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Glaciers and Glaciation

Chapter 15

Glaciers• A glacier is a large, permanent (non-

seasonal) mass of ice that is formedon land and moves under the force ofgravity.

• Glaciers may form anywhere thatsnow accumulation exceeds seasonalmelt.

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Types of Glaciers• Alpine glaciersAlpine glaciers form in the

mountains.– A valley glaciervalley glacier is one that

occupies a valley,– An ice capice cap is a mass of ice that

occupies a high mountain area andflows outward in severaldirections.

• Continental glaciersContinental glaciers (iceicesheetssheets) may cover largesections of continents as inGreenland and Antarctica.

Glacier Dynamics• A typical valley glacier will add snow at its

head and lose to melt at its foot.• The snow linesnow line is the line below which the

annual snow cover is lost in summer.• The region above the snowline is the zonezone

of accumulationof accumulation; the region below the zonezoneof wastageof wastage (melting, sublimation, calvingmelting, sublimation, calving).

• If it gains more than it loses, its terminuswill advance.

• If it loses more than it gains, it will retreat.

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Glacier Dynamics

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Properties of Ice• Ice has a density of 0.96 g/cm3. (It

floats.)• Ice is a weak solid and will not hold a

vertical cliff higher than 40m (130ft).• Ice will flow under it own weight.• Pressure can cause local melting.

Crevasses, Seracs, and Ogives• A crevassecrevasse is a fracture in the glacier

surface.– Crevasses form where the surface is in

tension.– Crevasses are rarely more than about 40

m deep.– A bergschrundbergschrund is a crevasse at the head

of a glacier.• A seracserac is a large, irregular block of

ice formed by intersecting crevasses• An ogiveogive is compressional wave in

the glacier surface.

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Crevasses

Serac

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Bergschrund

Ogives

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Moraines• A morainemoraine is a deposit of unsorted

(silt- to boulder-size) material (tilltill)left by a glacier.

– Lateral morainesLateral moraines are left at the sides ofglaciers

– Medial morainesMedial moraines are left in the centers.– Terminal morainesTerminal moraines are left at the ends.

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Lateral MorainesLateral Moraines

Continental GlacialLandforms

• A drumlindrumlin is a rounded, striatedoutcrop hill.

• An eskeresker is a low, sinuous hill oftill left by a sub-glacial stream.

• A kettle kettle is a depression in anoutwash plain left by a buriedblock of ice that has melted.

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Mountain Glacial Landforms• Glacial valleys have a characteristic U-U-

shapeshape.• A hanging valleyhanging valley is the valley of a tributary

glacier.• A cirquecirque is a steep-walled bowl-shaped

valley at the head of a glacier.• An arêtearête is a narrow ridge between two

cirques.• A hornhorn is a peak at the intersection of

arêtes.• A fjord fjord is a glacial valley that opens to the

sea.

MountainGlacial

Landforms:

U-shaped &HangingValleys

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MountainGlacial

Landforms:

Arête

Mountain Glacial Landforms:Horn

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Mountain Glacial Landforms:Nunatak

Mountain Glacial Landforms:

Cirque

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MountainGlacial

Landforms:

FjordKangerluqsuaqE. Greenland

Ice Ages• The last 1.8 million years of geologic time

(PleistocenePleistocene) are the Ice Ages.• The last major ice advance in North

America peaked about 18,000 years ago.• Climate warmed abruptly at 14,500 years

ago.• It cooled again and then warmed abruptly

(~6ºC) at about 11,500 to near currentlevels.

• The last remnants of the North AmericanIce sheets melted away less than 7000years ago.

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Previous Ice Ages

• Permian (300 million years)• Neo-Proterozoic (570 million

years)• Archaean-Proterozoic boundary?

PreviousIce Ages

Striationsfrom PermianGlaciation in

Namibia

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Causes of Ice Ages• Milankovich Cycles

– Eccentricity of orbit (100,000 ycycle).

– Tilt of rotation axis (41,000 ycycle).

– Precession of rotation axis (23,000y cycle).

• CO2 variation.• Continental drift and polar

continents.• Ocean Circulation patterns

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Eccentricity

Tilt

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Precession

Continental Drift

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Continental Drift

CO2 Variation• Carbon dioxide is a greenhouse gas.

– It transmits visible and UV radiation– It blocks IR radiation.

• CO2 variation correlates with globaltemperature.

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Glacial Terms

• Glacier• Alpine

Glacier• Ice Cap• Ice Sheet• Horn• Arête• Cirque• Fjord

• Snow Line• Sublimation• Calving• Crevasse• Bergschrund• Serac• Ogive• Till

• Drumlin• Esker• Kettle• Hanging

Valley• Moraine

– Terminal– Lateral– Medial