Lecture 15 Cold ClimatesLecture 15 Cold ClimatesGlaciers and Ice AgesGlaciers and Ice Ages
• 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 gravityinfluence of gravity
• 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
Davidson Glacier near Haines, AlaskaDavidson Glacier near Haines, Alaska
http://www.aad.gov.au/asset/webcams/mawson/
Formation of Glacial Ice from SnowFormation of Glacial Ice from Snow
snowball
iceball
Alpine Glacier: it is just a frozen river
Types of GlaciersTypes of Glaciers
Alpine
Continental
Alpine glaciation: found in mountainous regions Alpine glaciation: found in mountainous regions Continental glaciation: exists where a large part of a continent is covered by glacial iceContinental glaciation: exists where a large part of a continent is covered by glacial ice
Cover vast areas
Alpine GlaciersAlpine Glaciers– Cirque Glacier– Cirque Glacier
Mount Edith Cavell, Jasper National Park, Canada
Alpine Glaciers Alpine Glaciers – Valley Glacier– Valley Glacier
Tongas National Forest, Alaska
Lateral and medial moraines
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 air is too cold to hold much moisture.•The lowest recorded temperature is -89.2 °C.•There is no life in Antarctica except near the coast
http://www.aad.gov.au/asset/webcams/mawson/
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
Iceberg Calving Iceberg Calving – – Hubbard Glacier, Wrangell-St. Elias National Park, AlaskaHubbard Glacier, Wrangell-St. Elias National Park, Alaska
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)
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, it still flows downhill
Mechanics of Glacial FlowMechanics of Glacial FlowInternal deformationInternal deformation
Ice crystals slide past one anotherIce crystals slide past one anotherBasal Sliding Basal Sliding Entire glacier slides downhill on a thin film of Entire glacier slides downhill on a thin film of meltwater at its base.meltwater at its base.Glacier always flows toward zone of ablationGlacier always flows toward zone of ablation
Basal Sliding Discussion:Ice skates and sleds
Erosion 1: Glacial Abrasion in BedrockErosion 1: Glacial Abrasion in Bedrock
Source: Tom Bean
Glaciers cause erosion
Rock embedded in Ice
Glacial Erosion – Roche MoutonéeGlacial Erosion – Roche Moutonée
FROST WEDGING
2. Quarrying
Glacial Erosion – Roche MoutonéeGlacial Erosion – Roche Moutonée
Yosemite NP, Calif
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
Alpine Glacial ErosionAlpine Glacial Erosion
Cirque and valley glaciers form in river valleys, cut U-shapes
Alpine Glacial ErosionAlpine Glacial Erosion
Cirque glaciers erode uphill, widenValley glaciers enlarge
Arêtes, Horn, Col, HANGING VALLEYS
Yosemite Falls from Hanging ValleyYosemite Falls from Hanging Valley
U-ShapedU-Shaped Glacial Glacial Valley in Southeastern AlaskaValley in Southeastern Alaska
Valley glaciers erode a large quantity of bedrock and sedimentValley glaciers erode a large quantity of bedrock and sediment
Convert V-shaped stream valleys into U-shaped glacial valleysConvert V-shaped stream valleys into U-shaped glacial valleys..
Seawater Flooded U-ShapedSeawater Flooded U-Shaped Valleys: FjordsValleys: FjordsBela Bela Fjord, BC
Sea Level rose as glaciers melted
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
Erosion by Continental Glaciation (Great Lakes, Finger Lakes)Erosion by Continental Glaciation (Great Lakes, Finger Lakes)
Source: U.S. Dept. of Interior, USGS Eros Date Center
Superior
Michigan
Huron
Erie
Ontario
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 at glacier’s terminus as a
Terminal Moraine: hills of sediment left by a glacier’s retreat.
– Moraines may be reshaped by a later glacial advance into Drumlins: rounded elongated hills
Advance & Retreat: MorainesAdvance & Retreat: Moraines
Note moraine, no matter direction
Analogy: Escalator
Discussion: Advance & Retreat of Discussion: Advance & Retreat of Glaciers and Terminal MorainesGlaciers and Terminal Moraines
Large Granite Erratics Large Granite Erratics Favored by climbers with families
Medial MorainesMedial Moraines
Medial Moraines Medial Moraines – – Kennicott GlacierKennicott Glacier
Wrangell-St. Elias NP, SE AK
The Origin of DrumlinsThe Origin of Drumlins
Glacier retreats, leaving a terminal moraine. Then it advances again, and redistributes the drift as a spoon shaped hill called a drumlin.
Drumlins Drumlins Rochester,NYRochester,NY
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
Origin of EskersOrigin of Eskers
Eskers and Kettles in South DakotaEskers and Kettles in South Dakota
Effects of GlaciationEffects of Glaciation
• Change Climate – increase precipitation
pluvial lakes
• Depress continents & lateral rebound
• Drop sea-level: alter coastlines
• Moraines form Dams – Proglacial Lakes– Divert streams – Ohio and Missouri rivers
Formation of Terraces due to Crustal ReboundFormation of Terraces due to Crustal Rebound
Lowered Sea-level - Land bridgeLowered Sea-level - Land bridge
BeringGlacier grows, sea-level drops
Lowered Sea-level exposed continental shelfLowered Sea-level exposed continental shelf
Massive extinctions of shallow-water
marine organisms
The Creation of Glacial Lake MissoulaThe Creation of Glacial Lake Missoula
Purcell Lobe blocks Clark Fork River
The Draining of Glacial Lake Missoula The Draining of Glacial Lake Missoula
Repeated many times, last time 13000 kya
Giant Ripples of the Missoula FloodingGiant Ripples of the Missoula Flooding
Giant Ripples
Country road for scale
Flood kills everything in its path, 26 times
Max Glacier Distribution 20,000 yaMax Glacier Distribution 20,000 ya
Maximum glaciation occurs at coincidence of three astronomical cycles
if high land in polar latitudes
Causes of Ice AgesCauses of Ice Ages• Plate Tectonics
Moves Continents to Poles
Raises mountains above snowline
Albedo increases, colder, spread
• Orbit distance, Axis Tilt and Wobble– Moderates solar radiation past 65o Latitude– Croll-Milankovitch Cycles ~ every 100,000 years– With many smaller cycles between– Need low summertime radiation past 65o Latitude
less melting, glaciers expand
Milankovitch Cycles Milankovitch Cycles
100,000 years
The energy of solar radiation drops off as the square of the distanceFurther awayNot as hot
http://en.wikipedia.org/wiki/Milankovitch_cycles
41,000 years
Cold Dry Winter Hot SummerSnow meltsGlaciers shrink
Warm Wet Winter Cool SummerSnow doesn’t meltGlaciers grow
High latitude getting more sunlight in summer
High latitude getting less sunlight in summer
High latitude getting more sunlight in winter
25,700 years
at Perihelion
at Perihelion
Determines which hemisphere gets conditions suitable for glaciation
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
• Many glaciation peaks are about 100,000 years apart, but that is ONLY an average. This suggests that orbital ellipticity is important. The worst glaciations occur when minimum tilt coincides with maximum ellipticity.
Earth’s Past Ice AgesEarth’s Past Ice Ages
• Tertiary to Quaternary cooling – Pleistocene• None in Mesozoic• Late Pennsylvanian & Permian in southern continents (Gondwana)• Ordovician glaciation (Gondwana)
– Area that is now the Sahara at South Pole• PreCambrian Tillites (Lithified Till)
– Three, maybe four,episodes– Oldest 2.8 bya – 750 mya ice from poles to tropic “Snowball Earth”
Cenozoic
Mesozoic
Paleozoic
Paleozoic
PreCambrian
Permian Glaciation – Gondwana TillitesPermian Glaciation – Gondwana Tillites
Cenozoic CoolingCenozoic CoolingCentral America Forms
Southern Ocean forms
Foraminifera tests - Ice AgeForaminifera tests - Ice Age
1. Evap. water and CO2
removes 16O from oceans18O left in oceans used to make shells
3. Also spiral direction & diversity depends on Temp.
Nebraskan
Kansan
Illinoian
Wisconsinan
About 30 pulses in 4 or so major
groups
ColdWarm
2. Ice traps CO2 and water with light
oxygen
Continuous Ice Sheet 20 kyaContinuous Ice Sheet 20 kya
Scoured 30 M below sea-level
Global Temperatures Cycle, largest Global Temperatures Cycle, largest 100,000 Year (orbit eccentricity)100,000 Year (orbit eccentricity)
Smaller signals about 25K and 41KInterglacials get really WARM
The HoloceneThe Holocene
• Latest retreat began 10,000 years agoLatest retreat began 10,000 years ago• Climate varied. Hot Climate varied. Hot 8000 – 4000 ya8000 – 4000 ya
• Medievil Warm: 800 – 1200 ADMedievil Warm: 800 – 1200 AD
• Little ice age 1200 AD until 1850Little ice age 1200 AD until 1850
• Widespread Famine Widespread Famine
• 1253 Pueblo cliff-dwellings abandoned.1253 Pueblo cliff-dwellings abandoned.
• 1340s AD Black Death Bubonic Plague1340s AD Black Death Bubonic Plague
http://en.wikipedia.org/wiki/Little_Ice_Age
Thousands of Years BP
Mann’s Hockey Stick
http://en.wikipedia.org/wiki/Image:Holocene_Temperature_Variations.png
2004 oC
Holocene Temperature Variations
-1
+1
12 6 0
Waterfreezes
Sustained warming since 1850Sustained warming since 1850Athabaska Glacier, Columbia Icefield,
W. Canada2005 Greenland glacier
retreated 9 miles!
Antarctic Glaciers started melting this season