Exogenic Processes of the Earth GEO01 PDF

Exogenic Processes of the Earth Course Outcome 4 GEO01 – Earth Science Materials Prepared by: Ryo Jerome C. Tuzon, LPT Video Lecture Prepared by: Perseval S. Pineda, LPT Mass Wasting Mass Wasting Mass wasting is the downhill movement of masses of bedrock, rock...

Exogenic Processes of the Earth Course Outcome 4 GEO01 – Earth Science Materials Prepared by: Ryo Jerome C. Tuzon, LPT Video Lecture Prepared by: Perseval S. Pineda, LPT Mass Wasting Mass Wasting Mass wasting is the downhill movement of masses of bedrock, rock debris, or soil driven by the pull of gravity mass wasting is, with proper planning, perhaps the most easily avoidable of all major geologic hazards. Controlling Factors in Mass Wasting Table 9.2 Summary of Controls of Mass Wasting Triggers: (1) earthquakes; (2) weight added to upper part of a slope; (4) heavy rainfall Gravity Gravity – the driving force for mass wasting Normal Force. Shear Force. Shear Resistance. Shear resistance < shear force = landslide Steep slopes – shear forces maximized by gravity Shear Strength and Water Shear Strength – resistance to movement or deformation Saturated soil has reduced shear strength due to increased pore pressure. However, a small amount of water in soil can prevent downslope movement. Like building a sandcastle. Mass Wasting Triggers Seismic (earthquake) activity. Heavy Rainfall. Construction. Lack of vegetation – no roots to hold rock/soil in place. Landslide triggered by an earthquake in New Zealand, 2016 Classification of Mass Wasting Rate of movement < 1cm/year – >100 km/hour. Type of material Solid bedrock or debris (unconsolidated material at Earth’s surface). Type of movement flow, slide, or fall Some Types of Mass Wasting TABLE 9.1 Some Types of Mass Wasting Note: The type of material at the start of movement is shown in parentheses. Rates given are typical velocities for each type of movement. Creep Creep (or soil creep) - very slow downslope movement of soil Major contributing factors include water in soil and daily freeze – thaw cycles. Can be costly to maintain homes, etc., Source: C.F.S. Shape on creeping ground as foundations, walls, pipes and driveways crack and shift downslope over time. Flows: Earthflow and Solifluction Flow – descending mass moves downhill as a viscous fluid Earthflow – debris moves Earthflow downslope, slowly or rapidly, Source: Robert L.Schuster, U.S. Geological survey as a viscous fluid. Solifluction Permafrost – flow of water-saturated soil over impermeable material. Common in colder climates. Solifluction Flows: Debris Flow, Mudflow, Avalanche Debris Flow and Mudflow – flowing mixture of debris and water, usually down a channel. Mudflow is only soil and water. Debris Avalanches are very rapid and turbulent. Debris flow from Mudflow from Debris avalanche California, 2005 California, 2015 from Peru, 19 70 Falls Falls – material free-falls or bounces down a cliff Rockfall – a block of bedrock breaks free and falls or bounces down a cliff Commonly an apron of fallen rock fragments (talus) accumulates at cliff base. Slides Slides – descending mass remains relatively intact, and descends along well-defined surfaces Translational slide – movement along plane parallel to motion. Rotational slide (slump) – movement along a curved surface. Rockslide and Rock Avalanche – the rapid sliding of a mass of bedrock along an inclined surface of weakness Underwater Landslides Turbidity Currents. Can create a tsunami. Preventing Landslides Preventing Rockfalls and Rockslides on Highways Remove Loose material. Stitch slopes together. Preventing Mass Wasting of Soil Construct retaining wall with drains. Don’t oversteepen slopes during construction. Remove all rock that is prone to sliding. Add vegetative cover. Cover roads. Streams and Floods Hydrologic Cycle The Hydrologic Cycle – the movement and interchange of water between the sea, air, and land Evaporation – solar radiation provides energy Precipitation – rain or snow Transpiration – evaporation from plants Runoff – water flowing over land surface FIGURE 10.1 The hydrologic cycle. Water vapor evaporates from the land and ocean, condenses to form Infiltration – water soaking into clouds, and falls as precipitation (rain and snow). Water falling on land runs off over the surface as streams or the ground infiltrates into the ground to become groundwater. It returns to the atmosphere again by evaporation and transpiration (the loss of water to the air by plants). The distribution of water in the hydrosphere includes the oceans (96.5%), glacial ice (1.76%), groundwater (1.70%), lakes and streams (0.014%), soil moisture (0.001%), and the atmosphere (0.001%). Glacial ice and groundwater contain 98.8% of the fresh water on Earth. Running Water Stream – a body of running water, confined to a channel, that runs downhill under the influence of gravity Headwaters – upper part of stream near its source in the mountains Mouth – place where a stream enters sea, lake or larger stream Channel – a long, narrow depression eroded by a stream into rock or sediment Stream banks - sides of channel Streambed – bottom of the channel Floodplain – flat valley floor composed of sediment deposited by the stream Drainage Basins Drainage basin – the total area drained by a stream and its tributaries Tributary – a small stream flowing into a larger one. Divide – ridge or high ground that divides one drainage basin from another Continental Divide separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico Drainage Patterns Drainage pattern – the arrangement, in map view, of a stream and its tributaries Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream Dendritic – drainage pattern resembling the branches of a tree. Radial pattern – streams diverge outward like the spokes of a wheel, such as on conical mountains. Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles. Trellis pattern – parallel streams with short tributaries meeting at right angles. Factors Affecting Stream Erosion and Deposition Velocity Maximum velocity near center of channel. Higher stream velocities promote erosion and transport of coarser sediments. Floods involve increased velocity and erosion. Gradient (slope) Channel Shape and Roughness Discharge (volume of water passing a particular point in a stream over time) Stream Erosion Streams cut their own valleys, deepening and widening them over time and carrying away the sediment Hydraulic action – ability to pick up and move rock and sediment. Solution – dissolving of rocks. Abrasion – grinding away of stream channel by the friction and impact of the sediment load. Potholes are eroded into streambed by the abrasive action of the sediment load in the stream. Stream Transportation of Sediment Bed load – large or heavy particles that travel on the streambed Traction load – large particles that travel along the streambed by rolling, sliding or dragging. Saltation load – medium particles that travel by bouncing along. Suspended load – small/light sediment that remains above the stream bottom by turbulent flow for an indefinite period of time Dissolved load – dissolved ions produced by chemical weathering of soluble minerals upstream Stream Deposition Bars – sediments temporarily deposited along stream course Placer Deposits – concentrated heavy sediment Braided Streams Braided streams contain sediment deposited as numerous bars around which water flows in highly interconnected rivulets Resembles braids of hair or rope. Common for streams carrying a lot of sediment. C. Source: Earth Sciences and Image analysis laboratory at Johnson Space Center/N ASA Meandering Streams Rivers that develop pronounced, sinuous curves called meanders Water flows faster along the outside of bends causing erosion and created cut banks. Flows slower along the inside, depositing point bars on the insides of the meanders. Meandering Cutoffs Meander cutoffs may form when a new, shorter channel is cut through the narrow neck of a meander (as during a flood) Floodplains Floodplains are broad strips of land built up by sedimentation on either side of a stream channel Floodplain sediments are left behind as flood waters slow and recede at the end of flood events. Main channel has slightly raised banks with respect to the floodplain known as natural levees.  Deltas Delta – body of sediment deposited at the mouth of a river when flow velocity decreases Surface marked by shifting distributary channels. Shape of a delta depends on whether its wave- dominated, tide - dominated, or stream – dominated. (A) Source: Jacques Descloitres, MODIS Land Science Team/NASA; (B) ©M-Sat Ltd/Science Source; (C) Source: Japan ASTER Science Team/GSFC/METI/ERSDAC/JAROS, and U.S./N AS A Alluvial Fans Alluvial fan – large, fan – or cone-shaped pile of sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain Well – developed in desert regions, such as the southwestern U.S. Larger fans show grading from large sediments nearest the mountains to finer sediments farther away. FIGURE 10.28 (A) An alluvial fan at the mouth of a desert canyon. (B) This alluvial fan is from the Delaware Mountains of west Texas and formed, as sediment was washed out of the canyon by flash floods. (B) Moment Open/Getty Images Stream Valley Development: Downcutting Valleys are the most common landform on Earth Formed by stream erosion. Different valley morphologies depend on the erosional processes that created them. Downcutting – process of deepening a valley by erosion of the streambed V – shaped valleys typically form from downcutting combined with mass wasting and sheet erosion. Streams cannot erode below their base level. Stream Valley Development: Grading, Later Erosion, Headward Erosion Graded streams – have concave- up longitudinal profile, lack rapids and waterfalls, represent a balance between available sediment load and transport capacity Lateral erosion – widens stream valleys by undercutting of stream banks and valley walls as stream swings from side to side across the valley floor Headward erosion – the slow uphill growth of a valley above its original source by gullying, mass wasting, and sheet erosion Stream Valley Development: Terraces Stream Terraces – step – like landforms found above a stream and its floodplain Occurs when river rapidly cuts downward into its own floodplain. Represents relatively sudden change from deposition to erosion. Can be caused by rapid uplift, drops in base level, or climate changes. Stream Valley Development: Incised Meanders Incised meanders Retain sinuous pattern as they cut vertically downward. May be produced by profound base level changes, as when rapid tectonic uplift occurs. Flooding When water levels rise and overtop the banks of a river, flooding occurs Natural process on all rivers. Described by recurrence intervals. Can cause great damage in heavily populated areas. High velocity and large volume of water causes flood erosion. Slowing of waters as flood ends causes flood deposits to be deposited in the floodplain. Flooding and Urbanization Urbanization creates many impermeable surfaces which increases runoff flooding Water is delivered to streams faster which increases peak discharge and hastens occurrence of flood. Flash Flooding Flash floods are local, sudden floods of large volume and short duration Typically triggered by heavy thunderstorms. Source: W.R. Hansen U.S. Geological Survey Reducing Flood Risk Dams – designed to trap flood waters in reservoirs upstream and release it gradually over time Artificial levees – designed to increase capacity of river channel and works well until stream overtops levees, leading to extremely rapid flooding and erosion Wise land-use planning – including prevention of building within 100-year floodplains, is most effective Impact of Dams Societal Benefits Environmental Concerns Electricity production. Trapping of sediment and nutrients behind Flooding control. the dam. Reservoir for drinking water. Habitat destruction. Destabilizing the river valley when the reservoir fills. Can lead to landslides. Groundwater The Importance of Ground Water Ground Water – lies beneath the ground surface, filling pores in sediments and sedimentary rocks and fractures in other rock types Represents 1.7% of the hydrosphere (100x the fresh water in all lakes and rivers combined) Resupplied by slow infiltration of precipitation. Generally cleaner than surface water. Accessed by wells. Tremendously important resource. Growing population has a large impact on groundwater resources. Being removed at ever increasing rates. Pollution impacts are increasing. The Water Table Saturated zone – subsurface zone in which all rock openings are filled with water Water table – top of the saturated zone Water level at surface of most lakes and rivers corresponds to local water table. Unsaturated zone – unsaturated region above the water table Perched water table – above and separated from main water table by an unsaturated zone Commonly produced by thin lenses of impermeable rock (for example, shale or clays) within permeable ones. Porosity and Permeability Porosity - the percentage of rock or sediment that consists of voids or openings Measurement of a rock’s ability to hold water. Loose sand has approximately 30-50% porosity. Compacted sandstone may have only 10-20% porosity. Permeability - the capacity of a rock to transmit fluid through pores and fractures Interconnectedness of pore spaces. Most sandstones and conglomerates are porous and permeable. Granites, schists, unfractured limestones are impermeable.  Aquifers and Aquitards Aquifer - body of saturated rock or sediment through which water can move easily Unconfined – has a water table and is only partly filled. Confined – completely filled with water under pressure. Aquitard - rock/sediment that retards ground water flow due to low porosity and/or permeability Shale, clay, unfractured crystalline rocks. The Movement of Groundwater Movement of ground water through pores and fractures is relatively slow (cm to meters/day) compared to flow of water in surface streams Flow velocities in cavernous limestones can be much higher (km/day). Flow velocity depends upon: Slope of the water table. Permeability of the rock or sediment. Wells Well - a deep hole dug or drilled into the ground to obtain water from an aquifer For wells in unconfined aquifers, water level before pumping is the water table. Water enters well from pore spaces within the surrounding aquifer. Water table can be lowered by pumping, a process known as drawdown. Water may rise to a level above the top of a confined aquifer, producing an artesian well. Springs & Streams Spring - a place where water flows naturally from rock or sediment onto the ground surface Gaining streams - receive water from the saturated zone Gaining stream surface is local water table. Losing streams - lose water to the saturated zone Stream beds lie above the water table. Maximum infiltration occurs through streambed, producing permanent “mound” in the water table beneath dry channel. Contamination of Groundwater Infiltrating water may bring contaminants down to the water table, including: Pesticides/herbicides. Fertilizers. Landfill pollutants. Heavy metals. Bacteria, viruses and parasites from sewage. Industrial chemicals (P C Bs, T C E). Acid mine drainage. Radioactive waste. Oil and gasoline. Contaminated ground water can be extremely difficult and expensive to clean up Exa mpl es of Con tam inat ion Pollution Caused by Pumping Wells Balancing Withdrawal & Recharge If ground water is withdrawn more rapidly than it is recharged, the water table will drop Dropping water table can lead to ground subsidence. Subsidence can crack foundations, roads and pipelines. Areas of extremely high ground water pumping (such as for crop irrigation in dry regions) have subsided 7-9 meters. Source: Richard O.Ireland U.S Geological Survey  Geologic Effects of Groundwater Groundwater can easily dissolve soluble bedrock, such as limestone This creates cave systems, sinkholes, karst topography, and other effects. Caves Caves - naturally-formed underground chambers Acidic ground water dissolves limestone along joints and bedding planes Stalagmites – dripstone that forms on cave floors Stalactites – dripstone formations that hang from cave ceilings Cave Formation  Karst Topography Karst topography – area with rolling hills, disappearing streams, and sinkholes FIGURE 11.24 Karst towers in China’s Li River Valley. As indicated by the haze, this is a warm, humid climate. The towers are erosional remnants of pillars that separated caverns. The roofs of caverns collapsed and heavy, monsoon rainfall washed away the debris, leaving the present valleys between the pillars. Charles C. Plummer Sinkholes and Karst Topography Sinkholes – caves near the surface that have collapsed Source: Rick Duerling, Courtesy of Florida Geological Survey Other Effects of Groundwater Preservation of Fossils Petrified Wood. Concretions. Geodes Petrified Wood Concretions Geodes Hot Water Underground Hot springs - springs in which the water is warmer than human body temperature Ground water heated by nearby magma bodies or circulation to unusually deep (and warm) levels within the crust. Hot water is less dense than cool water and thus rises back to the surface on its own. Geysers - hot springs that periodically erupt hot water and steam Minerals often precipitate around geysers as hot water cools rapidly in the air. Geothermal Energy Geothermal energy – produced using natural steam or superheated water No CO2 or acid rain are produced (clean energy source) Some toxic gases given off (for example, sulfur compounds) Can be used directly to heat buildings Superheated water can be very corrosive to pipes and equipment Glaciers & Glaciation What is a Glacier? Glacier – a large, long-lasting mass of ice, formed on land, that moves downhill under its own weight Glaciated Terranes Alpine – found in mountainous regions. Continental – large parts of continents covered by glacial ice. approximately 70% of the world’s supply of fresh water is locked up in glacial ice Types of Glaciers Develop as snow is compacted and recrystallized, first into firn and then glacial ice Can only form where more snow accumulates during the winter than melts away during the spring and summer Alpine glaciation occurs in mountainous regions as valley glaciers. Continental glaciation covers large land masses in Earth’s polar regions in the form of ice sheets. Glaciation occurs in areas cold enough to allow accumulated snow to persist from year to year. Distribution of Glaciers Most extensive in polar climates but can occur anywhere where more snow falls than melts during the year. Approximately 10% of Earth’s surface is covered by glaciers. Approximately 85% of all glacial ice is in Antarctica. If all the ice on Antarctica were to melt sea level would rice approximately 65 meter (213 feet) flooding the worlds coastal cities. Formation and Growth of Glaciers Snowfall. Compaction of the snow removes air. Snowflakes recrystallize into granules. Firn – transitional between granular snow and glacial ice. Glacial Ice – formed once the firn is further compacted and more air removed and has a crystallize texture similar to the metamorphic rock quartzite. Gravity causes the glacier to move downslope. Ablation – loss of the glacier due to melting, evaporation, or calving of icebergs Glacial Budgets Advancing glacier – gains more snow than it loses, has a positive budget Receding glacier – loses more snow than it gains, has a negative budget Zone of accumulation – snow added. Zone of ablation – melting and calving of icebergs. Equilibrium line– separates accumulation and ablation zones, will advance or retreat depending on climate. Movement of Valley Glaciers Move downslope due to gravity Basal sliding – sliding of the glacier over the underlying rock Plastic flow – movement that occurs within the glacial ice due to its plastic nature Rigid zone – upper part of the glacier that moves rigidly downslope Crevasses – fractures formed in the upper rigid zone during glacier flow. Glacier flow is fastest at the top center of a glacier and slowest along its margins due to friction Movement of Ice Sheets Move downslope and outward from a central high area due to gravity Basal sliding, plastic flow and rigid zone movement similar to alpine glaciers Antarctica – two ice sheets separated by the Transantarctic Mountains (West Antarctic Ice Sheet and the East Antarctic Ice Sheet) Outlet glaciers – mountain valley glaciers that occur where the mountains are higher than the ice sheet Ice streams - flow zones that are much faster than adjoining ice Source: U.S. Geological Survey/N ASA Glacial Erosion Glaciers erode underlying rock by plucking of rock fragments and abrasion as they are dragged along Basal abrasion polishes and striates the underlying rock surface and produces abundant fine rock powder known as rock flour. Glacial Valleys Associated with Alpine Glaciation Several types: U-shaped valleys. Hanging valleys – smaller tributary glacial valleys left stranded above more quickly eroded central valleys. Truncated Spurs – ridges that have triangular facets. Cirques, Horns, and Arêtes Cirques – steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valleys Horns – sharp peaks remaining after cirques have cut back into a mountain on 3+ sides Arêtes – sharp ridges separating glacial valleys Landscapes Associated with Continental Glaciation Rounded topography is more common Weight and thickness of continental ice sheets produce more pronounced effects Rounded knobs. Grooved or striated rock (several meters deep and kilometers long). Thick enough to bury mountains rounding off ridges and summits  Glacial Deposition Till – general name for unsorted, unlayered glacial sediment Deposits of till left behind at the sides and end of a glacier are called lateral, medial and end moraines, respectively. FIGURE 12.24 Till transported on top of and alongside a glacier in Peru. View is downglacier. The lake is dammed by an end moraine at its far end. Charles C. Plummer Glacial Deposition: Moraines Lateral Moraines – elongate, low mounds of till along sides of valley glaciers Medial moraines – lateral moraines trapped between adjacent ice streams End moraines – ridges of till piled up along the front end of a glacier Recessional moraines – successive end moraines left behind by a retreating glacier Ground Moraine – thin layer of till at the base of the glacier Glacial Deposition: Outwash Outwash – sediment deposited by large amounts of meltwater flowing over, beneath and away from the ice at the end of a glacier Sediment-laden streams emerging from ends of glaciers have braided channel drainage patterns. Outwash Landforms Eskers – sinuous ridge Kettles – glacial depression Kames – low glacial mound Glacial Deposition: Lakes and Varves Glacial Lakes and Varves – annual sediment deposition in glacial lakes produces varves, which can be counted like tree rings Source: U.S.Geological Survey Past Glaciation In the early 1800s, past extensive glaciation of Europe was first hypothesized Hypothesis initially considered outrageous, but further observations by Louis Agassiz (initially a major opponent of the hypothesis) in Swiss Alps found much supporting evidence. Agassiz traveled widely in Europe and North America, finding more and more supporting evidence, eventually leading to the theory of glacial ages. Theory of glacial ages states that at times in the past, colder climates prevailed during which much more of the land surface of Earth was glaciated than at present Most recent glacial age was at its peak only ~18,000 years ago. Earth has undergone episodic changes in climate during the last 2-3 million years (Tertiary Period and later Pleistocene epoch).  Direct Effects of Past Glaciation Large-scale glaciation of North America during the most recent ice age produced the following effects: Most of the soil and sedimentary rocks were scraped off underlying crystalline rock in northern and eastern Canada, and lake basins were gouged out of the bedrock. Extensive sets of recessional moraines were left behind by retreating ice sheets in the upper Midwestern U.S. and Canada. Indirect Effects of Past Glaciation Glacial Lakes Lake Agassiz. Lake Missoula. Pluvial Lakes (formed in a period of abundant rainfall) existed in closed basins in Utah, Nevada and eastern California Lake Bonneville. Lake Missoula. Source. C.S. Denny, U.S. Geological Survey, and the Geological map of North America, Geological Society of America, and the Geological Survey of Canada More Indirect Effects Lowering and Rising of Sea level Fiords – coastal inlets formed by drowning of glacially carved valleys by rising sea level. Crustal Rebound Great Lakes region continues to rebound as crust adjust to removal of the last ice sheet.  Evidence for Older Glaciation Tillites – lithified glacial till, have distinctive textures that suggest emplacement of sediments by glaciers Paleozoic era in portions of the southern continents indicate that these landmasses were once joined. Snowball Earth hypothesis – Late Precambrian glaciation when the oceans were frozen over. Oldest glaciation evidence dates back to 2.3 billion years ago. strong evidence supporting theory of plate tectonics. Deserts & Wind Action Deserts Desert – any arid region that receives less than 25 cm of precipitation per year. Running water is the predominant force shaping most desert landscapes. Rare and often violent flash flood events produce most desert erosion. Where and How Deserts Form Deserts can be found anywhere that the atmosphere (air) is usually dry. Most deserts are associated with areas where air is descending. Most common near 30° north or south latitude. Source: U.S. Department of Agriculture Rain Shadow Deserts Rain shadow deserts form downwind of where moist air rises over high mountain ranges. Some Characteristics of Deserts Intermittent stream flow Streambeds are dry most of the year Lack through-flowing streams. Exceptions include the Colorado and Nile Rivers. Internal drainage – streams flow to land locked basins. Flash Floods – common in arid regions due to short-lived high volume rain storms. Desert washes or arroyos are commonly steep-sided, with flat floors covered by loose sediments - a result of rare but highly erosive flash flood events. Desert Features in the Southwestern United States Two distinct landscapes: Colorado Plateau. Basin and Range Province. Colorado Plateau – centered on the four corners region of Utah, Colorado, Arizona and New Mexico. 1500 meters above sea-level. Flat-lying sedimentary rocks that are heavily eroded into Source: Thelin and Pike, plateaus, mesas and U.S. Geological Survey buttes. Desert Landforms of the Southwestern United States Basin and Range province – has rugged, linear, fault-bounded mountain ranges separated by flat-floored valleys. Narrow canyons carry sediment down to valley floors during heavy rains. Sediment gets deposited into alluvial fans. Alluvial fans may overlap to form a bajada. Finest sediments travel to basin center where water ponds and evaporates in playas. Wind Large daily temperature and pressure differences lead to strong wind. Dust storms may occur if fine-grained sediments are readily available. Dust can be transported 1000s of km by atmospheric winds. Dust Bowl – continuing dust storms in the prairie states during the droughts of the 1930s. Saharan Desert sediments have carried across the Atlantic Oceana. Volcanic Ash. Source: National Oceanic and Atmospheric Administration Wind Erosion and Transportation Wind can keep dust in suspension, but larger sand grains move by saltation. Sand grains moving in high- speed winds can effectively sand-blast rocks into ventifacts. Deflation of fine sediments: Blowouts. Desert pavement. Wind Deposition: Loess Loess - deposit of wind-blown silt and clay composed of unweathered grains of quartz, feldspar and other minerals. Sediment sources include glacial outwash plains and desert playas. Thick loess deposits exist in China and the United States. Loess typically forms soils that are very fertile, yet easily eroded. Source: U.S. Department of Agriculture Wind Deposition: Sand Dunes Sand dunes – mounds of loose sand piled up by the wind. Most likely to develop in areas with large sand supply and winds that generally blow in the same direction. Small patches of dunes are common in southwestern U.S., but huge sand seas exist in the Sahara and Arabian deserts. Dunes may also form just inland of beaches along the coasts of seas and large lakes. Shaping Dunes Shapes depend upon: Wind velocity and direction(s). Amount of available sand. Distribution of vegetation cover. Types of Dunes: Barchan Barchan dunes – crescent-shaped, with horns that point downwind and a steep slip face on the concave side. Form in areas with one dominant wind direction and a limited sand supply. Exist on Mars. Types of Dunes: Transverse Transverse dunes – relatively straight, elongate dunes that form in areas with large sand supply and one dominant wind direction. Types of Dunes: Parabolic Parabolic dunes – deeply curved dunes convex in the downwind direction, forming around blowouts, and have horns anchored by vegetation. Types of Dunes: Longitudinal Longitudinal dunes – form in areas with large sand supply, parallel to the prevailing wind direction. Extremely long, high, straight and regularly spaced. Crosswinds may play a part in their development. Area between parallel dunes is swept clean of sand by winds. Formation mechanism still not fully understood. Source: NASA Reference: