cold climate - glaciers and ice ages. glaciers glacier: a large, long-lasting mass of ice, formed on...
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Cold Climate - Glaciers and Ice Ages
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GlaciersGlaciers
• Glacier: a large, long-lasting mass of ice, Glacier: a large, long-lasting mass of ice, formed on land that moves under the formed on land that moves under the influence of gravity and its own weight influence of gravity and its own weight
• Glaciers form by accumulation and Glaciers form by accumulation and compaction of snowcompaction of snow– Packed snow becomes Packed snow becomes firnfirn
– Then refreezes to iceThen refreezes to ice
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Formation of Glacial Ice from SnowFormation of Glacial Ice from Snow
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Glaciation TypesGlaciation Types
• Alpine glaciation: found in mountainous Alpine glaciation: found in mountainous regions regions
• Continental glaciation: exists where a Continental glaciation: exists where a large part of a continent is covered by large part of a continent is covered by glacial iceglacial ice
Cover vast areas
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Davidson Davidson Glacier near Glacier near Haines, Haines, AlaskaAlaska
An Alpine An Alpine glacier glacier systemsystem
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Types of GlaciersTypes of Glaciers
Alpine Continental
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Alpine GlaciersAlpine Glaciers
• Are confined by surrounding mountains
Types:
• Cirque Glaciers – erode basins in mountainsides
• Valley Glaciers – flow into preexisting stream valleys
• Icecaps – form on mountaintops
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Types of GlaciersTypes of Glaciers– Cirque Glacier– Cirque Glacier
Mount Edith Cavell, Jasper National Park, Canada
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Types of Glaciers Types of Glaciers – Valley Glacier– Valley Glacier
Tongas National Forest, Alaska
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Types of Glaciers Types of Glaciers – Icecap and Continental– Icecap and Continental
Sentinal Range, Antarctica
http://www.nmm.ac.uk/server/show/conWebDoc.1221
•Antarctica is the broadest high place on Earth, the ice cap is up to 4km thick and covers the continent•Antarctica is a desert, with only 15 cm (6 inches) of snowfall a year around the South Pole•The lowest recorded temperature is -89.2 °C.•There is no life in Antarctica except near the coast
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Types of Glaciers Types of Glaciers – Piedmont & Tidewater– Piedmont & Tidewater
Source: Jim Wark/Peter Arnold, Inc.
Piedmont: Originally confined alpine, spread at foot of mountains
Calving
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Iceberg Calving Iceberg Calving – – Hubbard Glacier, Wrangell-St. Elias National Park, AlaskaHubbard Glacier, Wrangell-St. Elias National Park, Alaska
Releases fresh water to
oceans, CO2 to atmosphere.
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A Glacier’s BudgetA Glacier’s Budget
• Budget = Gain – Loss
• Gains snow in zone of accumulation
• Loses ice in zone of ablation
• Budget can be positive (net growth)
• Static
• or negative (net melting)
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A Glacier’s BudgetA Glacier’s Budget
Summer Rain
Year round Snow
Note that a glacier is a river. Even if the terminus doesn’t advance, still flows
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Glacial FlowGlacial Flow
• Internal deformation– Ice crystals slide past one another
• Basal Sliding– Entire glacier slides downhill on a thin film of
meltwater at its base.
• Glacier always flows toward zone of ablation
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Mechanics of Glacial FlowMechanics of Glacial Flow
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Erosion by GlaciersErosion by Glaciers
• Abrasion– Rocks embedded in glacier’s base make
linear scratches and grooves in bedrock
• Quarrying– Glacier breaks off and removes large blocks
of rock. FROST WEDGING is important
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Glacial Abrasion in BedrockGlacial Abrasion in Bedrock
Source: Tom Bean
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Glacial Erosion – Roche MoutonéeGlacial Erosion – Roche Moutonée
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Glacial Erosion – Roche MoutonéeGlacial Erosion – Roche Moutonée
Yosemite NP, Calif
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Erosion by Glaciers (cont.)Erosion by Glaciers (cont.)• Alpine glaciers erode mountain slopes Alpine glaciers erode mountain slopes
into horseshoe shaped basins called into horseshoe shaped basins called cirquescirques– Melting forms cirque lake (Melting forms cirque lake (tarntarn))
• Erosion of two or more cirques erodes Erosion of two or more cirques erodes intervening rockintervening rock– Horns :pointy peaks made by triosHorns :pointy peaks made by trios
– Arêtes: long serrated ridges by pairsArêtes: long serrated ridges by pairs
– Cols: passes through the arêtesCols: passes through the arêtes
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Alpine Glacial ErosionAlpine Glacial Erosion
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Alpine Glacial ErosionAlpine Glacial Erosion
Origin of Hanging Valley
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Yosemite FallsYosemite Falls
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Valley GlaciersValley Glaciers
• Erode a large quantity of bedrock and sediment
• Convert V-shaped stream valleys into U-shaped glacial valleys.
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U-ShapedU-Shaped Valley in Tracy Wilderness, Southeastern AlaskaValley in Tracy Wilderness, Southeastern Alaska
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Seawater Flooded U-ShapedSeawater Flooded U-Shaped Valleys: FjordsValleys: FjordsBela Bela Fjord, BC
When glaciers melt, sea-level rises
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Erosion by Continental GlaciationErosion by Continental Glaciation
• Erosional Landforms much larger in scale than alpine glaciers
–Whalebacks – huge Roche Moutonée
–Huge U-shaped troughs
– Finger Lakes, Great Lakes, Puget Sound, Loch Ness were all once stream valleys excavated by Ice Sheets
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Erosion of Preglacial Lowlands (Finger Lakes)Erosion of Preglacial Lowlands (Finger Lakes)
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Erosion of Lowlands (Great Lakes, Finger Lakes)Erosion of Lowlands (Great Lakes, Finger Lakes)
Source: U.S. Dept. of Interior, USGS Eros Date Center
Superior
Michigan
Huron
Erie
Ontario
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Glacial Deposits - DriftGlacial Deposits - Drift
• Collectively called Glacial Drift• TYPE 1: UNSORTED• Glacial Till: unsorted, unstratified sediments
deposited by melting ice.– May contain glacial erratics– Often accumulates in glacier’s channel and at its
terminus as a Moraine: – Terminal Moraine: hills of sediment left by a glacier’s
retreat.– Terminal Moraines may be reshaped by a later glacial
advance into Drumlins: rounded elongated hills perpendicular to their original orientation
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Advance & Retreat: MorainesAdvance & Retreat: Moraines
Note moraine, retreat or stationary
Stationary Analogy:
Escalator
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Stationary & Retreat: Moraine at TerminusStationary & Retreat: Moraine at Terminus
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Large Granite Erratics Large Granite Erratics
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Lateral and Medial MorainesLateral and Medial Moraines
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Lateral and Medial Moraines Lateral and Medial Moraines – – Kennicott GlacierKennicott Glacier
Wrangell-St. Elias NP, SE AK
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The Origin of DrumlinsThe Origin of Drumlins
Glacier retreats, leaving behinda terminal moraine. Later it
advances again, and reshapes the moraine into a drumlin.
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Drumlins Drumlins Rochester,NYRochester,NY
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Glacial Deposits - DriftGlacial Deposits - Drift
• TYPE 2: SORTED
• Outwash: sorted stratified sediments deposited by meltwater streams
• Loess: wind erosion of drying outwash silt.
• Eskers: sinuous meltwater deposits of sand and gravel underneath ice
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Origin of EskersOrigin of Eskers
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Eskers in Coteau des Prairies, South DakotaEskers in Coteau des Prairies, South Dakota
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Effects of GlaciationEffects of Glaciation
• Change Climate – increase precipitation
locally - pluvial lakes
• Depress continents & lateral rebound
• Drop sea-level: alter coastlines
• Form continent-wide Dams– Divert streams – Ohio and Missouri rivers
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Formation of Terraces due to Crustal ReboundFormation of Terraces due to Crustal Rebound
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Lowered Sea-level - LandbridgeLowered Sea-level - Landbridge
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Glacier Distribution 20,000 yaGlacier Distribution 20,000 ya
Approximate Maximum
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Lowered Sea-level exposed continental shelfLowered Sea-level exposed continental shelf
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The Creation of Glacial Lake MissoulaThe Creation of Glacial Lake Missoula
Purcell Lobe blocks Clark Fork River
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The Draining of Glacial Lake Missoula The Draining of Glacial Lake Missoula
Repeated many times, last time 13000 kya
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Giant Ripples of the Missoula FloodingGiant Ripples of the Missoula Flooding
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Causes of Ice AgesCauses of Ice Ages
• Plate Tectonics
Moves Continents to Poles
Raises mountains above snowline
• Orbit distance, Axis Tilt and Wobble– Moderates solar radiation north of 65 N– Milankovitch Cycles ~ 100,000 years– Low summertime radiation 65 N, glaciers expand
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Milankovitch Cycles Milankovitch Cycles
100,000 years
Discussion: cool summers and wet wintersMoisture content of air masses
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41,000 years
Cold Dry Winter Hot Summer
Warm Wet Winter Cool Summer
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25,700 years
Discussion: Perihelion and Aphelion
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One More Point On ThisOne More Point On This
• The orbital affects that Milankovitch suggested as a partial cause for ice ages each have a different period.
• They combine at irregular intervals
• The average is about 100,000 years
but that is ONLY an average
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Earth’s Past Ice AgesEarth’s Past Ice Ages(oldest on bottom)(oldest on bottom)
• Tertiary- Quaternary cooling – Pleistocene 1.8 mya• None in Mesozoic• Late Pennsylvanian & Permian Ice covered part
“Gondwana” (South Africa, South America, India, Australia, Antarctica)
• Ordovician glaciation– Area that is now the Sahara at South Pole
• PreCambrian Tillites (Lithified Till)
At least three episodes. Interesting examples:– 750 mya ice from poles to tropic “SNOWBALL EARTH”– Oldest 2.8 bya
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Permian Glaciation – Gondwana TillitesPermian Glaciation – Gondwana Tillites
Poorly SortedUnstratified
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Cenozoic CoolingCenozoic Cooling
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The Late Tertiary and Quaternary oxygen
isotope record measured in marine fossil shells
Evap. water and CO2 during glacial timeremoves 16O to glacier ice leaving 18O in oceans for CaCO3 shell
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Pleistocene GlaciationPleistocene Glaciation
• Since 1.6 mya – more than 30 advances and retreats • In 4 large scale pulses. • Latest retreat 10,000 years ago Laurentide• Little ice age 700 to 150 years ago.• Sustained warming since 1850
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Foraminifera tests - Ice AgeForaminifera tests - Ice Age
Evap. water and CO2 during glacial
removes 16O to glacier ice leaving 18O in oceans for CaCO3 shell
Also spiral direction & diversity dep T
Nebraskan
Kansan
Illinoian
Wisconsinan
> 30 pulses in 4 or so major groups
warm cold
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Continuous Ice Sheet 20 kya then warmingContinuous Ice Sheet 20 kya then warming
Scoured 30 M below sea-level
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Cold pulse from about 1300 to 1850 AD Cold pulse from about 1300 to 1850 AD (The so-called “Little Ice Age”)(The so-called “Little Ice Age”)
Climate has been warming since then.Climate has been warming since then.
Athabaska Glacier, Columbia Icefield, W. Canada
Discussion: Global Warming
Worldwide melting, Worldwide melting, regardless of cause, releases regardless of cause, releases CO2 and H2O and exposes CO2 and H2O and exposes dark land. The atmosphere dark land. The atmosphere receives and holds more heat, receives and holds more heat, and temperature rise.and temperature rise.
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End of GlaciersEnd of Glaciers