Glaciers and Glaciation
In next two lectures:
• Glacier formation, movement
• Glacial erosion
• Glacial deposition
• Effects of glaciers
• Climate: past and future
Perito Moreno Glacier
Patagonian Andes
Advancing into Lago Argentino
January 2004
What is a Glacier ?
A large, long-lasting body of ice
„ Formed on land
„ Recrystallization of snow
– under gravity
„ Evidence of movement
„ Glaciated terrains
– Mountains - Alpine glaciation
– Continent scale - Continental glaciation
„
Glaciers-where they are ?
„
Develop where annual snow doesn’t melt away in
warm seasons
–
–
–
–
–
–
Polar regions
Heavy winter snowfall (Alaska, Southern Andes)
High elevations
85% are in Antarctica
10% are in Greenland
5% mountain belts
»
Andes, Himalaya, Alps, North American Cordillera
Types of Glaciers
Valley glacier
„ Ice sheet
„ Ice cap
„
Types of Glaciers
„
Valley glacier (Glacier Bay, Alaska)
Types of Glaciers
„
Ice sheet (polar regions)
Antarctica
Types of Glaciers
„
Ice cap (mountains)
Patagonian ice cap,
southern Andes
Theory of Glacial Ages
„
1830’s, Louis Agassiz
– Adopted “outrageous”
hypothesis:
» colder climates, larger
glaciers persisted in
Europe during the
recent past
„
Northern Europe & North
America heavily glaciated
– Peak of glaciation
18,000 years ago
– Thus, much of the
evidence is preserved
Erratic boulders of granite resting on
glacially scoured and polished sediments
East of Patagonian Andes
January 2004
Formation and Growth of Glaciers
„
„
„
Snow to firn to glacier ice
Wastage (or ablation)
– Melting, evaporation,
– Calving into icebergs
Glacial Budgets
– Negative budgetbudget- Receding
glacier
– Positive budgetbudget- Advancing
glacier
– Zone of accumulation vs. Zone
of wastage
»
–
Snow line or Equilibrium linelinedivides the zones
TerminusTerminus- movement reflects
budget
Formation and Growth of Glaciers
„
Recession or wastage of glaciers
– Melting
»
–
–
more melting at lower elevations
Evaporation
Calving into Icebergs
»
Snow
and
firn
where a glacier flows
into sea or lake
Zone of
accumulation
Equilibrium
line
Zone of ablation
Glacier ice
Glacier ice ablated during melting season
Perito Moreno Glacier
Patagonian Andes
Iceberg calving into Lago Argentino
January 2004
Formation and Growth of Glaciers
„
Advancing vs. Receding Glaciers
– Zone of accumulation
»
Where some snow remains after the melt season
– Zone of Wastage
»
Where all snow & some glacier melts
Snow
and
firn
– Advancing glacierglacier»
positive budget terminus moves forward
– Receding glacier
»
negative budget terminus retreats
Zone of
accumulation
Equilibrium
line
Zone of ablation
Glacier ice
Glacier ice ablated during melting season
Upsala Glacier, Patagonian Ice Cap
Largest S. Hemisphere glacier outside of Antarctica
Rate of recession accelerating, 10 km/75 yr
1928
2004
Yet 50 km to the south in 2004:
Perito Moreno Glacier is advancing into the same lake!
Movement of Glaciers
„
Valley Glaciers
– Gravity driving force
– Sliding along its base -basal sliding
– Internal flowageflowage- plastic flow
– Rigid zone
»
„
Crevasses may form here
Ice sheets
– Move downward & outward from central high
Movement of Glaciers
Ice
Bedrock
Rocks frozen
into glacier
Markers placed
Initial position
on glacier surface of pipe
at start of study
Ice
Bedrock
Rocks frozen
into glacier
Markers placed
Initial position
on glacier surface of pipe
at start of study
Markers on
Pipe after
glacier surface a period
after a period of of time
time
Ice
Bedrock
Amount of
Rocks frozen
Movement
due to sliding into glacier
Adjoining pairs of grains
of ice at different levels
in the glacier at the start
of the study period
+
– 40
meters
Rigid
zone
2 grains of ice locked
and moving together
Zone of
plastic flow
Upper grain moves
slightly farther than
lower grain
Upper grain moves
considerably farther
than lower grain
Crevasse formation in upper, rigid layer of glacier
Crevasses
Columbia Ice Cap
Canadian Rockies
Franz Josef Glacier South
Island
New Zealand
Movement of Glaciers
„
Valley Glaciers
– Gravity driving force
– Sliding along its base -basal sliding
– Internal flowageflowage- plastic flow
– Rigid zone
»
„
Crevasses may form here
Ice sheets
– Move downward & outward from central high
Glacial Erosion
„
Under glacier
– Abrasion & plucking
– Bedrock polished & striated
– Rock flour washes out of glacier
– Polishing and rounding
»
StriationsStriations- scratches & grooves on rock
Above glacier
– Frost wedging takes place
– Erosion by glaciers steepens slopes
–
„
“Sheep Rocks”
Abrasion & plucking
StriationsStriations- scratches
& grooves on rock
Erosional Landscapes Associated
with Alpine Glaciation
„
Glacial valleys
– U-shaped valleys
– Hanging valleys
– Truncated spurs
»
Triangular facets
Rock -basin lakes (tarns)
– Rounded knobs- rouche moutonnees
–
Hanging valleys
Truncated spurs
Tributary valley south of Upsala Glacier
Lago Argentino, Patagonian Andes
January 2004
U-shaped valleys
Fjord: glaciers carved U-shaped valley below sea level
Erosional Landscapes Associated with
Alpine Glaciation
„
Cirque- at head of valley glacier
–
Rock steps
»
Rock basin lakes
„
Horn
„
AreteArete- sharp ridge
Glacial Deposition
„
„
„
Till
– Unsorted debris
Erratic
MoraineMoraine- body of till on glacier, or left behind by glacier
– Lateral Moraine
– Medial MoraineMoraine- where tributaries join
– End morainemoraine»
»
–
Terminal
Recessional
Drumlin
Lateral and Medial Moraines
Lateral and End Moraines, Sierra Nevada
Glacial Deposition
„
Outwash
– StreamStream-deposited sediment
»
sorted
Braided streams typical
– Esker
– Kettle
Glacial lakes
– Varves
–
„
Deposition in front of receding glacier
Extent of Laurentide Ice Sheet in Wisconsin, ca. 15,000 yrs ago
Glacial Deposition
Relief Map: SouthSouth-Central Wisconsin
Drumlins
Green Bay Lobe Moraine
Wisconsin’s Glacial Features
Kames
Drumlin
Drumlins
Glacial Polish
Effects of Past Glaciation
„
Several glacial ages recognized
– Timing indicates periodic changes from glacial to
interglacial climate in Northern Hemisphere during
last 22-3 million years
„
Direct effects in North America
– Scoured much of Canada
– Cut Great Lakes
– Deposited till & flattened Midwest
– Extensive alpine sculpting of mountains
Effects of Glaciation
„
Indirect effects
– Pluvial lakes (wetter climate during glaciation)
glaciation)
– Lowering of sea level
»
Fiord
Crustal rebound
Evidence for older glaciation
– Tillite
– Late Paleozoic glaciation
–
„
»
–
Evidence for a supercontinent
Precambrian glaciation:
glaciation: The Snowball Earth
Records of Past Climate
Three major sources of information:
„
Marine environment
– Oxygen isotopes in foraminifera document ice volume
and temperature changes (ODP cores)
– Sea level changes
„
Ice cores (GRIP, GISP, Vostok,
Vostok, Taylor Dome)
– Continuous annual record of gas contents in atmosphere
– Oxygen isotopes reveal past temperatures
„
Glacial deposits on land
– Positions, timing of moraine deposition record local
changes in climate
Astronomical
Cause of Ice
Ages
„
Gravitational pull on Earth
by other celestial bodies +
tilt + precession
„
Effects on Earth’s orbital
position w.r.t.
w.r.t. the Sun:
– exceptionally well
know for the past
several tens of millions
of years
Astronomical Cause of Ice Ages
Records of Past Climate from Ice Cores
Vostok Ice Core, Antarctica
Methane
CO2
Change in
Temperature
Cross section of Pleistocene moraine sequence
East of Lago Buenos Aires, Southern Andes
Records of Past Climate
„
Glacial deposits on land
– Positions, timing of end
moraine deposition
record local changes in
equilibrium line altitude,
altitude,
thus climate