Glaciers and Glaciation

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Glaciers and Glaciation
Introduction
• Glacier – a permanent body of ice, consisting largely of
recrystallized snow that shows evidence of downslope or
outward movement due to the pull of gravity
• Glaciers occur anywhere more snow falls on land each
winter than melts each summer
Types of Glaciers
• Mountain Glaciers
– Also alpine or valley
– Flow down mountain valleys
• Large mountain glaciers may spread out on gentle slopes
at the base of the mountains to form piedmont glaciers
• Ice sheets or continental glaciers overwhelm nearly all the
land within their margins
Snow and Ice
• Glaciers only form above the snowline
• Snow “metamorphoses” to glacial ice
– Snowflakes
– Granular Snow
– Firn
– Glacial Ice
Snow to Ice
– New snow is light and fluffy
• Delicate points on a snowflake disappear by sublimation and
evaporation
• Water vapor that is produced condenses near the centers of snowflakes
– Snow that survives more than a year becomes denser and no
longer permeable to air
– It becomes glacial ice
Snow and Ice
• Changes in glacial ice with increasing depth
– Increasing pressure causes ice crystals to grow
• This is typical of metamorphism
• Increasing metamorphism results in crystal growth
Glacial Size
• Zone of Accumulation
– Upper zone
– Where snow accumulates
– Net gain in mass
• Zone of Wastage
– Lower zone
– Bare ice, snow, and melting
• Separated by firn limit
– At the snowline in valley glaciers
– Fluctuates annually
Glacial Size
• A balancing act
– Balance between accumulation and ablation
– Accumulation > ablation means the glacier expands
– Ablation > accumulation means the glacier retreats
Glacial Terminus
• Fluctuates with glacial equilibrium
– Advances when accumulation > ablation
– Retreats when accumulation < ablation
– Measure of climate change
• Response lags actual change
• Calving at the front of glaciers produces icebergs if ice
reaches the water
How Glaciers Move
• Surface ice is brittle and develops crevasses
• Basal sliding
– Meltwater at the bottom lubricates the base
– This only works in temperate or warm glaciers
• Plastic Flow
Glacial Movement
• Plastic flow
– Deformation of ice crystals
– Creep along internal crystal planes
– Crystals with planes parallel to direction of flow grow at the expense of
other crystals
– Results in crystal alignment
Glacial Movement
• Velocity and flow direction
– Velocity distribution similar to streams
– Glaciers only move in one direction
• Surges
– Exceptionally rapid movement of a glacier
– May be due to accumulation of water at the base of the glacier
Features of Mountain Glaciers
• Cirque–bowl-shaped depression
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Headwall–upslope end of cirque
Lakes in abandoned cirques are called tarns
String of tarns are pater noster lakes
Cirques are separated by aretes
Multiple cirques around a peak produce a horn
Features of Mountain Glaciers
• Glacial valleys
– U-shaped as opposed to V-shaped
– Hanging valleys are valleys occupied by small glaciers now left
hanging above the valley produced by a large glacier
Mountain Glaciers
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Cirque—Bowl-shaped depression
Tarn—Lake in abandoned cirque
Hanging valley—small valley above main valley
Horn—Peak eroded on at least 3 sides
Arête—Knife-edged ridge
Horn
Arête
Fiords
• Intense glacial erosion upstream deepens the valley
• Rising sea level then floods it
Streamlined Forms
• Roche moutonée
– Rock knob
– Up-ice end streamlined
– Down-ice end plucked
– Made of bedrock
• Drumlin
– Up-ice end steep
– Down-ice end streamlined
– Made of glacial till
Roche Moutonée
Drumlin
Glacial Deposits
• Glacial till
– Unsorted glacial debris (drift) and unstratified
– Tillite is lithified till
• Erratic–boulder carried from outside the local area
Moraines
• Lateral—on sides of glaciers
• Medial—in the middle, usually where two glaciers merge
• End moraines—at the down-ice end
– Terminal moraine–farthest down ice
– Recessional moraine–any time the icefront paused
• Ground moraine—covers the ground surface
Medial Moraine
Moraines
Dirty Ice
Ice Surface
Moraines
Stratified Drift
• Outwash
– Deposited by water derived from melting glacial ice
– Covers the area beyond that currently covered by ice
• Esker—sinuous ridge produced by streams flowing on or in the ice
• Kame—small hill of ice-contact stratified drift
• Kettle—depression created by melting of isolated block of ice
Esker
Field Lab
Outwash Plain
Ice Surface
Melt Water on the Ice
Glaciations
• Multiple glaciations in the Pleistocene
– Used to think of four major advances
– Oceanic data enables us to recognize something more like 20 advances
• Pre-Pleistocene glaciations
– 2 in the Precambrian
– Early Paleozoic
– Late Paleozoic–as many as 50
• Little Ice Age–mid 13th century to mid 19th century
Consequences of Glaciation
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Great landscapes
Lowering of sea level
Great Lakes
Diversion of drainage
Periglacial features
– Permafrost
– Loess
• Glacial lakes and their subsequent drainage
Causes of Glaciation
• Plate Tectonics
– Rearranges the continents and topographic highs and lows
– Alters circulation of air and oceans
• Atmospheric composition
– Accumulation of greenhouse gases
• Solar variations
– Changing output from the sun
– Blocking of suns rays by volcanic debris or ther material
Astronomical Cycles
• Milutin Milankovitch and John Croll
• Convergence of three effects
– Eccentricity of Earth’s orbit cycles every 100,000 and 400,000
years
– Tilt of rotation axis cycles 41,000 years
– Precession on a 23,000 year cycle
• Due to wobble
• Due to movement of the axis of Earth’s elliptical orbit
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