12 glaciers and glaciation

8/6/2019 12 Glaciers and Glaciation

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CHAPTER 18

GLACIERS AND GLACIATION

A.Glaciers:

- Aglacieris a thick ice mass that originates on land from the

accumulation, compaction, and recrystallization of snow.

- Where does snow come from?

It is part of the hydrologic cycle. Water evaporates into the

atmosphere, precipitates upon the land, and flows in rivers or

underground back to the sea.

Part of precipitation occurs at high elevations, and becomes

part of a glacier. Such water can be stored as glacial ice for

tens, hundreds, or thousands of years before melting and back

to sea.

Glaciers flow like running water and groundwater. They also

erode, transport and deposit sediments.


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B.Types of Glaciers:

i. Valley (Alpine) Glaciers:

Consists of glaciers in mountainous areas where it flows slowly

(few cm/d) through valleys that were occupied by streams. They

are basically streams of ice

ii. Ice Sheet (Continental Glacier):

Sheets of ice covering wide flat areas. The only two places that

exist nowadays are Greenland in the north, and Antarctica in the

South Pole.

These masses flow out in all directions and obscure the

topography of lands they flow through.

C.Glacier Budget:Glaciers originate from snow. Therefore, glaciers form in areas

where more snow falls in winter than melts during the summer.

Glaciers do gain and lose ice.


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Snow accumulates and ice formation occurs in thezone of

accumulation. It is where snow addition thickens the glacier and

initiates movement.

Its elevation varies from one region to another. In polar regions,

its at sea level. In tropical areas, it is only high in mountain

areas.

Where a glacier loses part of its snow/ice (melting) is thezone of

wastage.

Glaciers waste through calving. When a glacier reaches the

sea/lake, large pieces of ice break off and form icebergs.

Snowline presents the outer limits of zone of accumulation, and

separates this zone from zone of wastage. It is below

accumulation and above wastage.

The glacier is advancing if it gains, and retreating if it loses. It is

stationary if accumulation equals wastage (ablation)


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A glacier budget is the balance, or lack of balance, between

accumulation at the upper end of the glacier, and loss at the

lower end.

Whether the margin of a glacier is advancing, retreating, or

stationary, the ice within the glacier continues to flow forward.

D.Glacier Erosion:Glaciers are capable of great erosion. They can transport up to

boulder-sized particles. Glaciers erode the land in two ways:

i. Plucking:

Occurs when melt-water penetrates the cracks and joints of

bedrock beneath a glacier and freezes, expands, and breaks the

rock loose.

ii. Abrasion:

Occurs as the ice and its load slide over bedrock, they function

like sandpaper to smooth and polish the surface below. Theyform what is known as rock flour


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E. Erosional Landforms:Alpine glaciers move downvalley, so they generally accentuate

the irregularities in the topography by creating steeper canyon

walls and bold peaks.

Ice sheet override the terrain and subdue rather than accentuate.

i. U-shaped valleys:

Running water forms narrow V-shaped valleys. During

glaciation narrow valleys undergo a transformation as the glacier

widen and deepens them resulting in a U-shaped glacial trough.

ii. Hanging valleys:

During glaciation, glaciers cut their valleys deeper than their

smaller tributary glaciers. When glaciers retreat, the valleys of

tributary glaciers are left standing above the main glacial trough,

and are called hanging valleys.

iii. Cirque:

It is a bowl-shaped depression with steep walls on three sides but

is open on the downvalley side. It is where snow accumulatesand form ice.


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iv. Tarn:

When a glacier in a cirque melts, a small lake called tarn

occupies the cirque basin.

v. Arte and horns:

AnArte is a sinuous, sharp-edged ridge.

AHorn is a sharp, pyramid-like peak.

Both are formed due to enlargement of cirques through plucking

and frost action.

The divide between two cirques is reduced to a narrow knife-like

partition (arte).

F.Glacial Deposits:Glaciers do move slowly, and while moving they transport their

load of debris as they advance across the land.

This load will eventually be deposited when the ice reaches itterminus or when it melts.


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Glacial deposits are termedglacial drift. This term applies to all

sediments of glacial origin no matter how, where, or in what

shape they were deposited.

G. Depositional Landforms:

Till:

Unsorted material/sediment deposited directly by a glacier.

It is deposited as glacial ice melts and drops its load of unsorted

rock fragments of different particles (because ice can not sort its

sediments).

The particles are scratched and polished due to the fact that they

were dragged along by the glacier.

Moraine:

The most common term for landforms made of glacial deposits

is moraine.

It refers to a number of landforms, all of which are composed

primarily of till.


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End moraine:

is a ridge of till that forms at the terminus of a glacier.

Lateral moraine:

is a product of alpine glaciers (occurs in mountain valleys

exclusively).

It is formed due to ice erosion of the sides of the valley with

great efficiency.

When ice melts, it drops its load next to the valley walls, and

termed lateral moraine.

Medial moraine:

It is another unique feature of alpine glacier. It occurs when two

alpine glaciers converge (coalesce) and form a single ice stream,

or when a tributary joins the main stream.

The till that was carried along the sides of each glacier joins to

form a single load of debris. It is used as an indication of a

glaciers movement downvalley.


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Drumlins:

It is a depositional feature associated with ice sheet.

Smooth, elongate, parallel hills are termed drumlins. These are

streamlined asymmetrical hills made of till. They range in height

from 15-50m and up to 1 km long.

The steep sides indicate the direction from which the ice sheet

glacier advanced. The gentler, longer slope indicates the

direction the ice moved.

It is of unknown origin, however, it is thought that it originates

when glaciers advance over previously deposited drift and

reshape the material.

Kettles:

They are pits or depressions occur in deposits of till.

They form when blocks of stagnant ice become buried in drift

and eventually melts leaving pits in the glacial sediment.

They may be 2 km in diameter, and less than 10 m in depth.


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Kame:

These are glacial deposits in a form of steep-sided hills.

Some kames represent bodies of sediment deposited by

meltwater in openings within or depressions on tope of the ice.

Some originate as deltas or fans built outward from the ice by

meltwater streams.

Esker:

They are long, narrow, sinuous ridge composed largely of sand

and gravel. Some may be over 100 km long, and more than 100

m high.

Eskers are deposited by meltwater rivers flowing within, on top

of, and beneath a mass of stagnant glacial ice.

Tillite:

a sedimentary rock formed when glacial till becomes lithified.


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Glaciation (Ice Age):

The last major glaciation occurred during the Pleistocene epoch,

18,000 years ago. There were stages of glaciation and

deglaciation throughout the geologic time.

Glaciation means sea level low, exposure and erosion. Soil

formation is one aspect of glaciation (pedogenesis and

karstification). During glaciation, sea level may in negative

values compared with the current sea level.

Deglaciation (Interglacial) means ice melting, sea level high, and

deposition. Sea level may reach higher levels than the current

sea level.

12 glaciers and glaciation

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