chapter 18 - glacial systems

8/8/2019 Chapter 18 - Glacial Systems

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The Physical Environment: An Introduction to Physical Geography

CHAPTER 18: Glacial Systems

Cirque in the Front Range Rocky Mountains, Colorado

M. Ritter

Some of the most magnificent landscapes on Earth are created by the action of glaciers.

Throughout much of our geologic history, great sheets of ice have waxed and waned acrossthe Earth's surface. Glaciers in high mountains have created a craggy landscape of sharp

ridges, amphitheater-like depressions, and hanging valleys occupied by spectacular

waterfalls. Over the flatter plains of the Earth, ice sheets over a mile thick advanced, plowing

over and burying the surface in a great thickness of glacial sediment. Once retreated, the

glaciers left sinuous ridges, streamlined hills, and a pocked marked surface of depressions

and lakes.


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Glacial Systems Outline

y Glaciationo Causes of glaciationo Anatomy of a glaciero Glacier movement

y Geologic work of glacierso Glacial erosiono Glacier transport and

deposition

y Landforms of ContinentalGlaciation

y Landforms ofAlpine Glaciationy Review


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Glaciation

Aglacieris a natural accumulation of land ice showing movement at some time. Many times

during Earth's history, great ice sheets waxed and waned over the surface. What caused these

periods of glaciation is still not fully understood and no single reason will probably be found.

Causes of glaciation

The onset of a period orstage of glaciation is due to a change in Earth temperature and

circulation. It is generally accepted that a global decrease of 4o

to 5o

C, especially during the

summer, and a substantial increase in the amount of snowfall in subarctic and arctic regions is

necessary for the onset of a glacial episode. Several theories have been proposed for such a

change in climate -- reductions in solar radiation due to meteorite collisions with the Earth,

increased volcanism, the shifting location of continents, and the uplift of vast mountain

regions. Milutin Milankovitch proposed one of the most significant theories to account

for climate change by variations in Earth orbit. Changes in the eccentricity of earth orbit, the

degree of deviation of the orbit from a perfect circular path, is thought to cause the necessary

change in insolation to decrease global temperatures. Recall that the Earth's orbit is elliptical,but over periods of 100,000 years the shape varies. The changes in orbit have been correlatedwith ocean sediments that record the history of glacial stages. The cyclical nature of warming

and cooling correspond well with the estimated dates of glacial and interglacial periods. Inaddition to the change in orbit, the Earth "wobbles" on its axis which alters the amount of

insolation reaching the surface of the Earth. [For more about the causes and stagesof glaciation in earth history see: "Why were there four long, generally cool periods during

which continent-sized glaciers advanced and retreated?" from the Illinois State Museum.]

Anatomy of a Glacier

Whatever the cause, the main reason glacial advances are initiated is that winter accumulation

exceeds the summer loss of snow over a long period of time. Snow metamorphoses intoglacial ice under the increasing pressure of accumulated layers of snow. It first changes to a

granular form calledfirn, and ultimately into ice. Glacial ice sometimes looks blue because it

absorbs all colors of the visible light spectrum except blue, which it transmits and hence its

blue appearance. Glacier ice may also appear white because some ice is fractured with

pockets of air that indiscriminately scatters the visible light spectrum.

Figure GS.2Regions of a

glacier.


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The mass balance of a glacier determines if it will advance across the surface or not. The

mass balance is determined by the amount of gain and loss of ice from the glacier. The mass

balance is positive when it accumulates more ice than it loses. A glacier has a negative mass

balance if it loses more ice than it gains.

Glaciers form in the zone ofaccumulation, the portion of the glacier over which

accumulation exceeds ablation.Ablation is the loss of ice (or snow) from the glacier.

Ablation includes sublimation, wind erosion, melting, and evaporation. The zone of

accumulation for the large continental ice sheets resides at high latitudes. For mountain

glaciers, the zone of accumulation is at a high altitude where temperatures are cold prevent

complete summertime melt. Thezone ofablation is where loss of ice mass is greater than

accumulation. The boundary between these two zones is the firn orequilibrium line. Ifaccumulation exceeds ablation the glacier will grow. If ablation exceeds accumulation, the

glacier will retreat by melting in place. You can approximate the location of the equilibriumline by examinig an aerial photograph. The glacier looks dirtier below the equilibrium line

as glacial sediment is exposed on top of the ice.Above the line it is more white because freshsnow usually covers the surface. Listen to a glacier refreezing (Antarctica 2000).

Glacier Movement

Figure GS.3 Crevasses slice across

the surface of a glacier (CourtesyUSGS DDS21)

Once the ice reaches a thickness of about 20 meters (66 ft) it will begin to move under the

pressure of its weight. Glaciers move across the surface by internal deformation and basal

slip. Under the weight of accumulating ice, the ice is deformed and begins to move by

pseudo-plastic flow. Glaciers slip over the surface lubricated by meltwater at their base.

Generally speaking, flow velocity in a glacier is greatest near the surface of the ice anddecreases towards the bottom. The surface moves faster than the base does due to internal

deformation and basal slipping. The actual forward movement of a particle of ice in theglacier is about 1,000 feet per year. A typical glacier will move at about 10 inches a day,

though some move quite more rapidly like Greenland's Jakobshavn glacier. [ View "Fastest


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Glacier" fromNova scienceNow] Variations in the speed of the ice caused by surfaceirregularities results in differential expansion and compression of the ice and the development

ofcrevasses. A deadly situation for hikers, crevasses can open and close with little warning.

Aglacial surge occurs as an abrupt movement that can cover tens of meters per day. The

exact cause is not well-known, but may result from water pressure building at the base which

"floats" the glacier. In 1986 the Hubbard Glacier surged across the mouth of the Russell fjord

in Alaska cutting it off from Yakut Bay. Glacier movement exceeded 112 feet per day,

compared to a normal rate of 10 inches per day!

Types of glaciers

Figure GS.4 Ice sheet on

Ellesmere Island, Canada

(Courtesy: Geological Survey of Canada)

Continental glaciers are vast ice sheets which originate in high latitudes. Here, coldtemperatures allow snow to accumulate to great depths, metamorphosing into glacial ice. In

the not so distant past, geologically speaking, the great ice sheets waxed and waned,

penetrating into the midlatitudes as great lobes of ice. The continent ofAntarctic and

Greenland are the two major expanses of ice sheets on Earth today.

Figure GS. 5 Terminus ofNisqually Glacier in 1978

Mount Rainier National Park(Courtesy: USGS DDS-21)

Alpine glaciers are those that form at high altitudes where the environment is conducive toglacier formation. Pushing outward from their zone of accumulation, alpine glaciers fill

mountain valleys and sculpt the surface beneath. Upon retreat some of the most spectacularlandscapes on Earth are revealed.Piedmont glaciers form by the merging of alpine glaciers

at the base of mountains as they issue out of their valleys.


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Geologic work of glaciers

Glacial erosion

Glaciers themselves do relatively little significant erosion because ice is so soft. Under the

weight of an ice sheet thousands of feet thick continental glaciers detach material from thesurface by crushingthe underlying bedrock. Once the material is loosened from the surface,

ice can quarry (also known as plucking) the rock by freezing around and into fractures, then

lifting it from the surface. The rock embedded in the ice gouges and smoothes bedrock

surfaces by abrasion. Striations are fine scratches left in bedrock by abrasion. At a larger

scale, lineargrooves are ground into the bedrock in the direction of ice movement. Episodic

movement leaves crescent-shaped marks called chatter marks gouged into the bedrock. The

constant abrasion of exposed rock also createspolished bedrock.

Figure GS.6Grooved bedrock, Quebec

Courtesy USGS DDS21

Figure GS.7Chatter marks

Figure GS.8Striations

Courtesy USGS DDS21

Glacier Transport and Deposition

Glacial driftis the general term applied to materials eroded from the surface and deposited

by glaciers. Glaciers transport the embedded material towards the front of the glacier as if

they were on a conveyor belt, or is deposit directly beneath the ice. Most material is

embedded in the lowest few meters of the glacier and along its sides. Little drift material is

lodged in the interior as flow through most of the glacier is laminar, except at the nose where

thrust faulting of the ice occurs. When the ice becomes so burdened by its load of soil and

rock fragments, it deposits the mixture of fine and coarse textured material in place asglacial

till. Till is distinguished by its lack of sorting.


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Figure GS.9 Glacial till on Mt.Rainier.

Courtesy of USGS DDS21

Figure GS.10 Outwash along a road

cutCourtesy USGS DDS21

At the margin of the glacier, massive amounts of rock debris are deposited as the ice

stagnates and melts in place. The meltwater flushes through the accumulated debris,

spreading drift ahead of the decaying glacier as stratified drift.

Landforms of Continental Glaciation

Examine the diagrams of a region during glaciation and thesame region after glaciation while

reading the material below.

A moraine is a glacially formed accumulation of unconsolidated debris. Moraines often take

the form of a belt of low hills composed of till. Where the leading edge of the glacier was

located a terminalorend moraine can be found. The terminal moraine marks the furthest

advance of the ice sheet. Behind the terminal moraine is found a recessional

morainedeposited when the ice sheet receded and stopped for a period of time. Often,

uplands will cause an ice sheet to separate into lobes.


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Figure GS. 12

Aerial photo of a

portion of the

Northern Unit of

Kettle Moraine,

WI. The rugged

terrain and kettle

lakes ofthe

moraine is visible

in muchofthephotograph.

Figure GS.13 Wisconsin-age morainein northern Illinois

Interlobate morainesform between lobes of the ice sheet.Ground moraine is till that was

lodged beneath the glacier and generally found behind the terminal moraine. Ground

moraine. Wetland areas are often created in ground moraine which is a convenient way of

identifying them from a topographic map.


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Figure GS.14 Outwash plain, Copper River

region, Alaska(Courtesy USGS DDS21

An outwash plain forms ahead of the terminal moraine as melt water from the snout of a

glacier deposits stratified drift. The outwash plain is a relatively flat surface that may be pock

marked with depressions called kettles. If numerous kettles are present the surface is calledapittedoutwash plain.

Figure GS.15 Sinuous form of an esker is seen in

this aerial photograph

(Courtesy Geological Survey of Canada)

Eskers are sinuous ridges of glacio-fluvial material that form in tunnels in an ice sheet .

The sides of the tunnel act as part of the channel for a melt water stream. As the glacier

recedes, the support for the stream is removed and the stream deposits its load into a long

ridge-like form. Eskers are good sources for sand and gravel, and many of them have been

destroyed by mining for these materials.

Figure GS.16 Streamline profile of a drumlin in Alberta,

Canada

(Courtesy Geological Survey of Canada)

Drumlins are stream-lined hills that appear separately or in "swarms" . Their

formation is not well known but form by the deposition of till. As the ice rides over the till it

is smoothed into an inverted spoon-shaped feature. The steep side faces the direction the ice


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sheet came from while the more gentle slope of the tail points toward the direction of iceflow.

Figure GS.17 Kame in northern

unit of Kettle Moraine State Park,

WI

Kames are steep mounds or conical hills built by the deposition of stratified drift in or around

ice. Some kames form in holes in the ice where sediment accumulates. A mound of glacial

drift is left behind once the ice melts.

Figure GS.18 Kettle lake in moraine.(Courtesy USGS

DDS21)

Kettles are pits in the surface that may or may not be occupied by water . They formwhen an isolated block of ice is surrounded by till or stratified drift. After a period of time the

ice block melts away leaving behind a hole in the surface. Kettles are often found on outwash

plains or embedded in moraines (hence the name for Kettle Moraine State Forest inWisconsin). Go here to view kames and moraine topography in the Northern Unit of Kettle

Moraine, Wisconsin (Caution - 1.13 MB QuickTime file).

Landforms of Alpine Glaciation

Examine the diagrams of a region during glaciation and the same region after glaciation while

reading the material below.

The headwaters of stream tributaries serve as the birth place for alpine glaciers. The

headwaters of tributary valleys lie at the highest elevation in the drainage basin of a mountain

stream. Here snow accumulates to great thickness and starts to move down valley. As it

moves outward from the zone of accumulation, the glacier scours away at the valley sides.The material eroded from the surface is transported the length of the glacier to the zone of

ablation, where it is deposited.


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Figure GS.19 Pawnee Cirque, Front

Range, Colorado

At the zone of accumulation where ice is accumulating, the glacier plucks rock from the head of

the valley. The erosion creates a huge, amphitheater-like depression at the valley head calleda cirque. After the glacier melts away a tarn, or cirque lake , may be found on the floor.

Surrounding the floor are the massive curved side and headwalls of the cirque. A headwall

crevasse known as a bergschrund forms where snow and compacted firn pull away from ice that

remains frozen to the rock of the headwall. As alpine glaciers erode headward, they narrow the

upland between ice sheets from opposite directions and a pyramid-shaped peak, called ahorn iscreated. When two adjacent cirques on opposite sides of a divide cut back and remove part

of it, a sharp-edge notch or pass called a col is formed.

Figure GS.20 U-shaped trough in the Alps

The massive amount of ice that fills the main valley is from that which accumulates at the head of

the valley plus that from all the tributary valleys that enter it. The enormous weight of the ice

erodes away at the valley sides. The V-shape of the pre-existing stream valley now takes on the

characteristic U-shaped of a glacial trough.


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Figure FS. 21 Hanging valley in Yosemite National Park.

Courtesy USGS

Prior to glaciation, tributary streams entered the main stream at the elevation of the main channel.

But during glaciation, tributary glaciers feeding into the main valley are smaller and do not erodetheir valleys as deep as the glacier that occupies the main valley. Tributary valleys are left hangingat a higher elevation above the main valley floor as a result of more intensive downward cutting

by the main valley glacier. These hanging valleys create spectacular waterfalls.

Glaciers in adjacent tributary valleys scour away at the upland between them (an interfluve). As

erosion of the interfluve continues, it is narrows into a serrated ridge known as an arte. As

mountain glaciers flow down valley they encounter exposed bed rock of varying resistance to

erosion. The glacier will erode down into weaker rock but have to flow over the stronger rock.

This creates a series ofrock steps composed of the more resistant rocks with small depression

behind them where the weaker rock is exposed. Often these depressions are occupied with water to

form staircaselakes. Connected by a small stream these lakes are also know as paternosterlakes.

Figure GS.22Niwot Ridge arte, Colorado Front

Range


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Figure GS.23 Staircase lakes, Green Lakes Valley,

Colorado Front Range.

Figure GS.19 Lateral and end

moraine. (Courtesy Geological

Survey of Canada)

Glacial till is deposited along the valley sides as lateral moraine. Till is transported anddeposited at the nose of the glacier as an end moraine. The end or terminal moraine marks the

furthest advance of the glacier. Behind the terminal moraine are foundrecessional

moraines indicating positions of the glacier front during times of retreat. When lateral moraines

coalesce upon entering a main glacial trough, medial moraines are formed and run the length

of the glacier.

Figure GS.24 Medial moraine (Courtesy Geological

Survey Canada)

Explore some of these features by taking a virtual field trip to Niwot Ridge, Colorado in Google

Earth or with the embedded viewer below (Google Earth plugin required).


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Review

Use the links below to review and assess

your learning. Start with the "Important

Terms and Concepts" to ensure you know the

terminology related to the topic of the chapter

and concepts discussed. Move on to the

"Review Questions" to answer criticalthinking questions about concepts and

processes discussed in the chapter. Finally,test your overall understanding by taking the

"Self-assessment quiz".

y Important Terms and Conceptsy Review Questionsy Self-assessment quiz

Additional Resources

Use these resources to further explore the world of geography

Multimedia

"Glaciers" (Annenberg/CPB)from the site: "Many of the worlds most beautiful landscapes

were made by glaciers. This program shows how, explaining glacial formation, structure,

movement, and methods of gouging and accumulating earth. The program provides images of

glaciers and glacial landforms such as moraines, and discusses how study of glaciers may help us

understand ice ages and the greenhouse effect." (30:00) Go to the Earth Revealed site and scroll to

"Glaciers". One-time, free registration may be required to view film.Activity

"What's Happening to Alaska's Glaciers? Their Dynamic Response to Changing Climate and

Other Factors" Dr. Bruce Molina (USGS) Descriptive Flyer pdf.

Visualization

Satellite Image Atlas of Glaciers of the World (USGS)

Readings

Glaciers on Mt. Rainier (USGSOpen-File Report 92-474) Brief overview of Mt. Rainier's

glaciers including, glacier flow, glaciers and climate, and history of glacier fluctuations.

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