Chapter 14 Glaciers and Glaciation
Introduction Pleistocene Glaciations: A series of "ice ages" and warmer intervals that occurred 2.6 million to 10,000 years ago. The Little Ice Age was a time of colder winters and short, wet summers during which glaciers advanced and sea ice persisted. It occurred from about 1500 to the middle to late 1800s, and had its greatest effect on Northern Europe and Iceland. Geo-Insight 4, p. 342
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. 341
Introduction Snowfields exist year round in some mountains, but because they do not move, they are not glaciers. Icebergs are not glaciers because they float in water and not on land. Fig. 14.2, p. 341
The Kinds of Glaciers Valley Glaciers Confined within high mountain valleys Long, narrow tongues of ice Much smaller than continental glaciers Flow from higher to lower elevations Often have tributaries Geo-Insight 5, p. 342
The Kinds of Glaciers Valley glaciers are also called alpine glaciers. Tidewater glaciers are valley glaciers that flow into the sea. Fig. 14.2, p. 341
The Kinds of Glaciers Continental Glaciers or ice sheets are unconfined by topography and flow outward in all directions from a zone of accumulation. Currently found in Antarctica and Greenland They cover vast areas Often develop large ice shelves where they flow outward into the sea Fig. 14.3a, p. 341
The Kinds of Glaciers Ice Caps Similar to continental glaciers, but much smaller Some develop from valley glaciers when they grow out of their mountain valleys Others develop in flat areas, such as in Iceland Fig. 14.3b, p. 341
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. Besides melting, glaciers also sublimate, where ice converts directly to water vapor. Fig. 14.1, p. 340
Glaciers - Moving Bodies of Ice on Land 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, it recrystallizes (metamorphoses) to glacial ice and will flow under its own weight. Fig. 14.4a, p. 344
Snowflakes Glacial ice Granular snow Firn Stepped Art Fig. 14-4, p. 344
Glaciers - Moving Bodies of Ice on Land How Do Glaciers Originate and Move? Glaciers move through Basal Slip and Plastic Flow. If a slope is present, glaciers may slide over their underlying surface through basal slip. Meltwater helps to lubricate basal slip. Most of their movement is plastic flow, a type of deformation that takes place in response to stress. Fig. 14.5, p. 345
Glaciers - Moving Bodies of Ice on Land How Do Glaciers Originate and Move? The upper 40 m or so of a glacier is brittle and fractures. Tension within the glacier forms large crevasses on the surface of the glacier. Fig. 14.6a, p. 346
Crevasses Fig. 14.6, p. 346
Glaciers - Moving Bodies of Ice on Land Distribution of Glaciers Glaciers exist only where there is: Sufficient snow fall (excludes some polar deserts) Summer temperatures are low enough to hinder melting These conditions prevail in: High mountains (some even at the equator) High latitudes (such as in Alaska, the Canadian Arctic islands, Greenland, and Antarctica)
The Glacial Budget Glacial budget - A glacier's behavior depends on the balance between accumulation and wastage (melting). Fig. 14.7, p. 347
The Glacial Budget The upper part of the glacier, where the snow cover is year-round, is the zone of accumulation. The lower part, where losses from melting, sublimation, and calving of icebergs exceed gains, is the zone of wastage. The line separating the two is the firn limit. It shifts at least somewhat each year. Fig. 14.7, p. 347
The Glacial Budget 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. 14.7, p. 347
The Glacial Budget A persistent negative budget leads to the disappearance of a glacier. Glaciers are very sensitive to climatic changes. Most glaciers are in retreat because of global warming. In a few decades, the glaciers will probably be gone from Glacier National Park in Montana. Fig. 14.7, p. 347
The Glacial Budget 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. 14.8a, p. 350
The Glacial Budget How Fast Do Glaciers Move? The main valley glacier has more volume and flows faster than its tributaries. Plastic flow occurs year round, but basal slip is more important in the summer. Fig. 14.8a, p. 350
The Glacial Budget How Fast Do Glaciers Move? Like streams, glacial ice flows faster away from walls and floors. In general, glacial flow velocity increases down slope in the accumulation zone, but decreases in the zone of wastage. Fig. 14.8a, p. 350
The Glacial Budget How Fast Do Glaciers Move? Most continental glaciers flow at only a few cm to meters per day. Because they are at high latitudes, continental glaciers are usually frozen to their underlying surfaces and basal slip is minimal. Fig. 14.8a, p. 350
The Glacial Budget Glacial Surges are short-lived episodes of accelerated flow. Surges probably involve increased basal slip. They are more common in valley glaciers. Surges break the surfaces of glaciers into a maze of crevasses. Glaciers may surge at tens of meters per day for weeks or months before slowing down to their normal rates. Fig. 14.6b, p. 346
The Glacial Budget Hypotheses for the causes of surges: 1. Thickening of ice in the zone of accumulation and concurrent thinning in the zone of wastage increases the slope of the glacier and its velocity. 2. Pressure on soft sediment beneath the glacier squeezes fluids through the sediments and allows the overlying glacier to slide more effectively.
Erosion and Sediment Transport by Glaciers Glaciers effectively erode, transport and deposit huge amounts of sediments because they are moving solids. Glaciers deposit sediments of all grain sizes, from boulders the size of a house down to clay-sized rock flour. Fig. 14.11, p. 352
Erosion and Sediment 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. 14.9a, p. 351
Erosion and Sediment Transport by Glaciers Abrasion and plucking can form a roche moutonnée. Fig. 14.9, p. 351
Erosion and Sediment Transport by Glaciers Rocks and sediment in glaciers abrade underlying rocks and 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. 14.10, p. 352
Erosion and Sediment 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 deepen and widen so that they have a U- shaped profile with flat or gently rounded valley floors and near-vertical valley walls. Fig. 14.12 b, p. 353
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Mountain river valleys usually have a V-shaped profile in contrast to the U- shaped valleys produced by valley glaciers. A fiord forms when sea level rises and fills a U- shaped glacial valley with sea water. Fig. 14.13, p. 353
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Hanging Valleys: Create spectacular waterfalls Form when the floor of a former glacial tributary is higher than the main valley Fig. 14.12c, p. 353
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Hanging Valleys Fig. 14.14, p. 354
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Cirques At the upper end of the glacial trough, a scoopshaped depression, or cirque, eroded into a mountain side marks the place where a glacier formed and moved out into a trough. Fig. 14.12c, p. 353
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Cirques Fig. 14.15b, p. 354
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Arêtes An arête is a serrated ridge between U- shaped glacial troughs or between adjacent cirques. Fig. 14.12c, p. 353 Fig. 14.15a, p. 354
Erosion and Sediment Transport by Glaciers Erosion by Valley Glaciers Horns A horn is a pyramid-shaped peak left when headword erosion takes place by at least three glaciers in the same peak. Fig. 14.16, p. 355 Fig. 14.12c, p. 353
Erosion and Sediment 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. 14.17, p. 355
Glacial Deposits 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. 14.19a,b, p. 356
Glacial Deposits 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 meltwater, usually in braided streams; well-sorted
Glacial Deposits Landforms Composed of Till End moraines Crescent shaped deposits of till that form near the terminus of the glacier. That is, they form as a pile of rubble at the front of the glacier. Fig. 14.20, p. 357
Glacial Deposits Landforms Composed of Till Ground moraines deposits of till left by a retreating glacier Ground moraine has irregular, rolling topography, whereas end moraines are elongated ridges.
Glacial Deposits 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, deposits ground moraine 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. 14.18, p. 356 Glacial Deposits Recessional and Terminal Moraines
Glacial Deposits 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. 14.21, p. 357
Glacial Deposits Landforms Composed of Till Drumlins Streamlined hills of till shaped by continental glaciers or by glacial meltwater floods. Drumlin fields may contain hundreds of them. Fig. 14.18b, p. 356
Glacial Deposits 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. 14.18, p. 356
Glacial Deposits Landforms Composed of Stratified Drift Kames conical hills created when a stream deposits sediment in a depression on the glacier s surface Fig. 14.18, p. 356 Fig. 14.23a, p. 358
Glacial Deposits Landforms Composed of Stratified Drift Eskers snake-like deposits from sub-glacial streams Fig. 14.18, p. 356 Fig. 14.23b, p. 358
Glacial Deposits 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. Fig. 14.24a, p. 359
Glacial Deposits Deposits in Glacial Lakes The age and history of a glacial lake may be determined by counting and studying the layers. Fig. 14.24a, p. 359
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. Fig. 14.25, p. 360
What Causes Ice Ages? The Milankovitch Theory The 3 primary factors are: orbital eccentricity changes in axial tilt precession of the equinoxes Fig. 14.25, p. 360
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