Lecture 15 Cold Climates Glaciers and Ice Ages Glacier: a large, long-lasting mass of ice, formed on...

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