Ch. 11 - Glaciers
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Transcript of Ch. 11 - Glaciers
Glaciers
Ch. 11
Glaciers
Thick mass of ice
- accumulates on land
- flows downhill
- caused by large snowfalls in winter and ____________
Types of Glaciers
1) Valley (alpine)
- smallest glaciers
- occupy valleys formerly carved by streams
Types of Glaciers
2) Piedmont
“pied” = French for “foot”
- valley glaciers coalesce at base of steep mountain front
Types of Glaciers
3) Ice caps- cover upland & plateau areas- not confined to valleys- smaller in extent than ice sheetsEx: Iceland
Types of Glaciers
4) Ice Sheets
- largest glaciers
- cover continental regions
- today: Greenland & Antarctica
Ice sheets
- several 1000’s feet thick
- flow outward from central high spot
- covers all but highest elevations
Ice shelves
Occur where ice sheets flow into oceans
- large flat masses of ice attached to land by one or more sides
Glaciers:
Sediment Transport:
High capacity –
High competency –
Erosion Methods
1) Plucking – meltwater gets into cracks in bedrock & freezes
- pieces incorporated into base of ice (worn flat on one side)
Striations
Scratches in exposed bedrock
- indicate direction of glacial movement
Erosion Methods
2) Abrasion
- weight of moving rx & ice scrapes bedrock
- occurs at point of contact
Erosional Landscapes
Valley glaciers produce jagged mountain scenery
Ex: Rocky Mountains
Ice sheets smooth out landscape
Ex: Peoria area
Erosional Landscapes
1) Glacial trough (U-shaped cross-section)
Ice widens, deepens, & straightens former stream valleys
Erosional Landscapes
2) Hanging valley
- smaller valley from a tributary glacier is left higher than main valley
Erosional Landscapes
3) Cirque (“circle”)
- occurs at head of glacial trough
- steep walls around 3 sides but opens on 4th side
Erosional Landscapes
4) Horn
Sharp pyramid-shaped peak
Erosional Landscapes
5) Arete
Knife-like ridge separating adjacent glacial troughs
Erosional Landscapes
6) Tarn – lake occupying a cirque after glaciers melt
7) Paternoster lakes
- series of lakes that sit in glacial trough
Erosional Landscapes
8) Fiord
Drowned glacial trough after last Ice Age passed
Occur where mtns. are adjacent to oceans at high latitudes
Depositional Landscapes
Glacial drift = all sediment deposited by glaciers
Two types of drift:
a) Till = sediment deposited directly by glaciers (unsorted)
Two types of drift:
b) Stratified drift = sediments deposited by glacial meltwater
- sorted by size
Landforms composed of till
1) Erratic
- boulders that differ from underlying bedrock
- source area is outside region where they’re found
Landforms composed of till
2) Moraine – ridge of till
Several types:
a) Lateral moraine – found only w/ valley glaciers
- parallels sides of valley
Lateral Moraine
Material comes from:
a) ice scraping valley walls
b) rx from cliffs above
Types of Moraines
b) Medial moraine
- lateral moraines from joining glaciers merge
Types of Moraines
c) End moraine
- associated w/ stationary glaciers
Types of Moraines
d) Recessional moraines
- end moraines created as a receding glacier occasionally stabilized
Types of Moraines
e) Terminal moraine
- outermost end moraine that marks limit of glacial advance
Types of Moraines
f) Ground moraine
- associated w/ receding glaciers
- forms behind end moraines
- acts to level the land
Landforms composed of stratified drift
Deposited by glacier’s melt water
- flows through moraines & picks up sediment
- braided stream channels common
Landforms composed of stratified drift
1) Outwash plains
- broad ramp-like surface built in front of end moraines
- associated w/ ice sheets
(called “valley trains” w/valley glaciers)
Landforms composed of stratified drift
2) Kettle/kettle lakes
- large blocks of stagnant ice buried in sediment
- ice melts
- creates a depression
Depositional LandformsLoess
- windblown silt deposits
Loess
- associated with meltwater drainages from glaciation
- thickest near drainages and thins away from valleys
Movement of a glacier
Two methods:1) Plastic flow = internal flow- occurs in ice below 50 meters- occurs fastest in center of glacier
(less friction)
Zone of Fracture
Upper 50 meters of glacier
- brittle
- breaks into cracks called “crevasses”
Movement of a glacier
2) Basal slip
Melt water at base of glacier
- entire ice mass slips over surface
Glacial Budgets
Zone of Accumulation = area where snow accumulates & ice forms
Zone of Wastage = net loss as glacier advances into warmer climates (lower elevations)
Zone of Wastage
Loss of snow & ice is called ablation
Ablation is due to:
- melting
- calving
Glacial Budgets
Snowline = boundary between zone of accumulation & zone of wastage
Glacial Budgets
1) Advancing glacier – more accumulation than ablation
- glacial front advances
- snowline drops in elevation
Glacial Budgets
2) Retreating glacier
- ablation exceeds accumulation
- snowline rises in elevation
- caused by warming trend or decrease in snowfall
Glacial Budgets
3) Stationary Budget
- accumulation equals ablation
- total area of glacier is not changing
NOTE: Ice is always moving downhill!
Evidence for past glacial periods (“Ice Ages”)
Periodically, northern Europe & N. America covered by great ice sheets
Last one melted from Canada <10,000 years ago
Evidence for past glacial periods (“Ice Ages”)
1) Glacial erosion
Ice sheets: polished bedrock in northern regions, striations, recessional moraines
Ex: Central Park, New York City
Evidence for past glacial periods (“Ice Ages”)
Valley glaciers:
- Yosemite National Park
- Yellowstone National Park
Yosemite
Evidence for past glacial periods (“Ice Ages”)
2) Pluvial lakes
- form during cooler times w/ moderate rainfall
- occurred at lower latitudes where ice did not advance
Pluvial lakes
Most evident in Basin & Range region in western U.S.
Ex: Lake Manley, Death Valley
Ex: Lake Bonneville, Utah
(Great Salt Lake is a remnant)
Evidence for past glacial periods (“Ice Ages”)
3) Decreasing sea level
Water is locked onto land by ice
Estimated maximum 100 meters lower than present sea level
Decreasing sea level
Evidence:Submerged stream channels on
continental shelves
Evidence for past glacial periods (“Ice Ages”)
4) Crustal Rebound
Land readjusts upward after ice sheets melt
Hudson Bay region has uplifted 300 meters since end of last ice age
Crustal rebound
Evidence for past glacial periods (“Ice Ages”)
5) Lake Missoula
Ice dam blocked melt water
- formed huge lake that flooded western Montana
(half the size of Lake Michigan)
Lake Missoula Map
Lake Missoula
Melting weakened ice dam
- lake emptied in 1-2 days
- discharge ~ 386 million cfs (Amazon discharge = 6 million cfs)
Lake Missoula
Evidence:a) channeled scablands in western
Oregon & southern Idahob) giant ripples of coarse gravel
30’ high, 300’ apart, 2 miles long