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TRANSCRIPT
Glaciers and Glaciation Chapter 21
Why do we care 1. Erosion 2. Climate change
Glacier erosion and transport: who cares? 1. topography, sediments, rock cycle
Glacial Valley with LAKES
Glacier erosion and transport: who cares? 2. records of climate change
Glacier erosion and transport: who cares? 3. Sea level change
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
How snow turns to ice
AB
OU
T 50
MET
ERS
(160
feet
)
Glaciers, Glaciation and Landforms
• Glacier – a thick mass of ice that originates on land from the accumulation, compaction, and recrystallization of snow
• Types of glaciers • Valley (alpine) glaciers
– Exist in mountainous areas – Flows down a valley from accumulation
center at head
Valley Glacier Flows off Ice Sheet
Valley glacier off ice cap
• Types of glaciers • continental glaciers or ice caps
– Cover huge areas: Antarctica, greenland – Flows from accumulation center, but not
down a valley
The dry valleys
Alpine or valley glaciers
Greenland
Continental glaciers or ice caps
500 km
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
Glaciers move by basal sliding and internal flow
Movement of glacial ice
• Rates of glacial movement • Rates of up to several meters per day • Some glaciers exhibit extremely rapid
movements called surges
Record of a surging glacier
Part II. Glacier mass balance
Accumulation zone: snow > melt Ablation zone: melt > snow
Glacier mass balance
Change in ice volume = input – output Change in ice volume = snow – melt
What will happen if??? Change in ice volume = snow – melt If snow > melt, ice volume increases A) Terminus retreats B) Terminus advances
When does terminus ADVANCE
Change in ice volume = snow – melt If snow > melt, ice volume increases COLDER temperature or MORE PRECIPITATION
Sierra Nevada, California
Simulated ice thickness and distribution
When does terminus RETREAT
Change in ice volume = snow – melt If snow < melt, ice volume decreases WARMER temperature or LESS PRECIPITATION
• Budget of a glacier
• Balance between accumulation and loss – If accumulation exceeds loss the glacial front
advances – Of loss exceeds accumulation, front retreats
Why did glacier front retreat?
• A) colder temperature • B) warmer temperature • C) more precipitation • D) A and C
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?
Glacial deposition 1. Glacial ice flows towards terminus 2. Sediment deposited at terminus 3. Largest deposits when terminal position steady
TERMINUS
Terminal moraine
Moraines - Eastern Sierra Nevada, California
Glacier erosion and transport glaciers are world champions of erosion and transport
How do glaciers erode? Glaciers move by: 1. basal sliding 2. internal flow
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
PLUCKING
Glacial Polish
ABRASION
Glacial Striations
ABRASION
Mass movements
Glacial erosion produces
spectacular topography
Matterhorn in the Swiss Alps
Sharp Peaks, Horns and Cirques
Cirque
Glacial Valley with LAKES
Fjord - a drowned glacial valley
Hanging Valley - Yosemite Natl Park
Glacier transport lots of sediment in the ice and at the surface a wide range of grain sizes
Moraines (lateral or medial)
Glacial transport Boulders and other sediment
Where do glaciers deposit material they carry?
TERMINUS
Glacial deposition 1. Glacial ice flows towards terminus 2. Sediment deposited at terminus 3. Largest deposits when terminal position steady
TERMINUS
Terminal moraine
Lateral Moraines - Eastern Sierra Nevada, California
Terminal moraines from continental glaciers
Glacial deposits are typically unstratified and unsorted
Unsorted Glacial Till
Glacial deposits can be sorted and stratified: liquid water sorts material
Glacial depositional features
Drumlins – Occur in clusters called drumlin fields
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
1. Terrestrial records of glaciations Two types of glaciers
Alpine or valley glaciers
Greenland
Continental glaciers or ice caps
500 km
Shelton photo Field trip up a valley Another moraine
Trip up a valley Near top of valley Small, sharp moraine No vegetation
Cirque in top of valley may have a small glacier
Field Trip II. Massachusetts; Glacial erratic:
1. A big rock 2. different rock type than bedrock
Glacial deposits stretching for 100’s km 1. Poorly sorted, no stratification 2. Sorted with bedding
Large portion of N. America covered by continental glaciers
Terminal moraines from continental glaciers
Glacial deposits are typically unstratified and unsorted
Unsorted Glacial Till
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?
The little ice age: historical records
Why did glacier front retreat?
• A) colder temperature • B) warmer temperature • C) more precipitation • D) less precipitation • E) B & D
Little Ice Age 1800’s
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
What about the other glaciations?
3-4 distinct moraines; Relative dating cross-cutting
relationships
Soil development
Why did glaciers advance and retreat?
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?
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
Isotope of Oxygen
Oxygen 16 Oxygen 17 Oxygen 18
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
ICE SHEET
Water stored in ice sheets has relatively few H2O18 molecules
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
Maximum extent of ice during the last Ice Age
Last warming
trend leads to
rise in sea level
Present level is
very high!
Ice sheet reconstructions 21 ka 14 ka 12 ka
10 ka 8 ka 0 ka
Peltier (1994)
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
2. TILT
3. Precession
Comparison of orbital variations with isotope records
• 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)