GEOG 2051 FINAL STUDY GUIDE CHAPER 17 GLACIAL AND PREGLACIAL PROCESSES AND LANDFORMS CRYOSPHERE The frozen portion of Earth s waters including ice sheets ice caps and fields glaciers ice shelves sea ice and subsurface ground ice and frozen ground permafrost The Pleistocene Epoch of the late Cenozoic Era is when much of the Earth surface we know today was frozen It began 1 65 mya and is one of the prolonged cold periods in history It saw at least 18 expansions of ice An ice age is a time of generally cold climate that includes one or more glacials interrupted by brief warm spells known as interglacials The Illioian glacial and Wisconsinan glacial intervals with the Sangamon interglacial inbetween them are the events that span the 300 000 year interval prior to our present Holocene 10 000 ya Epoch During the Holocene temperatures increased 6 degrees Sea surface temps averaged between 1 4 1 7 degrees Celsius lower than today Air temperatures were as much as 12 degrees Celsius colder than today Evidence of glacials and interglacials is traced to ice cores layered ocean deposits of silts and clays in the extensive pollen record from ancient plants and in the relation of past coral productivity to sea level Medieval Warm Period warm is the mild climatic episode that lasted from about 800 1200 AD The shift to warmer wetter weather influenced migration and settlement northward From 1200 1900AD a Little Ice Age mixed cold and warm occurred Not consistently cold a time of rapid short term climate fluctuations that lasted only decades Milankovitch wondered whether the development of an ice age relates to seasonal astronomical factors Earth s revolution around the Sun rotation and tilt extended over a long period of time The interaction of Earth Sun relations creates a 96 000 year climatic cycle Changes affect the amount of insolation received Milankovitch cycles appear to be the primary cause of glacial interglacial cycles GLACIERS EXTENT AND LOCATION OF ICE COVERAGE Formed through Compaction Recrystallization and Growth 11 of Earth s land area covered by glaciers Mostly Greenland and Antarctica FORMATION Form in areas of permanent snow INPUT accumulation of snow in snowfield accumulation zone Snowfield highest elevation usually in a cirque bowl shaped recess at the head of a valley Snow accumulates increasing thickness thus increased weight and pressure on underlying ice Rain and summer snowmelt contribute water which seeps down into snowfield and refreezes Slowly transforms into glacial ice Ice recrystallizes and consolidates under pressure firn many years then glacial ice Metamorphic Process Snowline a temporary line marking the elevation where winter snowfall persists throughout the summer seasonally the lowest elevation covered by snow during the summer Firn snow of a granular texture that is transitional in the slow transformation from snow to glacial ice snow that has persisted through a summer season in the zone of accumulation MASS BALANCE Mass Budget net gains losses of glacial ice Positive expands Negative retreats Inputs snow Outputs ice melt water and water vapor Accumulation Zone where snowfall and other moisture feeds the upper reaches of the glacier usually ends at the firn line Ablation Zone the lower area of the glacier that is wasted reduced through several processes melting on the surface internally and at its base ice removal by deflation wind the calving of ice blocks and sublimation direct evaporation of ice Where accumulation gain balances ablation loss this is the equilibrium line coincides with firn line Positive Net Balance grows larger cold periods with adequate precipitation advance Negative Net Balance grows smaller warmer times equilibrium line migrates up retreat MOVEMENT Glacial ice behaves in a plastic manner distorts and flows in lower portion due to weight and pressure from above and degree of slope below Upper portions are brittle Greatest movement in a glacier occurs internally Basal Slip less rapid than internal plastic flow so upper portion flows ahead of the lower portion The base creeps and slides along due to temp and melt water Basal ice layer has a much higher debris content Glacial Surge lurch forward with little or no warning not as abrupt as it seems tens of meters a day Melt water gets to basal layer lubricating underlying soft beds of clay Some result from buildup of water pressure under the glacier ice quakes and ice faults Others from the presence of a water saturated layer of sediment beneath the glacier EROSION Plucking the glacier movement picks up rock from its bed and moves forward rocks freeze to basal layer and enable the glacier to scour and sandpaper the landscape as it moves abrasion Abrasion creates smooth surface on exposed rocks and glacial striations parallel to flow direction ALPINE LANDFORMS EROSIONAL FIG 17 10 PAGE 498 Horn a pyramidal sharp pointed peak that results when several cirque glaciers gorge an individual mountain summit from all sides Cirque a scooped out amphitheater shaped basin at the head of an alpine glacier valley an erosional landform Ar te a sharp ridge that divides two cirque basins Derived from knife edge in French these form saw tooth and serrated ridges in glaciated mountains Tarn a small mountain lake especially one that collects in a cirque basin behind risers of rock material or an ice gouged depression Paternoster Lakes one of a series of small circular stair stepped lakes formed in individual rock basins aligned down the course of a glaciated valley named because they look like a string of rosary beads Hanging Valley sites of waterfalls valleys carved by tributary glaciers that are left stranded high above the valley floor because the primary glacier eroded the valley floor so deeply U Shaped Valley glaciated valley greatly changed from previous stream cut V form DEPOSITIONAL After the glacier melts debris accumulates in moraines to mark the former margins of the glacier both its end and its sides Lateral Moraine debris transported by a glacier that accumulates along the sides of the glacier and is deposited along these margins Medial Moraine debris transported by a glacier that accumulates down the middle of the glacier resulting from two glaciers merging their lateral moraines forms a depositional feature following glacial retreat Terminal Moraine eroded debris that is dropped at a glacier s farthest extent Recessional Moraine consists of a secondary terminal moraine deposited during a