PowerLecture A Microsoft® PowerPoint® Link Tool for academic.cengage/com/earthsci Essentials of Physical Geology 5 th Edition Reed Wicander | James S. Monroe

Chapter 14 Glaciers and Glaciation

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

Introduction  The Little Ice Age  The Little Ice Age was a time of colder winters and short, wet summers during which glaciers advanced and sea ice persisted for long periods of time.  It occurred from about 1500 to the middle to late 1800s and had its greatest effect on Northern Europe and Iceland. Fig. 14.1a, p. 358

The Kinds of Glaciers By definition glaciers are moving bodies of ice on land, that flows downslope or outward from an area of accumulation. Sea ice and icebergs is nothing more than frozen seawater and are not glaciers because they do not form on land. Fig. 14.4b p. 361

The Kinds of Glaciers  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! Fig. 14.1b, p. 358

Valley Glaciers  Valley glaciers are also called alpine glaciers and mountain glaciers  Valley glaciers that flow into the sea are called tidewater glaciers. Fig. 14.2, p. 359

The Kinds of Glaciers  Continental Glaciers Continental glaciers flow outward in all directions from a zone of accumulation  Huge - cover vast areas.  Often develop large ice shelves where they flow outward into the sea. Fig a, p. 360 Fig a-b, p. 360

The Kinds of Glaciers  Ice Caps  Similar to continental glaciers but much smaller.  Some develop from valley glaciers when they grow over the top of a divide. Fig c, p. 360

Glaciers Moving Bodies of Ice on Land  Glaciers—Part of the Hydrologic Cycle  Glaciers are a reservoir in the hydrologic cycle where water is stored for long periods as it moves from the oceans to land and back to the oceans. Geo-Insight 8., p. 371

 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 80% air.  As the snow accumulates it thaws and refreezes, becoming a granular type of ice called firn.  When firn is buried and and recrystallized, it is metamorphosed to glacial ice and will flow under its own weight. Fig. 14.4a, p. 361 Glaciers Moving Bodies of Ice on Land

Fig. 14-4a, p. 361 Stepped Art Glacial ice Granular snow Firn Snowflakes

Glaciers Moving Bodies of Ice on Land  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. Fig. 14.5, p. 362

 Distribution of Glaciers  Glaciers exist only where there is  Sufficient precipitation in the form of snow  Temperatures low enough that they do not melt  These conditions prevail in  High mountains (some even near the equator or  High latitudes (such as in Alaska, the Canadian Arctic islands, Greenland, and Antarctica.) Glaciers Moving Bodies of Ice on Land

The Glacial Budget Accumulation and Wastage  Glacial budget - A glacier's behavior depends on the balance 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. Fig a-b, p. 366

The Glacial Budget Accumulation and Wastage  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. Fig a-b, p. 366

The Glacial Budget Accumulation and Wastage  Glacial budget - A glacier's behavior depends on the balance between accumulation and wastage (melting).  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. Fig a-b, p. 366

The Glacial Budget Accumulation and Wastage  How Fast Do Glaciers Move?  The rate of glacial movement depends on the slope, discharge and season.  In general, valley glaciers move more rapidly than do continental glaciers Fig. 14.5, p. 367

The Glacial Budget Accumulation and Wastage  Glacial Surges - During a glacial surge, accelerated flow into a glacier causes its terminus to advance rapidly  Its surface breaks into a maze of crevasses. Fig. 14.6, p. 363

The Glacial Budget Accumulation and Wastage  Theories for the cause of surges:  1. Water-saturated sediment below a glacier allows it to slide  2. A glacier’s slope increases due to thickening in the zone of accumulation and thinning in the zone of wastage.

Erosion and Transport by Glaciers  Glaciers effectively erode and transport significant amounts of sediments because they are moving solids.  They are very effective in eroding soil and unconsolidated sediment.  Glaciers deposit huge amounts of sediment of all grain sizes, from boulders the size of a house down to rock flour. Fig , p. 369

Erosion and Transport by Glaciers 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. Fig. 14.9, p. 368

Erosion and Transport by Glaciers Glaciers…  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 Fig , p. 368

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 b, p. 369

Erosion and Transport by Glaciers  Erosion by Valley Glaciers  A fiord forms when sea level rises and fills a U- shaped glacial valley with sea water. Geo-inSight 3-5, p. 370 U-Shaped Glacial Troughs

Erosion and Transport by Glaciers  Erosion by Valley Glaciers Hanging Valleys  Create some of the world’s most spectacular waterfalls  Form when a former glacial tributary reaches the main valley Fig c, p. 369 Geo-Insight 8., p. 371

Erosion and Transport by Glaciers  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. Fig c, p. 369 Fig a, p. 375

Erosion and Transport by Glaciers  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. 370 Fig c, p. 369

Erosion and Transport by Glaciers  Continental Glaciers and Erosional Landforms  Areas eroded by continental glaciers  Are smooth and rounded, ice-scoured plains  Create deranged drainages with swamps and lakes  Exhibit large areas of polished and striated bedrock Fig , p. 372

Deposits of Glaciers  Glacial Drift – a general term for all glacial deposits  Erratics – huge boulders derived from distant source areas, transported to their current location by glaciers Fig a-b, p. 373

Deposits of Glaciers  Glacial Drift – a general term for all glacial deposits  Two types of drift  1. Till – sediments deposited directly by glacial ice. Poorly sorted.  2. Stratified drift – sediments deposited by running water, usually in braided streams. Well-sorted. Fig b, p. 374 Fig a, p. 377

Deposits of Glaciers  Landforms Composedof Till  End moraines – Crescent shaped deposits of till that form near the terminus of the glacier.  Form a pile of rubble at the front of the glacier Fig , p. 374

Deposits of Glaciers  Landforms Composed of Till  Recessional moraine.  Suppose that a glacier reaches its maximum extent and has a balanced budget.  Accordingly it deposits a terminal moraine.  If it then has a negative budget,  Its terminus retreats and perhaps becomes stabilized once again if its budget is balanced  In this case another end moraine is deposited but it is called a recessional moraine. Fig. 14.7b, p. 366

Deposits of Glaciers  Landforms Composed of Till  Lateral and Medial Moraines – Ridge shaped deposits of till that form within the glacier.  Created by plucking rock from the valley walls  Lateral moraines form along the sides of the glacier  Medial moraines form where two lateral moraines meet Fig b, p. 375

Deposits of Glaciers  Landforms Composed of Till  Drumlins  Streamlined hills of till shaped by continental glaciers or by glacial meltwater floods. Form drumlin fields, with 100’s of drumlins present. Fig , p. 376

Deposits of Glaciers  Landforms Composed of Stratified Drift  Sediments deposited by glacial meltwater. Well sorted.  Outwash Plains – vast blankets of sediment, usually sand and gravel, that form in front of the glacier as it melts  Valley Trains – deposits of braided streams that form long, narrow deposits of stratified drift. Fig , p. 376 Fig a, p. 377

Deposits of Glaciers  Landforms Composed of Stratified Drift  Kames – conical hills created when a stream deposits sediment in a depression on the glacier’s surface.  Eskers – snake-like deposits from sub-glacial streams Fig , p. 376 Fig , p. 377

Deposits of Glaciers  Deposits in Glacial Lakes  The most distinctive deposits in glacial lakes are varves  Varves consist of couplets of dark and light, laminated, fine- grained sediment.  The dark layers form during the winter when small particles of clay and organic matter are deposited.  The light layers are made up of silt and clay that form during the warmer months.  The age of a glacial lake may be determined by counting the layers. Fig a, p. 378

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 Fig , p. 378

What Causes Ice Ages?  Short-Term Climatic Events  Milankovitch cycles can be measured in 10’s of thousands of years. They are too long to explain events like the Little Ice Age that lasted just a few hundreds of years  Several hypotheses have been proposed  Variations in solar energy due to solar flares or interstellar dust  Volcanic eruptions are known to cause short term climate change. A series of large eruptions could produce a prolonged event.

End of Chapter 14