Earth Science -

Glaciers & Ice Ages

Davidson Davidson Glacier near Glacier near Haines, Haines, AlaskaAlaska

GlaciersGlaciers

• 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 gravity and its own weight influence of gravity and its own weight

• 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

Formation of Glacial Ice from SnowFormation of Glacial Ice from Snow

Glaciation TypesGlaciation Types

• Alpine Glaciation: found in mountainous Alpine Glaciation: found in mountainous regions regions

• Continental Glaciation: exists where a Continental Glaciation: exists where a large part of a continent is covered by large part of a continent is covered by glacial ice - cglacial ice - cover vast areas

Types of GlaciersTypes of Glaciers

Alpine GlaciersAlpine Glaciers• Are confined by surrounding mountains

Types:

• Cirque Glaciers – erode basins in mountainsides

• Valley Glaciers – flow into preexisting stream valleys

• Icecaps – form on mountaintops

Types of GlaciersTypes of Glaciers– Cirque Glacier– Cirque Glacier

Mount Edith Cavell, Jasper National Park, Canada

Types of Glaciers Types of Glaciers – Valley Glacier– Valley Glacier

Tongas National Forest, Alaska

Types of Glaciers Types of Glaciers – Icecap and Continental– Icecap and Continental

Sentinal Range, Antarctica

A Glacier’s BudgetA Glacier’s Budget

• A story of gains and losses

• Gain snow in zone of accumulation

• Lose ice in zone of ablation

• Budget can be:– Positive (growth)– Static (unchanging)– 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, still flows

Iceberg Calving Iceberg Calving – – Hubbard Glacier, Wrangell-St. Elias National Park, AlaskaHubbard Glacier, Wrangell-St. Elias National Park, Alaska

Erosion by GlaciersErosion by Glaciers

• Abrasion– Rocks embedded in glacier’s base make

linear scratches and grooves in bedrock

• Quarrying– Glacier breaks off and removes large blocks

of rock

Glacial Abrasion in BedrockGlacial Abrasion in Bedrock

Source: Tom Bean

Glacial Erosion – Roche MoutoneeGlacial Erosion – Roche Moutonee

Yosemite NP, California

Erosion by Glaciers (cont)Erosion by Glaciers (cont)

• Alpine glaciers erode mountain slopes into horseshoe shaped basins called cirques

• Erosion of multiple cirques erodes intervening rock, causing:– Horns: pointy peaks

– Aretes: long serrated ridges

– Cols: passes through the ridges.

Alpine Glacial ErosionAlpine Glacial Erosion

Alpine Glacial ErosionAlpine Glacial Erosion

Origin of Hanging Valley

Yosemite FallsYosemite Falls

Valley GlaciersValley Glaciers

• Erode a large quantity of bedrock and sediment

• Convert V-shaped stream valleys into U-shaped glacial valleys.

U-ShapedU-Shaped Valley in Tracy Wilderness, Southeastern AlaskaValley in Tracy Wilderness, Southeastern Alaska

Seawater Flooded U-ShapedSeawater Flooded U-Shaped Valleys: FjordsValleys: FjordsBela Bela Fjord, BC

Erosion by Continental GlaciationErosion by Continental Glaciation

• Erosional Landforms much larger in scale than alpine glaciers

– Huge U-shaped troughs, including:

Finger Lakes, Great Lakes, Puget Sound,

and Loch Ness were all once valleys

excavated by glaciers

Erosion of Preglacial Lowlands (Finger Lakes)Erosion of Preglacial Lowlands (Finger Lakes)

Erosion of Preglacial Lowlands (Great Lakes of North America)Erosion of Preglacial Lowlands (Great Lakes of North America)

Source: U.S. Dept. of Interior, USGS Eros Date Center

Glacial Deposits or DriftGlacial Deposits or Drift• Glacial Till: unsorted, unstratified

sediments deposited by melting ice.

– May contain glacial erratics

– Often accumulate at glacier’s terminus as a Moraine: hills of sediment left by a glacier’s retreat.

Advance & Retreat: MorainesAdvance & Retreat: Moraines

Large Granite Erratics Large Granite Erratics

Medial Moraines Medial Moraines – – Kennicott GlacierKennicott Glacier

Effects of GlaciationEffects of Glaciation

• Change Climate – increased precipitation

• Drop in sea-level: alter coastlines

• Form continent-wide Dams– Divert streams – Ohio and Missouri rivers

Lowered Sea-level - LandbridgeLowered Sea-level - Landbridge

Lowered Sea-level exposed continental shelfLowered Sea-level exposed continental shelf

Giant Ripples of the Missoula FloodingGiant Ripples of the Missoula Flooding

Causes of Ice AgesCauses of Ice Ages

• Plate Tectonics

Moves Continents to Poles

Raises mountains above snowline

• Orbit Distances, Axis Tilt and Wobble– Moderates solar radiation past 65° N– Croll-Milankovitch Cycles ~ 100,000 years– Low summertime radiation causes glaciers to

expand

Glacier Distribution 20,000 yaGlacier Distribution 20,000 ya

Approximate Maximum

Milankovitch Cycles Milankovitch Cycles

100,000 years

41,000 years

Earth’s Past Ice AgesEarth’s Past Ice Ages

• PreCambrian Glaciation– 750 mya ice flowed from poles to tropics

• Late Pennsylvanian & Permian Glaciation– covered South Africa, South America, India, Australia

• Pleistocene Glaciation– most recent, ended about 10,000 years ago

Pleistocene GlaciationPleistocene Glaciation

• started 1.6 mya -30 advances and retreats

• Latest retreat ended 10,000 years ago

• “Little Ice Age” occurred 700 to 150 years ago, paralyzed Europe

• Next glaciation period - ???

Sustained warming since 1850Sustained warming since 1850

Athabaska Glacier, Columbia Icefield, W. Canada

End of GlaciersEnd of Glaciers