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Glaciers and Glaciation Chapter 21
Why do we care 1. Erosion 2. Climate change
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Glacier erosion and transport: who cares? 1. topography, sediments, rock cycle
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Glacial Valley with LAKES
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Glacier erosion and transport: who cares? 2. records of climate change
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Glacier erosion and transport: who cares? 3. Sea level change
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What causes the snow that falls on a glacier to become ice? A) changing humidities B) colder temperatures C) pressure from the overlying snow D) heat from the sun
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How snow turns to ice
AB
OU
T 50
MET
ERS
(160
feet
)
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Glaciers, Glaciation and Landforms
• Glacier – a thick mass of ice that originates on land from the accumulation, compaction, and recrystallization of snow
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• Types of glaciers • Valley (alpine) glaciers
– Exist in mountainous areas – Flows down a valley from accumulation
center at head
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Valley Glacier Flows off Ice Sheet
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Valley glacier off ice cap
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• Types of glaciers • continental glaciers or ice caps
– Cover huge areas: Antarctica, greenland – Flows from accumulation center, but not
down a valley
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The dry valleys
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Alpine or valley glaciers
Greenland
Continental glaciers or ice caps
500 km
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Movement of glacial ice (“flow”)
1. Plastic flow » Occurs within the ice » Under pressure, ice behaves as a plastic
material
2. Basal slip » Entire ice mass slipping along the ground » Most glaciers move this way by this
process
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Glaciers move by basal sliding and internal flow
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Movement of glacial ice
• Rates of glacial movement • Rates of up to several meters per day • Some glaciers exhibit extremely rapid
movements called surges
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Record of a surging glacier
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Part II. Glacier mass balance
Accumulation zone: snow > melt Ablation zone: melt > snow
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Glacier mass balance
Change in ice volume = input – output Change in ice volume = snow – melt
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What will happen if??? Change in ice volume = snow – melt If snow > melt, ice volume increases A) Terminus retreats B) Terminus advances
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When does terminus ADVANCE
Change in ice volume = snow – melt If snow > melt, ice volume increases COLDER temperature or MORE PRECIPITATION
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Sierra Nevada, California
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Simulated ice thickness and distribution
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When does terminus RETREAT
Change in ice volume = snow – melt If snow < melt, ice volume decreases WARMER temperature or LESS PRECIPITATION
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• Budget of a glacier
• Balance between accumulation and loss – If accumulation exceeds loss the glacial front
advances – Of loss exceeds accumulation, front retreats
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Why did glacier front retreat?
• A) colder temperature • B) warmer temperature • C) more precipitation • D) A and C
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Basic Idea: 1. Glaciers advance: when temperature lower or more
precipitation 2. Glaciers retreat: when temperature higher or less
precipitation
When did this happen? How many times did this happen? Why did it happen? could it happen again?
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Glacial deposition 1. Glacial ice flows towards terminus 2. Sediment deposited at terminus 3. Largest deposits when terminal position steady
TERMINUS
Terminal moraine
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Moraines - Eastern Sierra Nevada, California
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Glacier erosion and transport glaciers are world champions of erosion and transport
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How do glaciers erode? Glaciers move by: 1. basal sliding 2. internal flow
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3 main processes of glacial erosion
1. Plucking – lifting of rocks from “bed” 2. Abrasion – sliding over bed smoothes the surface
– Sediment within the ice is critical
3. Mass movements onto ice surface – Rock fall
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PLUCKING
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Glacial Polish
ABRASION
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Glacial Striations
ABRASION
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Mass movements
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Glacial erosion produces
spectacular topography
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Matterhorn in the Swiss Alps
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Sharp Peaks, Horns and Cirques
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Cirque
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Glacial Valley with LAKES
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Fjord - a drowned glacial valley
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Hanging Valley - Yosemite Natl Park
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Glacier transport lots of sediment in the ice and at the surface a wide range of grain sizes
Moraines (lateral or medial)
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Glacial transport Boulders and other sediment
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Where do glaciers deposit material they carry?
TERMINUS
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Glacial deposition 1. Glacial ice flows towards terminus 2. Sediment deposited at terminus 3. Largest deposits when terminal position steady
TERMINUS
Terminal moraine
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Lateral Moraines - Eastern Sierra Nevada, California
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Terminal moraines from continental glaciers
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Glacial deposits are typically unstratified and unsorted
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Unsorted Glacial Till
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Glacial deposits can be sorted and stratified: liquid water sorts material
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Glacial depositional features
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Drumlins – Occur in clusters called drumlin fields
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Glaciers and climate change Widespread evidence that size of glaciers
change through time Important because it tells us: 1. Climate varies 2. The impacts of climate change are
DRAMATIC (big sea level changes)
1. Terrestrial records 2. Glacier mass balance reviewed 3. Oceanic records and orbital forcing
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1. Terrestrial records of glaciations Two types of glaciers
Alpine or valley glaciers
Greenland
Continental glaciers or ice caps
500 km
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Shelton photo Field trip up a valley Another moraine
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Trip up a valley Near top of valley Small, sharp moraine No vegetation
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Cirque in top of valley may have a small glacier
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Field Trip II. Massachusetts; Glacial erratic:
1. A big rock 2. different rock type than bedrock
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Glacial deposits stretching for 100’s km 1. Poorly sorted, no stratification 2. Sorted with bedding
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Large portion of N. America covered by continental glaciers
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Terminal moraines from continental glaciers
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Glacial deposits are typically unstratified and unsorted
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Unsorted Glacial Till
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Basic Idea: 1. Valley glaciers larger in the past 2. Large portions of northern continents covered by ICE SHEETS
When did this happen? How many times did this happen? Why did it happen? could it happen again?
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The little ice age: historical records
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Why did glacier front retreat?
• A) colder temperature • B) warmer temperature • C) more precipitation • D) less precipitation • E) B & D
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Little Ice Age 1800’s
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What about the other glaciations?
Which order is correct, from oldest to youngest
a) tahoe, tioga I, tioga II b) tioga I, tioga II, tahoe c) Tioga II, tioga I, tahoe d) None of the above
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What about the other glaciations?
3-4 distinct moraines; Relative dating cross-cutting
relationships
Soil development
Why did glaciers advance and retreat?
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Basic Idea: 1. Glaciers advanced: when temperature lower or more
precipitation 2. Glaciers retreated: when temperature higher or less
precipitation
When did this happen? How many times did this happen? Why did it happen? could it happen again?
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The number of protons determines the element
The neutrons of a given element
may vary ISOTOPE: variations of the same
element, with different # of neutrons, and so different atomic weight
Isotope
Part III. Oceanic records
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Isotope of Oxygen
Oxygen 16 Oxygen 17 Oxygen 18
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Isotopes of H2O
H2O16 most common
Marine organisms make skeletons out of CaCO3
A diatom: SiO2 skeleton
H2O18 accounts for 1 in 1000 water molecules
Ratio of O16/O18 of sea water
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ICE SHEET
Water stored in ice sheets has relatively few H2O18 molecules
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Record of ice volume from ocean sediments Lots of ice: H2O18 relatively high in oceans Little ice: H2O18 relatively low in oceans
WOW 1. Many, many ice ages !!!!!!! 2. Interval varies; timescales of 10,000 to 100,000 years
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Maximum extent of ice during the last Ice Age
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Last warming
trend leads to
rise in sea level
Present level is
very high!
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Ice sheet reconstructions 21 ka 14 ka 12 ka
10 ka 8 ka 0 ka
Peltier (1994)
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Why do we have glaciers advance and retreated over and over again?
Hypothesis: linked to variations in Earth’s orbit and variations in solar input 1. eccentricity 2. precession 3. tilt
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2. TILT
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3. Precession
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Comparison of orbital variations with isotope records
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• What if the ice on Earth melted? • ~ 2 percent of the world’s water is in glaciers • Antarctic ice sheet
– 80% of world’s ice – 65% of Earth’s fresh water – Covers 1.5 times area of the United States – If melted, sea level would rise 60 to 70 meters – (wave goodbye to Florida, New York, coastal cities)