PHYSICALGEOG UNIT SIX

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Travel-Nature

Published on March 12, 2008

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Slide1:  Physical Geography by Alan Arbogast Chapter 17 Glacial Geomorphology: Processes & Landforms Lawrence McGlinn Department of Geography State University of New York - New Paltz Glacial Geomorphology:  Glacial Geomorphology Development of a glacier Types of glaciers Glacial landforms History of glaciers What causes glaciation? Impact of global climate change on glaciers Periglacial processes and landscapes Development of a Glacier:  Development of a Glacier Glacier – slowly moving mass of dense ice formed by gradual thickening, compaction, and refreezing of snow & water over time After summer melt, some snow left over With weight and partial melting, snow turns to Firn, crunchy transition from snow to ice Further compaction, ice crystals align, become dense glacial ice which flows slowly downslope At least 40-m thick to become glacier Glacial Mass Budget:  Glacial Mass Budget Glacial input : Snow Glacial output : ice, meltwater or water vapor Zone of Accumulation – top of glacier where temps are cooler - input > output Zone of Ablation – lower part of glacier where temps are higher – output > input Equilibrium line – point on glacier where input = output Glacial Mass Budget:  Glacial Mass Budget Glacial Movement:  Glacial Movement Glaciers move through internal deformation Interior of glacier like malleable plastic Glacier Types:  Glacier Types Mountain Glaciers Ice Cap – Continuous sheet of ice covering entire landscape Ice Field – Buries all but tallest mts – can be very thick Alpine Glacier – Flows down valleys away from high country Cirque - Bowl-shaped depression on mtn flank due to glacial erosion – snow source Alaskan Glaciers:  Alaskan Glaciers Hubbard Glacier Continental Glaciers:  Continental Glaciers Huge ice masses covering a large part of a continent or large island – also called ice sheets More than 3000 m deep in places Covers most of Antarctica and Greenland Weight of ice presses lithosphere down into aesthenosphere, called isostatic depression Glacial Landforms:  Glacial Landforms Rock & debris picked up by glaciers, transported in direction of movement & deposited Glacial erosion: Glacial Abrasion – scratch and gouge bedrock Glacial Striations – caused by glacial abrasion Glacial Grooves – deep striations Glacial Plucking – boulders ripped from ground by glacier – deposited by retreating glacier, called Glacial Erratics Glacial Erosional Landforms:  Glacial Erosional Landforms Roche Moutonnée – rounded hill, gradual on side toward direction from which glacier comes Glacial Striations Glacial Erratic Roche Moutonnée Alpine Erosional Landforms:  Alpine Erosional Landforms Glacial Erosion: Cirque – bowl-like feature on mtn flanks Tarn – small lake in bottom of cirque Arête – narrow, steep ridges between cirques Horn – mtn w/3 or more arêtes at summit Glacial Trough – u-shape valley eroded by glacier Hanging Valley – side trough above main trough – possible waterfall Alpine Erosional Landforms:  Alpine Erosional Landforms Cirque Horn “Matterhorn” Glacial Trough Glacial Depositional Landforms (Till):  Glacial Depositional Landforms (Till) Glacial Till – sediment directly deposited by glacier – many particle sizes Moraine – winding ridge formed by till at the front or side of glacier – Moraine types: Lateral – along former edges of glacier Terminal – along front of former glacier Recessional – formed as glacier recedes Medial – between 2 glaciers Ground – irregular deposition as glacier recedes Glacial Depositional Landforms (Till):  Glacial Depositional Landforms (Till) Glacial Depositional Landforms (outwash):  Glacial Depositional Landforms (outwash) Glacial Outwash – sediments deposited by water out & under a glacier as it melts – forms Outwash Plain, flat feature in front of former glacier Kame – large mound deposited near glacier front Esker – winding ridge from water flowing in tunnel through ice under glacier Kettle Lake – big ice block fallen off glacier front is buried by outwash, melts later forming lake Glacial Depositional Landforms:  Glacial Depositional Landforms History of Glaciation:  History of Glaciation As early as 2.3 B years ago, ice covered much of Earth, and off and on since then Most important Ice Age was Pleistocene Epoch, 1.8 M years ago till 10K years ago Glacial – period when glaciers expand from poles – cooler temps, lower sea level, Interglacial – period when glaciers recede: warmer temps, higher sea level Pleistocene Glaciations:  Pleistocene Glaciations Named for southern extent of ice sheet in North America Nebraskan – 1 million yrs ago Kansan – 625 K yrs ago Illinoisan – 300 K yrs ago Wisconsin – 35 K to 10 K yrs ago Laurentide Ice Sheet – eastern North America Cordilleran Ice Sheet – western North America Maximum Extent of Pleistocene Glaciation:  Maximum Extent of Pleistocene Glaciation 30% of earth’s surface covered by ice sheets (Only 11% coverage today) Evidence of More Glaciations?:  Evidence of More Glaciations? Ice core samples suggest more than the known 4 glaciations – show more cool, glacial periods Oxygen isotopes O-16 & O-18 both in water, but O-18 evaporates more in warmer climate, so ratio of O-16 to O-18 in ice cores can indicate relative warmth of climates over 1 million yrs ago! Causes of Glaciation:  Causes of Glaciation Summer temp (melting) is key to glaciation Possible Factors: 1. Variations in solar radiation (dust, sunspots…) 2. Reduced carbon dioxide (escaping heat) 3. Increased volcanic activity (reflective dust) 4. Variations in Earth-Sun geometry (axial tilt, shape of orbit, rotation) Milankovitch Theory:  Milankovitch Theory Dominant theory of causes of glaciation, based on Earth-Sun geometry: Orbital eccentricity – strongly elliptical orbit puts Earth farthest from Sun in summer, cooling it Tilt obliquity – Earth’s tilt varies from 22.1º to 24.5º - less tilt means lower angle Sun and less insolation at poles, thus cooler summers Orbital precession – wobbles of Earth’s axis - North Pole may point toward Sun at farthest point of orbit, creating a cool summer Milankovitch Theory:  Milankovitch Theory Orbital Eccentricity Axial Tilt Orbital Precession When three factors coincide, high probability of glaciation Glacial Geomorphology: Processes and Landforms Climate Change and Glaciers:  Climate Change and Glaciers Since mid-1800s glaciers have been receding, both alpine and continental Alps, Parts of Andes, Mt. Kilimanjaro melting Thousands of sq miles of Antarctica & Greenland ice sheet lost over last 30 years due to warming Melting area of Greenland has increased rapidly since early 1990s Periglacial Processes and Landscapes:  Periglacial Processes and Landscapes In near-glacial environments – constant freeze/thaw cycle effects on landscape Permafrost – ground that is permanently frozen Continuous – poleward of -7ºC mean annual isotherm – all surfaces frozen exp under water – avg 400 m thick, up to 1000 m thick Discontinuous – poleward of -1ºC mean annual isotherm – thinner than continuous, esp. on south facing slopes Extent of Permafrost:  Extent of Permafrost Permafrost Processes:  Permafrost Processes Active Layer – soil that melts & refreezes daily or seasonally – as thin as 10 cm in continuous permafrost, up to 2 m thick in discontinous Dramatic warming in arctic is making active layer much thicker & releasing tons of CO2 Talik – body of unfrozen ground within permafrost, e.g. under a lake, important for movement of groundwater Periglacial Landscape (Cross Section):  Periglacial Landscape (Cross Section) Ground Ice:  Ground Ice Ground Ice – distinct zones of frozen water within the ground – variable amts of water As these areas freeze & thaw, expand & contract, they cause physical weathering Ice Wedge – water enters crack in active layer Pingo – surface bulges because of ice under pressure below Patterned Ground – land broken into polygons as frost pushes coarser material to surface Ground Ice Landforms:  Ground Ice Landforms Ice Wedge Pingo Patterned Ground Slide32:  Physical Geography by Alan Arbogast Chapter 18 Arid Landscapes and Eolian Processes Lawrence McGlinn Department of Geography State University of New York - New Paltz Arid Landscapes & Eolian Processes:  Arid Landscapes & Eolian Processes Arid Landscapes Eolian Erosion & Transportation Eolian Deposition & Landforms Human Interactions with Eolian Processes Arid Landscapes:  Arid Landscapes 3 factors influence arid climates: Subtropical high pressure Rainshadow Distance from large bodies of water Desert Geomorphology:  Desert Geomorphology Water important to landforms in arid regions – little vegetation to slow intermittent erosion Arroyo – steep-sided gully cut into alluvium In undisturbed, horiz. rock layers more resistant sandstone or limestone forms flat caprock above easily eroded shale Result is landforms flat on top w/steep sides: Plateau -Canyon -Butte -Mesa Pinnacle -Playa Desert Landforms:  Desert Landforms Note: Tops of most landforms once part of same surface, since partially eroded away Eolian Erosion and Transport:  Eolian Erosion and Transport Wind-based processes important in deserts b/c: Strong winds common in desert Large supply of sand & silt to be blown Vegetation minimal – wind free to erode Fluid Behavior of Wind:  Fluid Behavior of Wind Wind acts like a fluid, like water, but less dense Faster wind can move larger particles Threshold Velocity for wind to carry different sized particles Particle Transport:  Particle Transport Silts and Clays carried in suspension Sand bounces along – saltation, or Sand rolls slowly along – creep Eolian Erosional Landforms:  Eolian Erosional Landforms 2 types of wind erosion: Deflation – wind blows loose soil away: leaves coarser pebbles & cobbles, called Desert Pavement when deflation causes basin to form, called Deflation Hollow Slide41:  Eolian Erosional Landforms Abrasion – wind blows sand along a surface to polish & abrade it Ventifacts – rocks shaped by abrasion: pitted, grooved, polished Yardangs – elongated, wind-sculpted ridges caused by abrasion Eolian Erosional Landforms:  Eolian Erosional Landforms Deflation/Desert Pavement Abrasion Ventifacts Yardangs Eolian Depositional Landforms:  Eolian Depositional Landforms Sand Dunes form based on 3 components: Backslope – windward surface, erosion Crest – high point of the dune Slipface – lee slope, deposition Sand Dune Types:  Sand Dune Types Loess:  Loess Fine-grained, wind-blown silt – high in calcium – usually from alluvial deposits or glacial till Can be transported farther than sand Loess Deposits around the World:  Loess Deposits around the World Arid Landscapes and Eolian Processes Human Impact/Desertification:  Human Impact/Desertification Desertification – transforming a vegetated landscape to one that is barren & susceptible to wind erosion Population pressure has forced more people to clear marginal, semi-arid-to-arid land for agriculture & firewood In wind, cleared land loses topsoil and nutrients Vegetation unlikely to reestablish Regions Prone to Desertification:  Regions Prone to Desertification Desertification in African Sahel:  Desertification in African Sahel Semi-arid region in transition region from Sahara Desert in north to rainforest in south Traditionally nomadic herders & small, sedentary farmers – north-south migrations to follow rain Into 20th century, European borders & resource exploitation made people more sedentary – over-cultivation of soil, overgrazing, and tree removal Add in extended drought since late 1960s, & you have desertification The Sahel:  The Sahel Desertification in Great Plains:  Desertification in Great Plains Great Plains lie east of Rocky Mts in semi-arid climate with short grass as dominant natural vegetation Desertification in Great Plains:  Early 1900s Americans moved to region to farm, plowing and clearing native grasses – unusually wet period 1930s – terrible drought hits – topsoil blows into dust storms – called “Dust Bowl” Many migrated to California & elsewhere Those who stayed have employed irrigation & soil conservation, including windbreaks, and conservation tillage Desertification in Great Plains Slide53:  Physical Geography by Alan Arbogast Chapter 19 Coastal Processes and Landforms Lawrence McGlinn Department of Geography State University of New York - New Paltz Coastal Processes and Landforms:  Coastal Processes and Landforms Oceans and Seas Nature of Coastlines: Intersection of Earth’s Spheres Coastal Landforms Human Impacts on Coastlines Oceans and Seas:  Oceans and Seas Oceans – largest bodies of water: Pacific, Atlantic, Indian, Southern, and Arctic Seas – next largest water bodies: Black, Mediterranean, Barents, etc. Gulf – next largest, usu. opens to larger water body: Mexico, Alaska, Guinea, etc. Bay – smaller still: Fundy, Biscay, etc. Slide56:  Oceans and Seas Water as Solvent:  Water as Solvent Salinity – concentration of dissolved solids in seawater – global seawater salinity 34-37 parts per thousand (‰) Brine – water with >35‰ salinity Brackish water - <35‰ salinity Shaping the Coastline:  Shaping the Coastline Key to shaping coastline is movement of water Eustatic Change - changes in water level in ocean – due to tectonic uplift or hydro cycle variation Land above sea level forms river valleys that extend to sea level – when sea level rises, valley floods, as with Chesapeake Bay or Delaware Bay Ria – river valley flooded by rising sea level Fjord – glacial valley flooded by rising sea level North American Coastline through Time:  North American Coastline through Time Lowest sea levels occurred during glaciations when water was tied up in glaciers (130k & 19k bp) – highest sea levels in interglacial periods (120k bp) Chesapeake Bay and Delaware Bay - Rias Tides:  Tides Regular, predictable oscillations of sea level – due to gravitation of moon (56%) & Sun (44%) High tide on side of Earth facing moon and on side away from moon – ellipsoid shape Long, narrow bays usu. have highest tides – up to 16 meters in Bay of Fundy in eastern Canada Waves:  Waves Oscillations in water due to force of friction from wind blowing across its surface Waves travel horizontally, but most movement of water is vertical, up-and-down Near coast seafloor slopes upward – wave base intersects ocean floor – wave pushes water up as it slows – waves pile up from behind - wave height exceeds 7X wavelength, and forms a breaker Only horizontal movement of water from breaker to beach, called surf – erosional agent Wave Compression:  Wave Compression Tsunami:  Tsunami Caused by undersea earthquakes with vertical displacement, volcanic eruptions, or landslides Vertical displacement of water causes fast wave with long wavelength – no harm at sea, but massive as it hits coastline Dec 2004 – India plate subducts under Burma plate which snaps upward along 1000 km stretch – massive tsunami hits Indian Ocean Prior to arrival, ocean recedes, giving warning 2004 Indian Ocean Tsunami:  2004 Indian Ocean Tsunami Before After Littoral Processes:  Littoral Processes Transport & deposition of sediment in shore zone Longshore current – forms when wave hits beach at oblique angle – water deflects downwind, parallel to beach Longshore drift – process of longshore current eroding & carrying sediment down shore Beach drift – zig-zag motion of sediment down beach due to swash & backwash Littoral Drift – longshore & beach drift together Littoral Drift:  Littoral Drift Coastal Landforms Erosional Coastlines:  Coastal Landforms Erosional Coastlines Breaking waves have great power to erode Headland – promontory that juts into ocean or sea – made of resistant rock Waves slow & pivot around headlands – erosive power of waves concentrated on headland Retrogradation – retreat of coastline due to erosion Wave-Cut Bluff – basic erosional landform of coastlines – near-vertical cliff at water’s edge Coastal Erosional Landforms:  Coastal Erosional Landforms Wave Refraction Wave Cut Bluff More Erosional Landforms:  More Erosional Landforms Marine Terrace (Falling Sea Level) Sea Stacks (Remains of Headlands) Evolution of a Rocky Coastline:  Evolution of a Rocky Coastline Submerged Coastline – Headlands Eroded Away Depositional Coastlines:  Depositional Coastlines Progradation – process of coastline extending outward into water through deposition Beach – dynamic transition from sea to land Offshore – under water, where waves break Foreshore – rise & fall of tides Offshore Bar – between offshore & foreshore Beach Ridge – at high water line Backshore – flat, only covered in storms Beach Cross Section:  Beach Cross Section Spits and Baymouth Bars:  Spits and Baymouth Bars Longshore current carries sediment down beach Current slows upon reaching bay – sand deposited as a Spit extending out into bay – current in bay turns spit toward land in hook shape Baymouth Bar – spit extends across bay, isolating it from ocean – bay now called Lagoon Tombolo – longshore currents from 2 directions meet – sand extends out to island or sea stack Depositional Landforms:  Depositional Landforms Hooked Spit due to northward current and flow into bay Common Depositional Landforms Longshore Processes and Depositional Coastlines Barrier Islands:  Barrier Islands Elongated bars of sand that form parallel to shore Likely formed from sand deposited on cont. shelf during last glaciation – waves & wind shaped sand Lagoons w/mudflats form behind barrier islands Mudflats develop into vegetated salt marsh Coral Reefs:  Coral Reefs Coral polyps excrete external skeletons of calcium carbonate (limestone) – Coral Reefs New reefs form on top of old, dead reefs 30º N - 25º S latitude, water warmer than 20º C 3 settings: Fringing reef – on shallows around island Barrier reef – line of coral parallel to shore Atoll – semicircular reef around degraded volcanic island Development of Atoll:  Development of Atoll Global Distribution of Coral Reefs:  Global Distribution of Coral Reefs Human Impacts on Coastlines:  Human Impacts on Coastlines 37% of world pop. live <60 mi. from shore (2 billion) – 50% within 120 miles of shore In US, 53% of pop. live near coastline Coastal Engineering Purposes: Protecting shore & property from hazards Stabilizing & nourishing beaches Maintaining traffic & trade into ports Mitigating Coastal Hazards:  Mitigating Coastal Hazards Raise buildings on stilts so waves roll underneath Sea wall – vertical, concrete wall to absorb energy of waves Revetment – slope covered with large rocks (rip-rap) to absorb energy of waves These work locally, but they aggravate erosion up and down shore from wall due to wave refraction Beach Nourishment:  Beach Nourishment Bringing sand to beaches that have eroded – over $336M spent in Florida since 1960s How? Trucking in sand from remote location Limit loss of sand through groins – low walls built at right angles to beach – intercept longshore drift Jetties – stone or concrete structures to keep channel open, keep sand to side Groins and Jetties:  Groins and Jetties Groins Jetties Impact of Global Climate Change on Coastlines:  Impact of Global Climate Change on Coastlines Melting ice sheets will lead to sea level rise – estimates of 5-35 inches – areas <5 ft elevation at greatest risk Pacific Islands & low-lying coastal areas will suffer greater erosion & flooding potential Coral bleaching by unusually warm water has also become a problem – ultimately kills coral Warming Threat on North Carolina Coast:  Warming Threat on North Carolina Coast

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