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 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 • Ice Sheets • Ice Caps
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 Sheets • Also called Continental Glaciers • Continental glaciers flow outward in alldirections from a zone of accumulation • Also 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 summer • melt 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 its • own weight.
Snowflakes Glacial ice Granular snow Firn Stepped Art Fig. 14-4a, p. 361
How Do Glaciers Originate and Move? • Glaciers move thru • Basal Slip and • Plastic Flow • If 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 deformation • that takes place in response • to 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 Glacier Glacial 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
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. The Glacial BudgetAccumulation and Wastage
Glaciers… 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 Glaciers
Glaciers… Polishrocks 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. Erosion and Transport by Glaciers Fig. 14.12 b, p. 369
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 • Cirques
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. • Erosion by Valley Glaciers • Arêtes and Horns Geo-inSight 10., p. 370 Fig. 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 !