earth rocks final exam.docx

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Western University
Earth Sciences
Earth Sciences 1022A/B
Stephen R Hicock

MASSWASTING Controls and Triggers: Gravity is most important but water is also critical; when water completely fills pores it reduces cohesion and allows grains to slide over each other; adds weight to soil Oversteepened slopes due to undercutting by waves, streams, and human excavation, resulting in loss of support for materials higher up the slope Vegetation anchors soil and regolith with plant roots; removing plants makes the slope unstable and susceptible to failure Earthquakes can dislodge rock and unconsolidated material, resulting in landslides Classification: based on types of materials, types of motion (freefall, sliding and flowing), and how fast they move Slump - rock or unconsolidated material that moves as blocks along curved surfaces commonly lubricated by water; gravity pulls the block in a rotational motion away from a scarp with the block's surface tilting back - happens when the slope is undercut by a stream, by waves, or by human excavation Rockslide - common in mountainous areas, some travel >200 km/hr and do a lot of damage; commonly occurs where strata or joints parallel the slope or where stronger rocks overlie weaker rocks - especially if the slope is undercut, sliding surface is lubricated after heavy rain or snowmelt, or shaken byan earthquake Debris flow - very wet mixture of soil and regolith that moves rapidlydown canyons and stream channels of semi-arid mountainous areas following heavy rain and soil saturation; or during and after volcanic eruptions where ash gets mixed with melted snow and ice, resulting in lahars rushing down valleys Earthflow - slower movement of mainly mud down hillsides in humid areas following heavyrain or snowmelt; forms a tongue-shaped mass that may continue to move slowly for years; commonly starts as a slump Creep - gradual down-slope movement of soil and regolith under gravity; caused byrepeated freezing and thawing, wetting and drying, or water saturation following heavy rain or snowmelt - results in tilted trees, fences and utilitypoles Solifluction - flow in saturated ground resting on impermeable rock, clay, or permafrost in polar areas; water cannot percolate downward below the impermeable surface and so material flows slowly downhill as lobes Submarine landslides – as slumps and debris flows down the flanks of volcanic islands, seamounts, and deltas; also as turbidity currents down continental slopes 10 RUNNINGWATER Hydrologic Cycle: 97% of Earth's water is stored in the oceans, evaporated then precipitated on land - some soaks in (infiltration), some flows over the land as streams (runoff), some evaporates back into the atmosphere and is transpired by plants, and some is stored in glaciers, but most returns to the oceans by streams Stream Flow: stream erodes, transports according to its velocityand channel shape Gradient - slope of stream channel over its length; cross-sectional shape determines how much of the channel contacts water, slowing it down; rougher, more friction Discharge - volume of water flowing through the stream’s cross-section per second; if discharge increases, stream usually gets wider, deeper, and faster downstream Work of Steams: they erode, transport, and deposit simultaneously over their lengths Erosion - by abrasion using particles in transport as cutting tools to scour channel walls (like sandblasting) and in circular eddies to cut potholes into the channel floor Transport- 3 types of sediment load: dissolved means ions in solution from chemical weathering and groundwater; suspended is sediment that remains above the bed; bed load is particles sliding and rolling along the channel floor; saltation involves jumping and skipping of particles that alternate between bed and suspended loads Deposition - occurs when stream can no longer carryits load and particles fall to the bed; can form a channel deposit of sand and gravel in bars, or floodplain deposit of mud beyond the channel StreamValleys: a stream cuts vertically but its banks cave in by mass wasting to develop narrow V-shaped valleys with waterfalls and rapids in rough channels Wide valleys form on gentle gradients and cut mainlysideways forming floodplains as the stream shifts back and forth across the valley leaving a thick fill of sediment; meanders are bends on floodplains where a stream erodes the outsides of bends at cut banks, deposits on the insides at point bars; the stream may cut across narrow meander necks at cutoffs, leaving oxbow lakes; a delta forms where a stream enters standing water at its mouth (splits into distributaries) Drainage Networks: determined by the network of tributaries feeding the stream; pattern of a basin depends on rock types and structures: dendritic occurs in uniform bedrock where channels follow local slopes; radial from a volcano or rock dome; rectangular where stream flows over joints or a fault system with right angle bends Floods and Flood Control: artificial levees of earth are built along stream banks to increase the volume the stream can hold, while flood control dams are built to store water then let it out slowly– resulting reservoirs are used for irrigation, power generation, and recreation but they flood river valleys, displace wildlife, drown forests, and fill with sediment that reduces their holding capacity and effectiveness 10 GROUNDWATER Distribution of Underground Water: takes a long time to get water underground and is most effective by steady rainfall soaking into the ground because heavy rain produces mostlyrunoff and little infiltration once plant requirements are met Zone of saturation - pores of sediment or rock are completely filled with water Zone of aeration - pores contain mainly air and minor water held as soil moisture; the surface between these two zones is called the water table that mimics the land surface – i.e. the water table is highest under hills and lowest under valleys Movement of Groundwater: very slow (cm/day) depends on the nature of materials Porosity - volume of open space in rocks or sediment, including fractures and openings; porosityis reduced by pores getting filled with cement and fine particles Permeability - connection of pore spaces and abilityof material to transmit water Impermeable material is called anaquitard; permeable material is called an aquifer - Groundwater flows under gravity in a pattern of long curving paths toward streams Springs - where water table meets the ground surface; springs may occur where an aquitard causes the local water table to be "perched" above the main water table Hot springs – where groundwater is heated at depth then rises to the surface Geysers - water boils under pressure, vaporizes and blasts water into the air, then water refills the pipe and cycle starts over again (Old Faithful erupts ~every hour) Wells – holes drilled into the zone of saturation; must be drilled deep enough below the water table to retrieve water during the dry season; wells cause cones of depression in the water table where it is drawn down toward the base of the well Artesian wells tap water under pressure in an aquifer that is sandwiched between aquitards; water naturally rises to the ground surface with minimal pumping Problems: Subsidence - pumping water for irrigation faster than it can be replenished causes underlying sediment to compact, resulting in gradual sinking of the ground surface Groundwater contamination - from septic tanks, fertilizers, toxic spills, buried chemical tanks, and leaking sanitary landfills; pollution can make its way into aquifers, therefore cities should dispose of their waste only in impermeable sites because it takes many years to flush out (cleanse) an aquifer Geologic Work of Groundwater: by solution of soluble rocks to form… Caverns - by groundwater solution enlarging cracks and bedding planes in soluble rocks just below the water table; needle- like stalactites form by water dripping from cave ceilings and stalagmites build up from the floor as posts; stalagmites and stalactites can eventually join to form columns Karst topography sinkholes can develop in limestone terrain bysolution and collapse of the ground surface; short streams commonly disappear into them 10 GLACIERSAND GLACIATION Glaciers: form by compaction, re-crystallization of snow that eventually turns into ice Types include: ice sheets that are radial-spreading large ice masses on continents (e.g.Antarctica); and valley in mountains, commonly flow down old stream valleys Movement of a Glacier: cm/day is normal; an upper brittle zone of fracture ~50m thick rides on a lower zone of plastic flow; the glacier may also slide over its bed Budget of a glacier: involves two zones - of accumulation (gain of snow and ice), and ablation (wastage or loss by melting, evaporation, icebergs calving in water): • if accumulation > ablation, the glacier's terminus commonlyadvances; • if accumulation < ablation, its terminus usuallyretreats internal movement within a glacier is always toward the front, even during retreat Glacial Erosion: occurs by plucking (freezeon of loose bedrock); and abrasion (using the glacier's load to scrape, scour, gouge the subglacial floor and sides) Striations (scratches) and grooves form by particles in ice scraping over bedrock; glaciated valleys (U-shaped) becom Fjords along coasts; arêtes (sharp ridges) and horns (pyramid-shaped peaks) form where multiple cirque glaciers gnaw into a mountain divide from the sides Glacial Deposits: glaciallyderived sediment is called glacial drift; two main types include till and stratified drift - both are released when the glacier ice melts Till: forms directly from glacier ice and contains a great varietyof particle sizes; its stones appear scratched, polished, and faceted from glacial transport - Forms lateral moraines built along the sides of valley glaciers; end moraines (curved ridges) at the glacier terminus where it piles debris by dumping and bulldozing; and ground moraine is plastered beneath glaciers Drumlins are mounds streamlined under the glacier (blunt ends point up-glacier) Stratified drift: comprises sand and gravel deposited byglacial melt-water; provides an excellent source of aggregate for building cities; associated landforms include: kettle: a depression in the ground left by an ice block that melted there kame: a debris mound lowered to the ground from the glacier surface as ice melted esker: a winding gravel ridge commonly formed in a subglacial tunnel Causes: plate tectonics and continental drift bring a major continent over a pole, allowing glaciation to start (by inducing global cooling) - glaciations are controlled by three cycles related to Earth's orbit around the Sun - when conditions combine for minimum contrast in seasons at mid to high latitudes, more snow falls during mild winters and less snow melts during cooler summers – then a major glaciation happens 10 DESERTSANDWINDS Deserts are dry places where there is low precipitation and little vegetation Geologic Processes: in spite of lack of water and vegetation, processes still occur: weathering – almost entirely mechanical and veryslow due to low precipitation; however, clays and thin soils form by oxidation of ferromagnesian silicate minerals water: ephemeral streams and flash floods from short, intense rainstorms do most of the erosion of desert surfaces, especially sediment not anchored by vegetation Transportation of Sediment by Wind: wind can move faster than water and carries sediment farther and higher than streams because flow is not confined to channels Bedload – sand moves by saltation (grain collisions) and by rolling over the ground Suspended load – dust particles are pushed into the air bysaltating sand grains, then swept up by the wind in dense clouds and carried high into the air Wind Erosion: occurs by deflation and abrasion Deflation - blowing away fine sediment to create blowouts, while leaving coarser particles behind that cover the desert floor as desert pavements Abrasion - mainly by saltation as particles sandblast things; wind polishes, pits, and facets stones (ventifacts) sitting on desert floor, and streamlines ridges (yardangs) Wind Deposits: deposition occurs when the wind slows down and is no longer able to carry its sediment load - produces landforms made of sand and silt Sand dunes - commonly form in wind shadows around obstacles where the sand builds small mounds that induce more deposition (self-perpetuating) an unstableslip face forms on the leeward (downwind) side which is always steeper (~34°) than the windward (upwind) side; the dune gradually migrates in the direction of the slip face on which primary cross-beds are deposited types of dunes: Barchan - crescentshaped with tips pointing downwind; form where supplies of sand are limited and the ground surface is flat and lacking vegetation Transverse – series of ridges and troughs elongated perpendicular to wind direction, commonly along coasts where wind is steady, sand abundant, and little vegetation Longitudinal - elongated parallel to wind in large deserts where sand is moderate Parabolic - crescentshaped with tips pointing upwind and commonly anchored byvegetation; form where lots of sand occurs along coasts and wind blows
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