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