Revised 8/ 06 (Monroe 6th ed.)

Glaciers: Ice with an attitude!




Development of the Glacial Theory

Distribution of Glaciers

Formation of Glacial Ice

Glacial Budget

Glacial Movement

Alpine Glaciers

Continental Ice Sheets

Miscellaneous features




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 (Monroe; fig 17-2, pg. 534)

Continental glaciers (Monroe; fig. 17-3, pg. 535)

Evidence for repeated episodes in geologic past

Lake Bonneville (Monroe; fig. 17-22, pg. 555)

Great Lakes & into Canada (Monroe; fig. 17-23, pg. 556)

Spokane Flood & Channeled Scablands (Monroe; Fig. 17-24, pg. 557)

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

Hydrologic cycle changed during ice ages

Precipitation as snow - doesn't melt - Locked up on land

Sea level drops - book says up to 130 meters (430')

Profound effect on world geography

Continental shelves exposed

Land bridges

DIGRESS TO: Noah's flood and Carribean cultures


Development of the Glacial Theory

Much early work in geology by English (Hutton, Playfair, etc.)

Not many glaciers in England

So they were unable to explain some of the features they observed

Drift & Erratics (Monroe; fig. 17-15, pg. 548)

Attributed to Noah's flood

Louis Agassiz (early 1800's)

Swiss naturalist - did much of the early work

Exported his theories to U.S in 1837 - "met with resistance"


Distribution of Glaciers

Cover 10% of land surface (15 X 106 km2)

Most in 2 large "continental" chunks

Most of the rest as relatively small alpine glaciers

Ties 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 temps 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, and several other factors

Temperature ranges (seasonal and diurnal)

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 (Monroe; fig. 17-4, pg. 536)

Snow is frozen water vapor - lots of open space

After falling, snow can change in several ways

Sublimate - solid to vapor

Melt and run off

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 lose mass

Advance vs. retreat depends on how the budget is doing

Where do gains and losses lake place?

Snowline - a.k.a. Firn Limit

Lower limit of any year's permanent snowfall (Monroe; fig. 17-7, pg. 539)

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 (Monroe; fig. 17-8, pg. 540)

But several fundamental differences

Generally non-turbulent (see medial moraines: Monroe; fig. 17-17b, pg. 550)

No mixing like streams

Generally pretty slow - 10 to 1000 feet/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 toe hold glacier in place - suddenly breaks loose

The reality of the process difficult to study - under the ice!

Basal slip

Actual detachment at ice/rock interface

Results in weathering and erosion of the bedrock at the ice/rock interface

Striations, glacial polish

Plastic flow (Monroe; fig. 17-5, pg. 536)

Permanent deformation due to pressure

DIGRESS TO: diamond core drilling thru glacier

The primary way glaciers move

IMPORTANT NOTE: Not always downslope!

Flow away from centers of accumulation

Glaciers can actually "flow" uphill!

Generally occurs deep within the glacier

Upper portion of ice is different

Brittle - breaks instead of flow (Monroe; fig. 17-6, pg. 537)

Cracks & crevasses - can be quite deep (down to 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

Your basic glacier

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 abrasion - easiest to understand

Like sandpaper - rock is ground down

By rocks of all sizes frozen into the base and sides of the ice

Striations and glacial polishing common (Monroe; fig. 17-11, pg. 543)

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

Erosional landforms (Monroe; fig. 17-13, pg. 546; and pgs. 544-545 for examples)

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


Col - pass between the heads of 2 glaciers

Headward erosion & plucking tear down the arête


Hanging valleys

Truncated spurs - like recent fault scars


Glacial deposition - Alpine

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 (Monroe; fig. 17-17, pg. 550)

Debris carried at the ice/rock interface

Medial moraine (Monroe; fig. 17-17, pg. 550)

Merged lateral moraines

Terminal moraine (End moraine) (Monroe; fig. 17-16, pg. 549)

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

Much less now than in the "recent" geologic past

Minimum of 4 major advances during Pleistocene (Monroe; fig. 17-21, pg. 554)

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 (Monroe; fig. 17-3, pg. 535)


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!


Similar in concept to the Greenland sheet, but MUCH larger

Again, the base extends below sea level

Must not be uncommon - look at Hudson Bay

And it's rebounded 900 feet!

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

DIGRESS TO: cold/high pressure air masses

Little or no melting - way too cold here

Lose mass by "calving"

Landforms Associated with Continental Glaciers (Monroe; fig. 17-18, pg. 551)

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

Incredibly efficient erosional machines

Basically scour the landscape smooth

Softer areas can still be more easily eroded - lakes

EXAMPLE: Devonian sediments in Michigan basin

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 250,000,000,000,000,000,000 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?

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


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 (Monroe; fig. 17-19b,c, pg. 552)

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

Miscellaneous features associated with continental & alpine

Outwash plains and Valley trains

Massive amounts of debris downslope from terminus

Moved around by meltwater

Braided streams common (Monroe; fig. 15-10, pg. 467)

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



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