Lecture 15 Cold Climates Lecture 15 Cold Climates Glaciers and Ice Ages Glaciers and Ice Ages • Glacier: a large, long-lasting Glacier: a large, long-lasting mass of ice, formed on land that mass of ice, formed on land that moves under the influence of moves under the influence of gravity gravity • Glaciers form by accumulation and Glaciers form by accumulation and compaction of snow compaction of snow – Packed snow becomes Packed snow becomes firn firn – Then refreezes to ice Then refreezes to ice Davidson Glacier near Haines, Alaska Davidson Glacier near Haines, Alaska http://www.aad.gov.au/asset/webcams/mawson/
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Lecture 15 Cold Climates Glaciers and Ice Ages Glacier: a large, long-lasting mass of ice, formed on land that moves under the influence of gravityGlacier:
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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
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
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
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
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”