Geology Lecture Outline –
Glaciers – (Ch18)
I. Lecture Content
The Hydrologic Cycle
Origin and Nature of Glacial Ice
Types of Glaciers
Behavior of Glaciers & Glacial
Budgets
Glacial Erosion and Transport
Glacial Deposition and Deposits
Causes of Glaciation and Global Climatic
Cycles
Pleistocene Ice Age
Glaciers and Isostacy
II. Introduction
A. Glaciation, Ice Ages and Earth's
Changing Climate
1. Defined: Glaciers are masses of ice, which move over
the land by plastic flow and basal slip
2. Glaciers and ice caps
critical part of the hydrologic cycle
· Around 2% of hydrosphere
· Roughly 75% of Earth's freshwater
· "Inactivated" part of the short-term hydrologic cycle
3. Intimately related with global climatic changes
4. Profound effect on atmospheric and oceanic conditions
5. We are currently in what is called a relatively warm
interglacial period, which is part a much longer duration,
ongoing 1.6 million year-old Ice Age event
B. Glaciation and Earth's
Changing Surface
1. A glacier is like a cross between a moving frozen river,
a bulldozer, and a gigantic piece of very rough sandpaper
2. Some of Earth's most spectacular landscapes are the
result of glacial action over tens of thousands of years
3.
Humans started evolving on Earth just prior to the latest
Ice Age event, which started 1.6 million years ago.
III. Origin and Nature of Glacial Ice
A. Formation and Growth of
Glaciers
1. Principle mechanism by which glaciers form and grow
§ Winter snowfall exceeds summer snowmelt
§ Snow begins to slowly accumulate year after year
2. Transformation of snow to glacial ice a multi-step process
v Snowflakes → Granular snow → Firn → Glacial ice
20% solid 80%air 90%
solid 10% air
3. Thawing-freezing (firn) cycles combined with compaction
4. Glaciers typically form at both high elevations (mountain
valleys) and in polar regions
IV. Types of Glaciers
A. Valley Glaciers
1. Defined: Glaciers that are confined to mountain
valleys
· High elevations
· High latitudes
2. Several smaller tributary glaciers typically merge to form
a much larger glacier
· Very similar to river systems
3. Valley glaciers flow from higher to lower elevations
4. Excellent examples of mountain valley glaciers are found
at some of the most scenic locations on Earth:
· Alaska and the Canadian Rockies
· New Zealand
· European Alps
· The South American Andes
5. Worldwide, many valley glaciers are actively retreating
· Consequence of global warming?
B.
Continental Glaciers
1. Defined: Thick, continental-size sheets of ice presently
found in the northern and southern polar regions
· Greenland
· Antarctica
2. Take form as very thick and extensive sheet-like bodies
of glacial ice called ice sheets, ice caps and ice shelves
· Up to 3000 meters thick near the center of sheet
· Edges typically meet the ocean in Antarctica
· Ice shelves are ice sheets that float on the ocean
· "Calving" of icebergs occur along margins of ice
sheet where it meets the ocean
3. Volume of ice making up the polar ice sheets is HUGE!
· Antarctic ice sheet = ~ 30,000,000 cubic kilometers
· Greenland ice sheet = ~ 2,600,000 cubic kilometers
4. Unlike valley glaciers, continental ice sheets flow radially
outward in all directions from its thickest central region
outward toward its margins (thinnest)
5. Presently there is a scientific debate over whether the
polar ice sheets are shrinking, growing or stable
V. Behavior of Glaciers and Glacial
Budgets
A.
Movement of Glaciers
1. Valley glaciers start to move when accumulation of snow
& ice reaches about 40 meters in vertical thickness
2. Two primary mechanisms of glacial movement
· Plastic flow occurs inside the glacier
· Basal slip occurs at the bottom surface of glacier
· Primary moving force is gravity
· See Figure 17.3 for illustration
3. Velocity
of a glacier depends on several factors
§ Thickness of glacier
§ Downhill slope or gradient
§ Presence of water along basal surface
§ Roughness and relief of ground surface
4. Velocity profile of a glacier is much like a regular river
· Swiftest near the top and middle line
· Slowest
near the bottom and sides
5. Average speeds of glaciers have a considerable range
· Centimeters per day (slow ones)
· 10's of meters per day (fast ones)
· Valley glaciers are generally faster than continental
ice sheets
B.
Glacial Mass Budget - Accumulation and Wastage
1. Change in a glacier's size and shape is controlled by
the dynamic balance between the accumulation and
removal (wastage) of its snow and ice mass
2. Zone of Accumulation - Upper region of a glacier where
yearly snow accumulation (increase) occurs
3. Zone of Wastage - Lower region of a glacier where yearly
snow and ice wastage (decrease) occurs
· Melting and Sublimation
4. The annual snow line (firn limit) marks the boundary
between the two zones
5.Glacial growth -- accumulation > wastage
6. Glacial retreat -- wastage > accumulation
7. Stable glacier-- accumulation = wastage
VI. Erosional Processes and Features of
Glaciers
A. Erosional Processes
1. Glaciers act like a gigantic grinding, scraping, milling,
Polishing, gouging, tearing, yanking, pushing machine
·
Abrasion
·
Plucking
· Bulldozing
2. Glaciers use their weight + momentum + abrasives to do
their erosional work on the underlying earth surface
3. Glacial erosion action generates huge amounts sediment
called glacial drift, having with a wide range of sizes
4. Mass wasting also occurs on the mountainous flanks of
a glacier, which loads material on to the glacier's surface
5. Most of a glacier's eroded sediment is carried at its base
and along its margins
B. Erosional Features
1.
Valley glacier erosion produce very distinctive landscape
features (below) - most notably in mountainous regions
· U-shaped valleys
· Hanging valleys
· Fiords
· Arêtes
· Horns
· Cirques
· Striated and polished bedrock
2. Continental glacier erosion also produces distinctive
landscape features
· Flat to rolling hill landscapes
· Extensive striated bedrock surfaces with little/no soil
· Deranged drainage patterns
· Numerous kettle lakes
VII. Depositional Processes and Features
of Glaciers
A. Depositional Processes
1. Glaciers transport large quantities of eroded sediment
material, called drift, using several methods
· Dragged
· Carried
· Pushed
2. Glacial drift is concentrated into sheet- and ribbon-shaped
structures in and around a glacier, called moraines
3. There are several types of glacial moraines - each named
for their location in respect to the glacier
· Ground moraine
· Lateral moraine
· Medial moraine
· End moraine
4. Glaciers deposit their sediment load during the wastage
process which occurs primarily at it's leading end of
the glacier, called the terminus or snout.
· Wastage process = melting ice
5. Deposition of glacial sediment is done in two ways
· Directly from melted glacial ice
ü Drops to the ground as an unsorted, mixed-up
mixture called till
· Indirectly from running glacial streams originating
from the glacier
ü Streams carry sediment for some distance and
then deposits it as sorted and layered alluvium
called stratified
drift
6. Stratified drift is a depositional product of braided
stream channel activity
·
Mainly layers of poorly sorted sand and gravels
B. Depositional Features
1. Glaciers produce very distinctive depositional features
2. Glacial depositional features derive their uniqueness
from several glacial processes
· Bulldozing effect
· Melting in place (till drop) effect
· Intensive braided stream action
3. There are several types of glacial deposit features
· Ground moraines
· End moraines
ü Terminal moraine
ü Recessional moraine
· Drumlins
· Kames and Eskers
· Kettle lakes
· Outwash plains
· Glacial erratics and Dropstones
4. Ancient glaciation events can be recognized in Earth's
rock record by its unique erosion and deposition clues
· Glacial till deposits
· Extensive polished striations on bedrock surfaces
· Gondwanaland glaciation record
VIII. Global Climate Cycles and the Origin
of Ice Ages
A. Earth Goes Through Cycles of Warming and Cooling
1. Back and forth shifts between the "Greenhouse Effect"
and the "Icehouse Effect"
2. Several terrestrial and extraterrestrial phenomena appear
to form a complex relationship that controls cyclic short-
and long -term changes in the Earth's climate
§ Plate tectonics
Ø Volcanism
Ø Orogenies
Ø Plate configurations
§ Ocean circulation and temperature patterns
§ Atmospheric aerosols and circulation patterns
§ Biosphere activity
§ Earth's rotation and orbit patterns
§ Solar energy flux
§ Comet and meteor impacts
B. Initiator of the Major Ice Ages - The
Milankovitch Theory
1. The Milankovitch Theory proposes that irregularities in
the Earth's orbit and rotation axis are sufficient to alter
the amount of solar energy that the Earth receives
· Change in orbital eccentricity - 100,000 year cycle
· Change in axial tilt - 40,000 year cycle
· Precession of the equinoxes - 22,000 year cycle
· Season changes versus aphelion and perihelion
2. Other theories that may explain glacial cycles
· Spikes in global volcanism
· Solar anomalies
· Meteor impact
IX. The Pleistocene Ice Age
A. Most Recent Global Ice Age Event
1. Very first signs of global refrigeration showed up nearly
40 million years ago in the Southern Hemisphere
· Cold ocean waters moving toward the equator
· Permanent ice sheet in Antarctica; est. by 15 MYA
2. Full-blown global ice age began about 1.6 MYA
· Termed the Pleistocene Ice Age
· Officially ended 10,000 years ago
3. The Pleistocene Ice Age includes several major ice sheet
advances and retreats
· Cold-warm cycle mechanism
· Major ice sheet retreat termed interglacial periods
· Last major advance climaxed around 18,000 years ago
and ended 10,000 years ago
B. Nature of the Pleistocene Ice Age
1. Maximum extent of ice sheets in North America reached
down into the U.S. 48 states
2. Pleistocene climate belts were somewhat like today's
but were shifted toward the equator
§ Northern U.S. and Europe were very cold and dry
§ Southern U.S. and Northern Africa were very wet
3. Abundance of large lakes to the south of the ice sheets
· Pluvial lakes
ü Far from the glacial front
ü Example is Lake Bonneville
· Proglacial lakes
ü Near margin of the great ice sheets
ü Some form behind giant ice dams
ü Example is Lake Missoula
4. High precipitation and cool climate facilitated lake growth
X. Glacier Vocabulary - Ch 17
Abrasion
Arête
Basal slip
Cirque
Continental glacier
Drumlin
End moraine
Esker
Fiord
Firn
Glacial budget
Glacial drift
Glacial erratic
Glacial ice
Glacial polish
Glacial striation
Glacier
Ground moraine
Hanging valley
Horn
Ice age
Icehouse effect
Lateral moraine
Medial moraine
Milankovitch theory
Outwash plain
Plastic flow
Stratified drift
Terminal moraine
Till
U-shaped valleys
Valley glacier
Zone of accumulation
Zone of wastage