Transcript Glaciers and Ice Ages - The Naked Science Society

Earth Science Glaciers & Ice Ages
Davidson
Glacier near
Haines,
Alaska
Glaciers
• Glacier: a large, long-lasting mass of ice,
formed on land that moves under the
influence of gravity and its own weight
• Glaciers form by accumulation and
compaction of snow
– Packed snow becomes firn
– Then refreezes to ice
Formation of Glacial Ice from Snow
Glaciation Types
• Alpine Glaciation: found in mountainous
regions
• Continental Glaciation: exists where a
large part of a continent is covered by
glacial ice - cover vast areas
Types of Glaciers
Alpine 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 Glaciers– Cirque Glacier
Mount Edith Cavell, Jasper National Park, Canada
Types of Glaciers – Valley Glacier
Tongas National Forest, Alaska
Types of Glaciers – Icecap and Continental
Sentinal Range, Antarctica
A 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 Budget
Year round Snow
Summer Rain
Iceberg Calving –
Hubbard Glacier, Wrangell-St. Elias National Park, Alaska
Erosion 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 Bedrock
Source: Tom Bean
Glacial Erosion – Roche Moutonee
Yosemite NP, California
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 Erosion
Alpine Glacial Erosion
Yosemite Falls
Valley Glaciers
• Erode a large quantity of bedrock and
sediment
• Convert V-shaped stream valleys into Ushaped glacial valleys.
U-Shaped Valley in Tracy Wilderness, Southeastern Alaska
Seawater Flooded U-Shaped Valleys: Fjords
Bela Bela Fjord, BC
Erosion 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 (Great Lakes of North America)
Source: U.S. Dept. of Interior, USGS Eros Date Center
Glacial 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: Moraines
Large Granite Erratics
Medial Moraines – Kennicott Glacier
Effects 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 - Landbridge
Lowered Sea-level exposed continental shelf
Giant Ripples of the Missoula Flooding
Causes 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 ya
Approximate Maximum
Milankovitch Cycles
100,000 years
41,000 years
Earth’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 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 1850
Athabaska Glacier, Columbia Icefield, W. Canada
End of Glaciers