Office Hours Tuesday Thursday 11 00am 12 00pm or by appointment GEOL 1001 3 Instructor Dr Tiffany Roberts Room Howe Russell 130 Office E209 Howe Russell Office Phone 225 578 2801 Email tiffanyroberts lsu edu TA Abah Omale Email aomale1 lsu edu Office location 205 Howe Russell Office Hours M W 1 00 2 00pm 4 8 14 Chapter Mass Wasting Important concepts Blah Mass wasting Presents a natural hazard Influenced by gravity Controls or triggers causes cid 127 Water increases cohesion Saturates sand rock Reduce friction Downslope movement of rock regolith and soil Cause flow debris flows cid 127 Over steepened slopes Every material has an angle of repose cid 127 Maximum stable angle Sand 30 degrees Increased by Stream action Terracing cid 127 Weathering and erosion can steepen the angle Can be problematic with building roads Removal of vegetation Plants bind soil Prevent some rain infiltration Remove vegetation by cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Forest fires Human activity Logging Farming Development Dislodge loose materials rocks Earthquakes Liquefaction Random event No trigger Unstable slopes cid 127 Many events are fast Avalanches rock Landslides rockslides Debris flows Slow events creep Particles flow downhill a few mm per year Three ways for mass wasting to occur 1 Fall 3 Slide Purely by gravity Look up term in book cid 127 Occurring on a discreet surface plane of weakness 5 Creep flow cid 127 Ground surface behaves viscously Desert a region that is so arid that it contains to permanent streams except for those that bring water in from temperate regions elsewhere 4 10 14 Chapter 17 Dry Region The geology of deserts Important concepts cid 127 What is a desert cid 127 What factors are involved in their formation cid 127 What are the erosional Deserts Cover 25 of land surfaces THe primary control on deserts is Plate tectonics Characterized by extreme dryness can be hot or cold Specialized ecosystems and low human populations Unique geological processes What is a desert Deserts are lands so extremely arid that cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Have evaporation rates preventing permanent surface water Vegetation covering less than 15 of the surface Annual rainfall amounts of less than 10 inches 5cm But can exist in both hot and cold climates Desert climates Hot Low latitudes Low elevations Far from oceans Cold High latitudes High elevations Near cold ocean currents Types Five types with distinctive landscapes and biota Subtropical deserts 20 30 N to S Earth s largest Form due to patterns of atmospheric convection Equator 0 latitude Solar energy evaporates water which rises as hot moist air Rising are cools and expands forming abundant rain The air stripped of moisture flows to the N and S Subtropics 20 to 30 N to S Cool sinking air wicks water from the surface The air heats up and the landscape dries out Desert bracket the rainforest near equator Found across geologic time Rain shadow deserts most ocean winds are driven over mountains cid 127 WIndward air is forced to rise expand and cool cid 127 Moisture condenses rains out and creates a rain forest Leeward air stripped of moisture sinks and Coastal deserts cool air over cold ocean water holds little moisture THis air absorbs moisture when it interacts with land The Atacama desert peru is the driest place on earth Interior deserts air loses moisture as it crosses continents Land far from ocean moisture can be arid THe Gabi desert on mongolia is a prime example Polar deserts above 66 N and S latitude there is little moisture Air circulation carries dry air to polar regions It is so cold the air can t hold moisture Desert type review Deserts are so arid they host only sparse vegetation cid 127 Occur in several settings Subtropical dry climates Raind shadows Coasts border by cold currents COntinental interiors Desert Processes Deserts are characterized by special processes Erosion by water and wind cid 127 Weathering cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Soil formation Deposition Physical weathering dominates Joints split rocks into pieces Rare chemical weathering leaches ions Calcite precipitates beneath surface forming calcite Evaporation salts both break and cement grains Desert varnish Horizons Exposed surfaces develop desert varnish Forms very slowly by bacterial activity dust and water Native americans left petroglyphs in desert varnish Desert soils are thin with poorly defined horizons Iron oxides in rock weather to produce vibrant colors Trace elements orchestrate wide color variations Painted Desert AZ cid 127 Water erosion Though rare water shapes landscapes Lack of roots magnifies sediment erosion and transport Torrential rains generate dangerous flash floods Rapid flow of thick muddy and viscous water Flash floods quickly infiltrate dry stream beds cid 127 Wind Sparsely vegetated ground is scoured by wind High winds can carry dust across entire oceans cid 127 Wind carries two types of sediment load Surface load coarser sand sized particles Suspended load finer grained silt sized dust Sand forms dunes inside deserts Coarse clasts cannot be lifted and moved by wind Lag deposits form when finer sediment is removed Form desert pavement a surface layer of stones Desert pavement is resistant to erosion Deflation lowering the land surface via erosion Plants roots protect against erosion cid 127 Review Deposition desert sediments accumulate in many settings Joint bounded blocks break off ledges and cliffs Talus aprons at the angle of repose collect cliff debris Allucial fans conical accumulations of sediment Playas desert lakes efvaporate leaving flat dry lake bed Desert landscapes Cliffs and mesas bedrock controls landforms cid 127 Structure governs landforms Desertification Aridification of non desert areas Human activity expands desert areas Deseret soils are thin to absent so bare rock debris and desert pavement