General Statements
Evidence indicates that ice has covered much of the land in the recent past
As well as in the more distant geologic past
Glacial ice is a major force in the development of landforms
Two basic types of glaciers
Alpine (valley) glaciers
Continental glaciers
Evidence for repeated episodes in the recent (and distant) geologic past
Lake Bonneville
Great Lakes & into Canada
Spokane (Bretz) Flood & Channeled Scablands
Isostatic readjustment
Land depressed by weight of ice
Vertical readjustment of surface
Ex: Hudson Bay - up 900' since last major advance
South end still rising 6.5' per 100 years
Fluctuations in sea level
Hydro cycle changed during ice ages
Precipitation as snow - doesn't melt - Locked up on land
Sea level drops - up to 130 meters (430')
Profound effect on world geography
Continental shelves exposed
Land bridges
Distribution of Glaciers
Cover 10% of land surface (15 X 106 km3)
Most in 2 large "continental" chunks
Most of the rest as relatively small alpine glaciers
Tie up quite a bit of water
Approx. 30 X 106 km3 (2.15% of earth's water resources)
Difficult number to estimate - basal configuration uncertain
What if it all melted?
Minimum rise in sea level of 225' (I've heard much greater predictions)
This would be a problem!
Currently in retreat - "sea level up 6 in. to 12 in. since 1890's"
Formation of Glacial Ice
Glaciers are big masses of naturally occurring ice
Ice is a mineral in this case
To be a true glacier it must flow internally
Or it's just a big ice cube
Starts with snow - no snow, no ice, no glacier
Summer temperatures must remain low so some snow remains year-round
Also, enough must continue to accumulate each year to maintain the glacier
Total amount of snow needed obviously varies with locality as well as...
Temperature ranges
Local environment
Topographic slope of land
North vs. south facing slopes
Windward vs. leeward slopes - more snow on windward
Prevailing wind direction - redistribution of snow
Conversion of snow to ice
Snow is frozen water vapor - lots of open space
After falling, snow can change in several ways
Melt and run off
Sublimate - solid to vapor
Melt and re-freeze
Leads to granular snow and 'firn' (German/Swiss term)
Firn accumulates and gradually changes to glacial ice
A true metamorphic change
The result of pressure and temperature adjustments
A solid state change
Too much melting would destroy the glacier
Firn builds up in sedimentary layers
Intergranular spaced filled with air
Compression reduces pore space
Specific gravity increases (new snow 0.1, glacial ice 0.9)
Glacial Budget
Glaciers continually gain and loose mass
Advance vs. retreat depends on how the budget is doing
Where do gains and losses lake place?
Snowline - lower limit of any year's permanent snowfall
Obviously this varies quite a bit from year to year
Zone of Accumulation - above snowline
Zone of Ablation (Wastage) - below snowline
Advance or retreat of glacier depends on position of snowline
Stable glacier: Accumulation = ablation
Advancing glacier: Snowline drops - accumulation > ablation
Retreating glacier: Snowline rises - accumulation < ablation
Glacial Movement
A lot like stream flow
Slower along sides & bottom
Differs from streamflow in several important ways
Generally non-turbulent
Generally pretty slow - 10 to 1000 feet per year
Can go faster in certain situations
Glacial surges - common in stagnant or receding glaciers
De-couples from rock floor
Rapid movement - up to 20,000' per year
Causes vary
Short term increase in snow at head of glacier
Lubrication by percolating meltwater
Frozen blocks at base of ice hold glacier in place - suddenly breaks loose
The reality of the process is difficult to study because it all happens under the ice.
Capable of flowing uphill
Glaciers can also transport debris uphill
Plastic flow
Permanent deformation due to pressure
DIGRESS TO: diamond core drilling thru glacier
The primary way glaciers move
IMPORTANT NOTE: Not always downslope!
Creep away from centers of accumulation
In the case of alpine glaciers this is generally downslope, but continental ice sheets do not necessarily have a major elevation difference between the Zones of Accumulation and Ablation
Basal slip
Actual movement at ice/rock interface
This slip results in erosion due to material frozen into the base and sides of the ice
Also "glacial quarrying" where glacial ice frozen into cracks in the bedrock literally "plucks" rock material from the valley floor as the glacier moves
Upper portion of ice is different
Brittle - breaks instead of flow
Cracks & crevasses - can be quite deep (down to approximately 100')
Down to where the pressure results in plastic flow
Where have we seen this concept before (Brittle-Ductile Transition Zone)
Remember: this is a true metamorphic environment!
Alpine Glaciers
Mountain slopes & summits above snowline
Advance downslope due to gravity
Probably follow pre-existing stream courses
Carve and accentuate topography (not like ice sheets)
Glacial erosion
2 main methods
Glacial quarrying (or plucking)
Chunks of rock pried out of sides and bottom
Poorly understood - tough to observe in real-time
Meltwater seeps into cracks/fractures
Freezes to ice sheet
Pries out chunks as glacier moves
Lots of evidence for this in glaciated areas
This material becomes part of the glacier and leads to...
Glacial abrasion - easiest to understand
Like sandpaper - bedrock is ground down by rocks of all sizes frozen into the ice
Striations and glacial polishing common
Erosional landforms
Can be quite spectacular!
Deepen & straighten stream channels
V-shaped vs. U-shaped valleys
Cirque - head of glacier
Probably the result of glacial plucking
Rotational slumping of ice mass
Tarn - lake in cirque
Arête
Col - pass between the heads of 2 glaciers
Headward erosion & plucking tear down the arête
Horn
Hanging valleys
Truncated spurs - like recent fault scars
Fiord
Glacial deposition - Alpine glaciers
Glaciers are like big conveyor belts
Transport material away (usually down) from centers of accumulation
Moraines - rock/debris deposited along margins
Several main categories
Lateral moraine
Debris carried at the ice/rock interface
Medial moraine
Merged lateral moraines
Terminal moraine (End moraine)
At the terminus - easy to identify
Somewhat tougher to define for a continental sheet
Can be large if glacier stabilizes for a long time
Obviously destroyed if glacier advances beyond the end moraine
If glacier retreats, can lead to
Recessional moraine - intermediate terminal moraines
Can be large if glacier stabilizes during a general retreat
Many can develop in a valley during a long retreat
Paternoster lakes common
Ground moraine
Debris dumped during rapid retreat
Also called till, or drift
Generally very poorly sorted
With (initially) very irregular topography
Rock glaciers
Basically an ice-poor glacier
Surface debris tends to act like insulation
Allows glacier to extend well below the snowline
Generally move very slow - 3 feet/year
IDEA: do most glaciers turn into rock glaciers as they move into the zone of ablation?
Continental Ice Sheets
Broad regional sheets of ice
Can be truly immense during maximum glacial advance
Minimum of 4 major advances during Pleistocene
Two (?) major zones of accumulation in North America
Canadian Shield & Cordilleran
Would merge in times of maximum advance
Controlled climate & migration routes
Compressed climatic belts between the southern end of the glaciers and the equator
We are now in an "inter-glacial" period
2 major sheets left
Greenland
Central zone of accumulation ringed with zone of wastage
Greater than 2 miles thick in the center
Almost certainly has eroded to below sea level
Try and get a stream to do this!
Antarctica
Similar in concept to the Greenland sheet, but MUCH larger
Again, the base extends below sea level
DIGRESS TO: this must not be uncommon - look at Hudson Bay
And it's rebounded 900 feet!
Different from Greenland in that it's grounded on the continental shelf
Can only advance if sea level drops
The glacial budget is all fouled up
Very arid climate - <2" precipitation/year
Therefore, very little additional ice per year
Little or no melting - way too cold here
Loose mass by "calving"
Landforms Associated with Continental Glaciers
Quite a bit different from alpine landforms
Much more complex, much larger scale, much more subtle
Tend to even out the landscape, not accentuate it
Basically scour the landscape smooth
Softer areas can still be more easily eroded - lakes
EXAMPLE: Devonian sediments in Michigan basin
Incredibly efficient erosional machines
Just how much does one of these ice sheets weigh?
1 cubic foot of water weighs 62.4 lbs./ft3
Ice (@ .9 specific gravity) 56.16 lbs./ft3
1 square mile (5280' X 5280') 2.8 X 107 ft2
(2.8 X 107 ft2) X 56.16 lbs./ft2 = 1.57 X 109 lbs./mi2
Canadian Shield sheet (4000 mi2 X 4000 mi2) 1.6 X 107 mi2
(1.57 X 109 lbs./mi2) X (1.6 X 107 mi2) 2.5 X 1016 lbs./vertical foot
Assume 10,000' thick (2.5 X 1016 lbs.) X (1 X 104) 2.5 X 1020 lbs.
or 200,000,000,000,000,000,000 pounds!
(Assume ice sheet was 10,000' thick 561,600 lbs./ft2)
or 281 tons of ice per square foot (3900 lbs./in2)
And this is assuming that the entire sheet is composed of ice. If we assume that 30% of it is rock material, at an average specific gravity of 3.0, we can essentially double the total weight of the sheet.
Glacial till
LARGE expanses of till associated with continental ice sheets
Both along margins and as
Ground moraine - till deposited beneath the ice
Or dropped during retreat
Composition & texture very erratic
Terminal (End) Moraines
Reflect times of equilibrium at maximum advance
Generally poorly sorted, but not as poor as alpine glaciers
Why is this?
(Less energy in system, less gradient, lots like stream flow!)
Lakes are abundant!
Differential scouring - see above
Irregular topography leads to lots of low spots which can fill up with water
Kettle lakes - holes left in ground moraine by melted blocks of ice
Moraine dams
Drumlins
Form beneath moving ice
Low, rounded elongate hills
Long axis parallel to direction of ice flow
Usually contain a large percentage of clay
Occur in sub-parallel groups
Eskers
My personal favorite
Long narrow ridges of stratified sediment
Up to 100' high and hundreds of miles long
Represent streams which flowed beneath the ice!
Can meander and have tributaries
Imagine the environment along one of these streams!
What sorts of plants/animals existed here?
Miscellaneous features associated with continental & alpine
Outwash plains and Valley trains
Massive amounts of debris downslope from terminus
Moved around by meltwater
Rock flour
Finely ground rock material
Commonly transported away from the glacier by the generally strong winds found along the margins
Called "loess deposits"
Generally thickest near glacier - thin out with distance