to a lack of moisture or a layer of desert pavement and water doesn’t sink in. A dried up streambed is called a wash. After a precipitation event occurs, standing water evaporates quickly, leaving salt behind. Areas where this occurs frequently become salt flats known as playas. Alluvial fans occur near playas when a river coming out of a canyon loses velocity very abruptly and deposits all sediment before reaching a body of water. A bajada is formed where multiple alluvial fans grow together The Colorado River contained much more water in the past and also transported much more sediment, but now it stops about a kilometer short of where it used to flow Deserts contain specially adapted plants and animals that have evolved and learned to subsist in their harsh environments, but not all deserts are the same. North American deserts in general have more plant cover than Asian deserts Deserts contain a wide variety of differentially eroded rocks. Rocks that are erosional remnants above surrounding remaining area are known as Inselbergs or monadnocks. Relatively low-relief areas with rugged topography are called badlands. Lithified dunes in ancient deserts are referred to as cross-bedded or cross-stratified Internal drainage occurs when rivers drain into lakes with no outlets. When the water in the lake evaporates, it leaves behind salt with nowhere to go, so the lakes end up being incredibly salty Desertification is the expansion of the earth’s desert lands, and occurs along the margins of current deserts. It can be caused by poor agricultural practices, improper soil moisture management, erosion and salinization, deforestation, and global climate change
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