Geology lecture 20

Chapter 22

Glaciers and Ice Ages

Chapter 22

Outline• Ice and glacial ice, glaciers and their formation.

• Glaciers-Types: mountain and continental-Thermal categories: temperate and polar-Ice movement, glacial advance & retreat-Ice in the sea

• Effects of glaciers:-Erosion, sediment transport, deposition-Depositional landforms

• Glaciation-Theory of glaciation and ice ages-Subsidence & rebound, sea level change, drainage modification.-The Pleistocene Glaciation and earlier ones-Causes

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Ice: The Water Mineral• Ice is solid water (H2O).

• Forms when water cools below freezing point.• Natural ice is a mineral; grows in hexagonal forms.

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Formation of Glacial Ice

• Snow is transformed into ice.• Flakes accumulate.• Buried by later falls. • Compression expels air. • Pressure causes melting &

recrystallization. • Snow turns into granular firn.• Over time, firn melds into

interlocking crystals of ice.

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Glaciers & Their Formation• Thick masses of recrystallized (glacial) ice.

• Last all year long, flow via gravity.

• 3 conditions necessary:• Cold local climate• Snow must be abundant; more snow must fall than melts• Snow must not be removed by avalanches or wind

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Forming a Glacier

• Glacier-sustaining elevation is latitude controlled• Polar regions -> glaciers form at sea level• Equatorial regions -> glaciers form >5km elevation

• This elevation is marked by the “snow line”

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Glaciers

2 main types: Continental and mountain.

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Mountain Glaciers• Flow from high to low elevation in mountains• Include a variety of types.

• Ice caps cover mountain peaks• Cirque glaciers fill mountain top bowls

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Mountain Glaciers

• Valley glaciers flow like rivers down valleys

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Mountain Glaciers• Piedmont glaciers spread out at valley terminus

Greenland

Alaska

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Continental Glaciers• Ice sheets covering large land areas.• Flows outward from thickest portion• 2 major ice sheets on Earth:

Greenland, Antarctica

Minor: Iceland

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Thermal Categories

• Used to classify glaciers; determined by climate.• Temperate glaciers – ice at/near melting temperature• Polar glaciers – ice well below melting temperature

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Glacial Ice Movement• How do glaciers move?

• Wet-bottom glaciers – water flows along base• Basal sliding – ice slips over a meltwater/sediment slurry

• Dry-bottom glaciers – cold base is frozen to substrate• Movement is by internal plastic deformation of the ice

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Glacial Ice Movement• 2 types of mechanical behavior.

• Brittle – Uppermost 60 m• Tension initiates cracking• Crevasses open and close with movement

• Plastic – Lower than 60 m• Ductile flow occurs in deeper ice• Ice flow heals cracks

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• Ice flows downhill via gravity. • Gravity (g) can be resolved into 2 vectors

• Parallel to slope component (gs) drives flow

Glacial Ice Movement

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• Ice flows downhill via gravity. • Ice flows from thickest part of continental glaciers

• Analogous to honey flowing…


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• Variable flow rates (10 to 300 m/yr). • Rarely, glaciers surge (20 to 110 m/day)• Flow rate controlled by:

• Slope -> steeper = faster• Basal water: wet-bottom = faster• Location within glacier

• Greater velocity in ice center• Friction slows ice at margins


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• Glaciers behave like bank accounts• Accumulation zone – Area of new snow addition

• Colder T prevent melting• Snow remains even in summer

• Ablation zone – Area of net ice loss• Zones abut at equilibrium line

Glacial Advance and Retreat

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Glacial Advance and Retreat• Toe (leading edge) position:

• If accumulation = ablation the toe is stable

Note curved path of ice flow.

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• Toe position:• If accumulation < ablation, toe retreats upslope

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• Toe position:• If accumulation > ablation, the toe advances

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Glacial advance and retreat is determined by the balance between the accumulation of snow and the removal of ice by sublimation, melting, and calving (ablation). When the rate of ablation below the snowline equals the rate of accumulation above it, the glacier is stationary, as in View 1. During glacial retreat, View 2, the rate of ablation exceeds the rate of accumulation, and the position of the toe retreats toward the origin of the glacier. Glacial advance, View 3, occurs when the rate of accumulation exceeds the rate of ablation. For all views, pay attention to the motion of the stones. Note that in all cases, ice flows downhill. For more information, see Section 22.2 Ice and the Nature of Glaciers starting on p. 758 and Figure 22.14 on p. 768 in your textbook.


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Ice in the Sea• In polar regions, glaciers flow out over ocean water.

• Tidewater glaciers – valley glaciers entering sea• Ice shelves – continental glaciers entering sea• Sea ice – non-glacial ice formed on frozen seawater

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Ice in the Sea• Large areas of the polar seas are covered with ice.

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Ice in the Sea

• Marine glaciers have both grounded and floating ice• Ice debris calves off edge forming icebergs

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Glacial Erosion• Glaciers erode substrates in

1. Incorporation – rock surrounded and carried off

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Glacial Erosion

2. Plucking – ice breaks off and removes bedrock fragments• Ice melts by pressure against up-ice side of obstruction• Enters cracks in bedrock, water re-freezes to the ice• Glacial movement “plucks” away bedrock chunks

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Glacial Erosion3. Abrasion – a “sandpaper” effect on substrate

• Rock fragments in moving ice abrades and polishes bedrock• Leaves scratches called striations

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• Various erosional features of glaciated valleys.Glacial Erosion

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Glacial Erosion

• Cirque – bowl-shaped basin near mountain top• Forms at uppermost portion of valley• Erosion at glacier base over-steepens into bowl• After ice melts, cirque is often filled with a lake (tarn)

Glacier, Nat’l Park, MT

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Glacial Erosion• Arête – a “knife-edge” ridge

• Formed by 2 adjacent cirques or • Valley glaciers

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Glacial Erosion• Horn – a pointed mountain peak

• Formed by 3 or more cirques that coalesce

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Glacial Erosion

• U-shaped valleys.• Glacial erosion creates

very steep walls• Unlike V-shaped fluvial

valleys

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Glacial Erosion• Hanging valleys.

• -Intersection of tributary glacier• With trunk glacier• -Trunk glacier incises deeper• -Trough bases at different• -Elevations• -A waterfall results

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Glacial Sediment Transport

• Glaciers carry sediment of all sizes (lots of it!)• Some falls onto from adjacent cliffs• Some entrained from substrate erosion

• When ice melts, material is dropped

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Glacial Sediment Transport

• Moraines – unsorted debris carried/dumped by glaciers• Lateral – along valley glacier flank• Medial – mid-ice merging lateral moraines

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Glacial Sediment Transport• Glaciers are large-scale conveyor belts

• Pick up, transport, deposit sediment. • Transport is always downhill• Debris at toe is an end moraine

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Glacial Deposition• Many sediment types derive from glaciation.• Called glacial drift, these include...

• Till.• Erratics.• Marine sediments.• Outwash.• Others (lake seds, loess)

• Stratified drift is water-

sorted; unstratified drift

isn’t.

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Glacial Deposition• Glacial till – Sediment dropped by glacial ice.

• Consists of all grain sizes• Unmodified by liquid water, hence:

• Unsorted • Unstratified

• Accumulates…• Beneath glacial ice• At glacial toe• Along glacial flanks

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Glacial Deposition• Erratics – boulders dropped by glacial ice

• Different than underlying bedrock• Often carried from long distances

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Glacial Deposition

• Glacial marine – sediments from an oceanic glacier• Calving icebergs raft sediments away from ice• Melting bergs drop stones into marine muds• Dropstones…

• Differ from ambient sediment• Indicate glaciation

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Glacial Deposition

• Glacial outwash – sediment transported in meltwater • Muds removed• Abraded and rounded• Dominated by sand and gravel

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Depositional Landforms

• Glacial sediments create distinctive landforms.• Moraines.• Drumlins.• Ground moraine.• Kettle lakes.• Eskers.

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Depositional Landforms• End moraines form at stable glacier toe• Terminal moraines form at farthest edge of flow• Recessional moraines form as retreating ice stalls

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• Drumlins – long aligned hills of molded till• Asymmetric form – steep up-ice; tapered down-ice• Aligned parallel to ice flow direction

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• Eskers – long, sinuous ridges of land and gravel• Form from meltwater channels below ice

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The Theory of Glaciation• European farmers broke plows on large rocks.

• Buried in fine-grained soils, often of enormous size. • Unlike local bedrock, they were from 100s of km away.

• Rocks became known as erratics• Origins were a mystery…

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The Theory of Glaciation• Louis Agassiz, a Swiss geologist, observed glaciers.

• Saw glaciers as landscape change agents• They carried snad, mud, huge boulders long distances• They dropped these materials, unsorted, upon melting

• Realized glaciers could explain the erratics

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The Theory of Glaciation

• Agassiz proposed an ice age had frozen Europe• Ice sheets covered land• Ice carried and dropped…

• Erratics and• Unsorted soil (till)

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Ice Ages• Glaciers presently cover ~10% of Earth• During ice ages, coverage expands to ~30%• Most recent ice age ended ~11 kyrs ago

• Covered New York, Montreal, London, Paris, Seattle• Ice sheets were 100s to 1,000s of m thick.

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Glacial Consequences

• Subsidence & rebound.• -Ice sheets depress lithosphere

into mantle• -Subsidence follows

asthenosphere flow• -Ice melts, depressed

lithosphere rebounds• -Continues slowly today

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Glacial Consequences• Sea level changes due to ice ages

• Water stored on land during ice age –> sea level fall• Deglaciation returns water to oceans –> sea level rise• Sea level was ~100 m lower during last glacial maximum

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Glacial Consequences

• Can cause drainage basin reorganization.• N. America: Glaciation completely changed drainage.

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The Pleistocene Glaciation

• Young (< 2 Ma) glacial remnants are abundant.• North America• Scandanavia and Europe• Siberia

• Distinctive glacial landscapes.

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The Pleistocene Glaciation• Glaciation chronology.

• Several Pleistocene glacial advances• In North America, 4 recognized – youngest to oldest…

• Wisconsinan, Illinoian, Kansan, Nebraskan

• Ice ages are separated

by interglacials

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The Pleistocene Glaciation

• Glaciation chronology.• Oxygen isotopes from plankton

suggest more ice ages• They reveal 20 or more

glaciations

• Higher 18O/16O = colder.• Lower 18O/16O = warmer.

• The “original 4” ice ages

may simply be the largest

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Earlier Glaciations• Glaciation recurs across Earth history. • Evidence? Fossil til (tillite), striated bedrock

• Tillite at equatorial latitiudes suggest an ice-covered world: Snowball Earth

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Causes of Glaciation• Long-term causes – set stage for ice ages

1. Plate tectonics – controls factors that influence glaciation• Continents at high latitudes• Sea level flux by mid-ocean ridge volume changes• Oceanic currents

2. Atmospheric chemistry.• Changes in greenhouse gas concentrations

• Carbon dioxide (CO2)

• Methane (CH4)

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Causes of Glaciation

• Short-term causes – govern advances and retreats• Milankovitch cycles – climate variation over 10-100s kyrs

predicted by cyclic changes in orbital geometry1. Earth’s orbit shape varies (~100,000 year cyclicity)

2. Earth’s axis tilt caries from 22.5degrees to 24.5degrees (~41,000 years)

3. Earth’s axis wobbles (23,000 years)

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Causes of Glaciation• Milankovitch – cyclic changes in orbital geometry.

• Variations lead to excess warming/cooling.• Ice ages result when cooling effects coincide.

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Geology lecture 20