Review Questions and Answers; Glaciers

A glacier is a thick mass of moving ice deforming internally (under its own weight) and being pulled (flowing) downslope by the force of gravity.

Glacial ice forms in areas where, year in and year out, the accumulated snow and frost exceed the amount melted, evaporated, or otherwise lost. Each year the mass thickens and the deeply buried snow turns to ice. Eventually when the thickness is great enough, the interior ice begins to deform and flow, producing a glacier.

2. Describe how glaciers fit into the hydrologic cycle. What role do they play in the rock cycle?

The water in ice sheets and glaciers can be viewed as removed from the oceans and temporarily stored on land. Glacial ice, like groundwater, does eventually return to the sea, but the recycling time is hundreds to thousands of years compared to months or a few years for surface water runoff from rainfall events and melting snow. By a large margin, glacial ice represents the largest freshwater reservoir in the hydrologic cycle. Lowered sea level accompanies net growth of ice sheets; net shrinking of ice sheets causes sea level to rise. As global temperatures rise, glaciers undergo net loss of ice, sea level rises, and the surface area of the oceans increases. These factors work in favor of an increase in the total amount of water evaporated into the atmosphere. With cooler temperatures and declining sea level, the total amount of water evaporated from the oceans is also expected to decrease.

In addition to the well-known greenhouse effect of carbon dioxide, recent studies have focused on water vapor and methane as potentially powerful global climate modifiers. Methane has a larger "greenhouse effect" than CO2 and large quantities are currently tied up in shallow sediments as gas hydrates. Should these be "released" by lowered sea levels or warming of the oceans, the methane would contribute to increased net absorption of infrared radiation from Earth and warming of the atmosphere. The role of water vapor as the dominant energy carrier in the atmosphere is well-known; thus an increase in atmospheric water vapor content could have important if complex effects on global climate varying from warming at high latitudes to increased cloud cover and corresponding changes in the atmosphere's albedo. Thus glacial ice is an integral long-term player in the hydrologic cycle, and changes in the amount of glacial ice have important, if not completely understood effects on global climatic change.

As powerful agents of erosion, transport, and deposition, glaciers contribute to the accumulation of sediments directly, till for example, and indirectly, sand and gravel from meltwater streams and loess from windblown glacial rock flour. Given the proper conditions for preservation and burial, these sediments can eventually lithify to rock as shown by occurrences of tillites, clastic rocks of glaciofluvial origin, and loessite in the rock record. Thus glaciers play an important role in the rock cycle as well as in the hydrologic cycle.

Ice sheets such as in Greenland and Antarctica also play a major role as repositories for extraterrestrial lithic debris and particles. Without an ice cover, these particles are soon destroyed by weathering and/or widely dispersed and hidden as trace particles in clastic sediments and terrestrial regolith. The famous "meteorite from Mars", recovered from the Antarctic ice sheet, is a good example. These samples plus meteorites constitute lithic samples from the solar system, and rare small particles with novel isotopic compositions that must have come from beyond our solar system. Thus the ice sheets help us to study the "rock cycle" out in space.

3. Each statement below refers to a particular type of glacier. Name the type of glacier.

(a) The term continental is often used to describe this type of glacier. - This refers to continental glaciers or continental ice sheets like the ones that cover most of Greenland and Antarctica today.

(b) This type of glacier is also called an alpine glacier. - This would be a valley glacier, a long ice stream that flows downslope along a valley in a mountainous region.

(c) This is a stream of ice leading from the margin of an ice sheet through the mountains to the sea. - The statement describes an outlet glacier.

(d) This is a glacier formed when one or more valley glaciers spread out at the base of a steep mountain front. - a piedmont glacier

(e) Greenland is the only example in the Northern Hemisphere.- an ice sheet or continental ice sheet

4. Where are glaciers found today? What percentage of Earth's land area do they cover? How does this compare to the area covered by glaciers during the Pleistocene?

Today, glaciers cover about 10 percent of the land area. Valley glaciers are found in high, mountainous regions at all latitudes. Ice sheets and ice caps are found only at high latitudes in such areas as Iceland, Greenland, and Antarctica; the Antarctic ice sheet is by far the largest. During the height of the Pleistocene glaciations, about 30 percent of the land area was ice covered. Due to lowered sea level, the Antarctic ice sheet was slightly larger than at the present time; most of the other ice covered lands were beneath the continental ice sheets of North America (Fig. 12.31) and Europe.

5. Describe the two components of glacial flow. At what rates do glaciers move? In a valley glacier, does all of the ice move at the same rate? Explain.

Glacial flow involves two mechanisms (Fig. 12.5). One is basal sliding, in which the entire glacier moves forward by sliding or slipping along the bedrock at the base of the glacier; the other flow mechanism involves internal deformation and plastic flow. Ice in the interior parts of the glacier slowly deforms and recrystallizes, producing a net, downslope movement of all the ice above the zone of deformation. Ice higher in the zone of deformation moves faster than the ice beneath it, and surface ice in the center of the glacier moves faster than ice near the margins. Glaciers move very slowly on average; velocities may be less than a meter per year. Periods of unusually rapid glacial movements are called surges (Box 12.2). As the temperature at the base of a glacier reaches the melting point, quantities of liquid water accumulate at the ice-bedrock contact. Frictional resistance is greatly reduced and the ice tends to "float" on the meltwater. Thus basal slip is accelerated. Temperatures below freezing at depth in the glacier result in low basal slip and internal flowage velocities.

6. Why do crevasses form in the upper portion of a glacier but not below 50 meters?

Crevasses are transverse, open cracks or fissures in a glacier that extend from the surface to depths of about 50 meters (Fig. 12.6). The cracks are widest at the surface and taper downward. The bottom tip of the crevasse marks the base of the brittle ice zone and the top of the plastic flow zone. The brittle, surface ice layer, while being carried passively downslope by flowage in the plastic zone, responds to stresses by cracking and fracturing.

7. Under what circumstances will the front of a glacier advance? Retreat? Remain stationary?

Excluding surges and other, unsteady flow movements, the glacier will advance (its snout will move downslope) when the snow and ice accumulated during many, consecutive years exceed that lost by melting and other forms of ablation. The glacier will retreat (the snout melts back to higher elevations) when the reverse is true, ablation exceeds accumulation. The snout will be stationary if accumulation and ablation are exactly balanced year after year.

8. Describe the processes of glacial erosion.

Glaciers are powerful agents of erosion. The melting and refreezing of water at the base of the glacier can dislodge large blocks of rock (plucking); rock particles entrained in the ice at the bottom and sides of the glacier scratch and gouge the bedrock. Areas of unconsolidated materials and soft bedrock can be deeply eroded, the debris being carried away by the glacier. Rock and soil particles of all sizes, entrained in the glacier through plucking, erosion, abrasion, and mass wasting, are carried along and eventually deposited at the snout of the glacier.

9. How does a glaciated mountain valley differ in appearance from a mountain valley that was not glaciated?

Nonglacial valleys were cut by streams and widened by mass wasting; they have V-shaped cross sections; sinuous, longitudinal profiles; and lots of sharp ridges that extend downslope to the valley floor and stream. Glacial valleys were strongly scoured by the moving ice. They have U-shaped cross sections with wide, relatively flat floors; very steep walls; and straight, longitudinal profiles (Figs. 12.11 & 12.12). Truncated spurs are blunt facets eroded from ridges that extended to the valley floor before the valley was glaciated.

10. List and describe the erosional features you might expect to see in an area where valley glaciers exist or have recently existed.

Large, open, bowl-shaped, erosional basins (cirques) are present at the heads of the larger valleys. The highest mountains are horn peaks, and sharp, knife-edged ridges (aretes) form common boundaries between neighboring cirques. Valleys are fairly straight with U-shaped, cross-valley profiles and numerous, truncated ridge spurs; hanging valleys and waterfalls may be evident where tributary canyons were left dangling high above the floor of the main valley.

11. What is glacial drift? What is the difference between till and stratified drift? What general effect do glacial deposits have on the landscape?

Glacial drift denotes any sedimentary material deposited from melting ice or meltwater streams. Till is the unsorted, unstratified drift deposited directly as the ice melts. Stratified drift (also called outwash) denotes sand and gravel beds deposited from glacial meltwater streams. Many glaciated landscapes exhibit low, irregularly shaped hills, mounds, and ridges of till standing above lower, marshy areas. Outwash plains are typically flat; like moraines, they may be pitted by kettles (depressions formed by collapse of drift into voids formed by melting of buried ice blocks).

Glaciers can excavate deep valleys and lake basins, obliterate pre-glacial drainage systems, deposit moraines and outwash plains, carve mountain regions into alpine peaks and valleys, and leave behind the flat, silt covered floors of once immense lakes. Glaciation profoundly alters the morphology and appearance of landscapes.

12. List the four basic moraine types. What do all moraines have in common? What is the significance of terminal and recessional moraines?

The four types of moraines are end, lateral, medial, and ground. All are composed of till; except for ground moraine, they are prominent, irregularly shaped mounds and ridges. End moraines form around snouts of glaciers. A recessional moraine is any end moraine left by a retreating glacier, and a terminal moraine is a special end moraine that marks the position of the glacier's farthest advance. Lateral moraines accumulate along the sides of valley glaciers. Medial moraines are longitudinal, debris streams in interior parts of valley glaciers; they form by merging of lateral moraines at the junction of the main glacier and a tributary glacier. No rock debris is added below the junction. Medial moraines are easily seen as prominent, dark streaks in valley (alpine) glaciers; they are very rarely preserved as landforms. Ground moraine is a general term to describe the generally thin and irregularly distributed, till deposits left by continental ice sheets.

13. Why are medial moraines proof that valley glaciers must move?

Medial moraines are streaks of rock debris embedded in the interiors of valley glaciers. At glacier junctions, the outer lateral moraines of each combining glacier stay separated and continue downstream as the lateral moraines of the combined glacier, augmented in volume by mass wasting of rocky debris from the valley walls. The two inner lateral moraines of the combining glaciers join together as a medial moraine, a streak of debris embedded in the interior part of the combined glacier. Rock debris in the medial moraine is supplied by mass wasting above the junction; the medial moraine has no direct additions of debris below the junction. Thus for the medial moraine to continue "intact" on down the combined glacier, the ice must be moving and carrying rocky debris supplied from upstream.

14. How do kettles form?

Kettles are circular to elliptical, closed depressions in areas underlain by till or outwash. They form at the snout regions of glaciers where blocks of stagnant ice get buried or partly buried by till or outwash. When the ice melts, the till or outwash collapses into the void formerly occupied by the ice, leaving a depression in the land surface. Kettles are commonly occupied by lakes or marshes (Fig. 12.27).

15. What direction was the ice sheet moving that affected the area shown in Figure 12.26? Explain how you were able to determine this.

The narrow, elongated hills including Hill Cumorah are drumlins. The elongation parallels the direction of ice movement, and the more gently sloping, tapered end points in the downstream direction. The contour lines are spaced farther apart on south-facing slopes than on the north-facing slopes, showing that the north-facing slopes of the drumlins are steeper. Therefore the ice was moving from north to south.

16. What are ice-contact deposits? Distinguish between kames and eskers.

Ice-contact deposits are composed of sand and gravel deposited by meltwater streams. The sediments are laid down in contact with stagnant ice near the snout of a glacier. After the ice melts, the deposits are left standing above the surrounding countryside as mounds and ridges. Eskers are sinuous ridges of sand and gravel deposited by streams flowing in meandering tunnels cut through the ice. Kames are rounded, conical hills of sand and gravel deposited in ponds with steep-sided, ice walls. Such sediments are deprived of lateral support when the ice melts. Thus they collapse and spread laterally, resulting in locally tilted and distorted bedding.

17. The development of the glacial theory is a good example of applying the principle of uniformitarianism. Explain briefly.

During the late eighteenth and early nineteenth centuries, northern Europeans contributed many of the fundamental ideas and concepts that underlie modern geology, including uniformitarianism. Most northern European landscapes have been strongly modified by the Pleistocene ice sheets, and active glaciers in the Alps had been observed for many centuries. Undoubtedly, the numerous, "peculiar", glacial landforms such as eskers, erratics, and moraines had sparked much curiosity and interest as to their mode of origin. The then prevailing views focused on erosion and deposition during the Noahan flood or in a since vanished ocean. These explanations may have satisfied the pious and contrite; but others, especially those familiar with active glaciers, may not have been so readily convinced. Swiss observers, such as Ignaz Venetz, were the first to "put two and two together" regarding the connection between glaciers and landscapes.

The next intellectual step (more like a leap, actually) was recognizing that surface topography and landforms in low lying areas far from active glaciers had originated through the action of continental-sized glaciers. The ice sheets covering Greenland and Antarctica were known but had hardly been studied. Ice sheets large enough to have covered much of northern Europe had to be "invented". Extrapolating local, observational evidence was one thing, but challenging entrenched ideas and convictions with a new, continent wide, ice-age idea was another. Louis Agassiz (Box 12.3) was initially a staunch defender of the status quo. During the course of field work and observations in the Swiss Alps undertaken to refute the new glacial theory, he became a dedicated convert and one of its major proponents, first in Europe and later in North America as a well-respected professor at Harvard. His theory of the "Ice Ages" was soundly based on observational data but involved an enormous extrapolation (leap!) beyond the context of those observations. His Ice Age theory vastly enlarged the scope of the uniformitarian concept and set a precedent for guaranteeing fair, scientific hearings to later, far more radical, global-scale ideas such as continental drift and asteroidal impacts.

18. During the Pleistocene epoch the amount of glacial ice in the Northern Hemisphere was about twice as great as in the Southern Hemisphere. Briefly explain why this was the case.

High latitude land areas are much more extensive in the Northern Hemisphere. Thus, during the Pleistocene glacial advances, ice sheets covered larger land areas in the Northern Hemisphere and, despite the extensive Antarctic ice sheet, contained twice the volume of ice as glaciers in the Southern Hemisphere. The Antarctic ice sheet expanded slightly due to lowered sea levels, but its thickness could not have increased much over present-day values because precipitation decreases rapidly with increased elevation and plastic flow rates increase as the ice sheet thickens.

19. List three indirect effects of Ice Age glaciers.

Drastic changes in climate forced migrations of fauna and flora. Sea level dropped as vast quantities of water were removed from the ocean and stored as ice; and lowered sea levels caused some downcutting by rivers and streams. As sea level rose again when the ice melted, the deepened valleys and canyons were filled in with sediments. Also vast, freshwater lakes formed in what are now dry areas of the western United States and central Asia; large lakes, such as the Great Lakes, formed along the edges of the retreating ice sheets or filled in depressions gouged out by the ice. Land that had subsided under the great weight rose (rebounded) as the ice melted.

20. How might plate tectonics help explain the cause of ice ages? Can plate tectonics explain the alternation between glacial and interglacial climates during the Pleistocene?

Glacial episodes in the geologic history of Earth coincide with times when large, continental areas were situated at high latitudes (consider Greenland and Antarctica today). The ice ages were made possible by the large land masses at high latitudes (northern Europe, northern Russia and Siberia, Canada, and Alaska). However, plate movements during the Pleistocene are not large enough to have caused the glacial-interglacial climatic fluctuations. Thus they must have had some other cause. Possibilities include variations in the Sun's output and variations in received solar energy caused by slight variations in Earth's orbit (Fig. 12.36).