glaciers ch 13. introduction – glaciers are masses of ice which move over land by plastic flow and...

Glaciers

Ch 13

Introduction– Glaciers are masses of ice

which move over land by plastic flow and basal slip.

– Glaciers presently contain 2.15% of all water on Earth and cover about 10% of the land surface.

Fig. 14.2, p. 359

Types of Glaciers

• Valley Glaciers • Ice Sheets• Ice Caps

By definition glaciers are moving bodies of ice on land, that flows downslope or outward from an area of accumulation.

Note: Sea ice and icebergs is nothing more than frozen sea water and are not glaciers because they do not form on land.

Valley Glaciers

Valley glaciers are• Long, narrow tongues of ice• Typically much smaller than

continental glaciers• Flow from higher to lower

elevations• Confined within mountain

valleys.• Create spectacular scenery!

Ice SheetsAlso called Continental GlaciersContinental glaciers flow outward in alldirections from a zone of accumulationAlso called Ice Sheets

• Huge - cover vast areas.• Often develop large ice

shelves wherethey flow outward into the sea.

Ice Caps

• Similar to continental glaciers but much smaller.

• Some develop fromvalley glaciers whenthey grow over thetop of a divide.

How Do Glaciers Originate and Move?

Glaciers form when winter snowfall exceeds summermelt and snow accumulates yearly.Ice is a crystalline solid. Fresh snowflakes are about

As the snow accumulates it thaws and refreezes, becoming a granular type of ice called Tim.

When firn is buried and and recrystallized, it is metamorphosed to glacial ice and will flow under itsown weight.

Fig. 14-4a, p. 361

Stepped Art

Glacial ice

Granularsnow Firn

Snowflakes

How Do Glaciers Originate and Move?

Glaciers move thru Basal Slip and Plastic FlowIf a slope is present glaciers may slide over their underlying surface, a phenomenon called basal slip

Most of their movement is accomplished by plastic flow, a type of deformationthat takes place in responseto stress.

In a glacier the pressure comes from the weight of the ice pilled above; it forces the ice crystals to slip past one another

The Mass Balance of GlacierGlacial budget - A glacier's behavior depends on thebalance between accumulation and wastage (melting).• The upper part of the glacier, where the snow cover is year-

round is the zone of accumulation.• The lower part, where losses exceed gains is the zone of

wastage.• The line separating the two is the firn limit. It shifts each year.

• Glacial budget - A glacier's behavior depends on the balance between accumulation and wastage (melting).

– Glaciers having a balanced budget have a stationary terminus. The firn limit changes very little from year to year.

– Positive and negative budgets result in advance and retreat of the terminus, respectively.

Glacial Budget

The Glacial BudgetAccumulation and Wastage

– A valley glacier with a balanced budget will deposit a terminal moraine at its base.

– If it has a negative budget a recessional moraine may develop.

Erosion and Transport by GlaciersGlaciers…

– Push or bulldoze loose materials in their paths– Erode by abrasion - that is, the movement of

sediment-laden ice over rock surfaces– Erode by plucking when ice freezes in or

around bedrock projections and pulls them loose.

Erosion and Transport by GlaciersGlaciers…

– Polish rocks as they grind them into a fine powder called rock flour.

– Abrasion also results in glacial striations – scratches made by rocks scraping against one another as the glacier moves

Erosion and Transport by Glaciers• Erosion by Valley Glaciers• Valley glaciers carve angular peaks and deep valleys

– U-Shaped Glacial Troughs

– When mountain valleys are eroded by glaciers they are deepened and widened so that they have flat or gently rounded (U-shaped) valley floors and near-vertical valley walls.

Fig. 14.12 b, p. 369

V-shape vs U-Shape

• Erosion by Valley Glaciers– Cirques

– At the upper end of the glacial trough, a scoop-shaped depression, or cirque, eroded into a mountain side marks the place where a glacier formed and moved out into a trough.

• Erosion by Valley Glaciers– Arêtes and Horns

– Both are landforms generated by valley glacier erosion.

– An arête is a serrated ridge between U-shaped glacial troughs or between adjacent cirques

– A horn is a pyramid-shaped peak left when headword erosion takes place by at least three glaciers in the same peak.

Geo-inSight 10., p. 370Fig. 14.12c, p. 369

What Causes Ice Ages?

• The Milankovitch Theory• An explanation for the onset of the

glacial episodes• Milankovitch claimed that

irregularities in Earth’s rotation and orbit bring about complex climatic changes that provide the triggering mechanism for glacial episodes.

• The 3 primary factors are– orbital eccentricity– changes in axial tilt– precession of the equinoxes

Homework

• Describe the theory of Snowball Earth Theory

• And describe the Albedo effect

• Due Next Wednesday !