earth science - glaciers & ice ages. davidson glacier near haines, alaska
TRANSCRIPT
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