“GLACIAL ICE”

GLACIAL ICE

Glacial ice is the largest reservoir of

freshwater on Earth, supporting one third of the

world's population. Glaciers slowly deform and

flow due to stresses induced by their weight,

creating crevasses, seracs, and other

distinguishing features. They also abrade rock and

debris from their substrate to create landforms

such as cirques and moraines. Glaciers form only

on land and are distinct from the much thinner sea

ice and lake ice that form on the surface of bodies

of water.

Glacier- A large, long-lasting mass of ice that forms on land and moves downhill under its

own weight.

The Aletsch Glacier, the largest glacier of the Alps, in Switzerland

Gorner Glacier in

SwitzerlandIce calving from

the terminus of the Perito

Moreno Glacier, in

western Patagonia,Argentina

The Quelccaya Ice Cap, is the

largest glaciated area in

the tropics, in Peru

St. Mary Lake, Glacier National Park.

The Origin of Glaciers

Glaciers form where the accumulation of snow andice exceeds ablation. The area in which a glacier forms iscalled a névé - a typically bowl-shaped geological feature(such as a depression between mountains enclosed by arêtes)- which collects and compresses through gravity the snowwhich falls into it. This snow collects and is compacted bythe weight of the snow falling above it, crushing theindividual snowflakes and squeezing the air from the snow.Once the air has been squeezed from it the snow is turnedinto extremely dense 'glacial ice'. This glacial ice will then fillthe névé until it 'overflows' through a geological weakness orvacancy, such as the gap between two mountains. When themass of snow and ice is sufficiently thick, it begins to movedue to a combination of surface slope, gravity and pressure.On steeper slopes this can occur with as little as 15 m (50 ft)of snow-ice.

•Snow line – it is the elevation above

which snow remains all year, varies

with different locations.

•Snow field – are found in low latitudes

only at high latitudes.

Types of Glaciers

Valley glaciers – also called alpine glaciers. Pillingup of snow in mountain valleys above the snow linemay produce valley glaciers.Alpine glaciers, alsoknown as mountain glaciers or cirqueglaciers, form onthe crests and slopes of mountains. An alpine glacierthat fills a valley is sometimes called a valley glacier.

- a number of valley glaciers may also blendtogether to form a huge piedmont glacier as theymove out onto a plain at the base of the mountains.

Continental glacier – is found only in Polar Regions.This type of glacier covers the entire landsurface.Glacial bodies larger than 50,000 km² arecalled ice sheets or continental glaciers.

CONTINENTAL GLACIER VALLEY GLACIER

The Baltoro Glacier in

the Karakoram,Baltistan,

Northern Pakistan. At 62 kilometres

(39 mi) in length, it is one of the

longest alpine glaciers on earth.

SAMPLE OF

CONTINENTAL &

VALLEY/ALPINE

GLACIER

MOVEMENT OF GLACIERS

The complete explanation for glacial

movement probably involves the fracturing of

ice, the flowing of ice under pressure, and

melting and refreezing, as well as other

processes not yet understood.

• Glacier speed varies from millimeters to tens of meters

per day (depends on slope).

• The base of a glacier slides over underlying rock on a

thin layer of meltwater (basal sliding).

• The thicker parts of glaciers (the centers) move faster

than thinner parts of glaciers (the edges) which

experience more frictional drag.

• The lower part of glaciers flows plastically as individual

ice grains move relative to each other (the zone of

plastic flow). Here the ice grains deform to

accommodate movement.

• The upper part of the glacier moves rigidly. Here

fractures called crevasses may develop here.

Crevasses - are often more than a hundred feetdeep, and may be concealed by a thin crust ofsnow which breaks at the slightest weight.

Tide-water glaciers – when the edges ofglaciers that reaches the shore. This is becausethe rise and fall of the tide snaps off largesections of the ice that float away as icebergs.Tidewater glaciers are glaciers that terminate inthe sea. As the ice reaches the sea, piecesbreak off, or calve, forming icebergs. Mosttidewater glaciers calve above sea level, whichoften results in a tremendous impact as theiceberg strikes the water.

SAMPLE OF CREVASSE AND TIDEWATER GLACIERS

Sightseeing boat in front of a tidewater glacier, Kenai Fjords National Park, Alaska

Crossing a crevasse on the Easton Glacier, Mount Baker, in the North Cascades,United States

Glacial Erosion

Meltwater at the base of a glacier may work its wayinto cracks and freeze. Pieces of bedrock may beworked loose and picked up by the moving glacierin a process is called plucking.

Rocks carried along at the base of the glacier maygrind and polish the bedrock (abrasion). Largerocks may produce glacial striations in the bedrockas they move over it.

DIAGRAM OF GLACIAL PLUCKING AND ABRASION

*

1. U-shaped valley – The characteristic cross section profile of a valley carved by glacialerosion.

2. Hanging valley – A small valley that terminates abruptly high above a main valley.

3. Truncated spur – The triangular facet produced where the lower end of a ridge has beeneroded by glacial ice.

4. Rock-basin lake – A lake occupying a depression caused by glacial erosion of bedrock.

5. Cirque – A steep-sided, amphitheater-like hollow carved into a mountain at the head of aglacial valley.

6. Horn – A sharp peak formed where cirques cut back into a mountain on several sides.

7. Arètes – A sharp ridge that separates adjacent glacial valleys.

A Glacier carves a U- shaped valley

Bridal Veil Falls in YosemiteNational Park flowing from ahanging valley.

Horn and arète in the Fairweather Range, Alaska.

FEATURES OF A GLACIAL LANDSCAPE

Glaciers pick up rock fragments from theunderlying bedrock and from the valley walls.

Glaciers carry the debris without tumbling orsorting. The deposited debris is called glacialtill (angular, poorly sorted, and unlayered). Whentill becomes lithified, it is called tillite.

Large boulders transported large distances byglaciers are called erratics.

Moraine- An elongate deposit of glacial till.

There are several types:

1. Lateral moraine- Forms along the sides of a valley glacier asrocks fall from the steep cliffs of glacial valleys.

2. Medial moraine- Form where two tributary glaciers cometogether and adjacent lateral moraines join.

3. Recessional moraine- Deposits produced as glaciers recededuring melting.

4. End moraine- A type of recessional moraine that forms at theterminus of a glacier during glacial recession as debris piles uplong the front of the ice. They tend to be crescent shaped.

5. Terminal moraine- Special type of end moraine that marksthe farthest extent of the glacier.

• Drumlin- A streamlined hill of till produced by

continental glaciers (the origin is not entirely

understood). Drumlins point in the direction of ice

movement.

• Outwash Deposits- When glaciers melt, braided

streams develop on the surface of land and carry

away material known as outwash.

• Eskers- Under the ice, meltwater moves in tunnels

and carries sediments that can be deposited in

sinuous ridges of sediment called eskers (well-sorted

and cross-bedded).

Kettle- A small depression in outwash formed when a

block of glacial ice gets buried by sediment. When the

ice melts, a depression remain. These may fill with

water creating small lakes.

Varves- Seasonal deposits of silt and clay that form

alternating light and dark layers.

Each silt-clay set represents one year:

-The silt is deposited during the warmer part of the

year when more melting and sediment transport

occurs.

-The clay is deposited during the colder part of the

year when the lake is frozen and the clay can settle

out of suspension.

Varves can be used to determine how long a glacial

lake lasted.

The Glacial Periods

A glacial period (or alternatively glacial or glaciation) is

an interval of time (thousands of years) within an ice

age that is marked by colder temperatures

and glacier advances. Interglacials, on the other hand, are

periods of warmer climate between glacial periods. The last

glacial period ended about 15,000 years ago;

The Holocene epoch is the current interglacial.

The first glacial period occurred about 800 million yearsago. Then there was a period of about 300 million years offreedom from general glacial activity.

The second glacial period came about 500 million years agoand was followed by a warm spell of about 300 millionyears.

Then the ice sheets formed again to the third time about200 million years ago.

The latest ice invasion took place only about 1 million yearsago. Our very distant ancestors were no doubt chilled andforced to migrate by the advancing glaciers.

POSSIBLE CAUSES OF THE GLACIAL PERIODS

1. Variation in the amount of heat received from

the sun.

2. Topographical factors.

3. Melting of the Arctic ice.

A glacier moves as a solid rather than as a liquid, as isindicated by the formation of crevasses (see crevasse). Thecenter of a glacier moves more rapidly than the sides and thesurface more rapidly than the bottom, because the sides andbottom are held back by friction. The rate of flow dependslargely on the volume of ice in movement, the slope of theground over which it is moving, the slope of the upper surfaceof the ice, the amount of water the ice contains, the amount ofdebris it carries, the temperature, and the friction itencounters. Glaciers are always in movement, but the extentof the apparent movement depends on the rate of advanceand the rate of melting. If the ice melts at its edge faster thanit moves forward, the edge of the glacier retreats; if it movesmore rapidly than it melts, the edge advances; it is stationaryonly if the rate of movement and the rate of melting are thesame.

THAT’S ALL…

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