Page 601 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

5. How are icebergs generated? Describe their buoyancy characteristics using this chapter’s discussion and the section on ice in Chapter 7. Why do you think an iceberg overturns from time to time as it melts?
■ Illustrate the mechanics of glacial movement.
A glacier is an open system. The firn line is the eleva- tion above which the winter snow and ice remain intact throughout the summer melting season but below which melting occurs. A glacier is fed by snowfall and is wasted by ablation (losses from its upper and lower surfaces and along its margins). Accumulation and ablation achieve a mass balance in each glacier.
As a glacier moves downhill, vertical crevasses may develop. Sometimes a glacier will move rapidly, an event known as a glacier surge. The presence of water along the basal layer appears to be important in glacial movements. As a glacier moves, it plucks rock pieces and debris, in- corporating them into the ice, and this debris scours and sandpapers underlying rock through abrasion.
firn line (p. 539) ablation (p. 539) crevasse (p. 540) glacier surge (p. 541) abrasion (p. 541)
6. What is meant by glacial mass balance? What are the basic inputs and outputs contributing to that balance?
7. What is meant by a glacier surge? What do scientists think produces surging episodes?
■ Describe characteristic erosional and depositional landforms created by glaciation.
A roche moutonnée is an erosional landform produced by plucking and abrasion. It is an asymmetrical hill of ex- posed bedrock, gently sloping on the upstream end and abruptly sloping on the downstream end.
Extensive valley glaciers have profoundly reshaped mountains worldwide, transforming V-shaped stream val- leys into U-shaped glaciated valleys and producing many other distinctive erosional and depositional landforms. As cirque walls erode away, sharp arêtes (sawtooth, serrated ridges) form, dividing adjacent cirque basins. Two erod- ing cirques may reduce an arête to a saddlelike col. A horn results when several cirque glaciers gouge an individual mountain summit from all sides, forming a pyramidal peak. A bergschrund forms when a crevasse or wide crack opens along the boundary between flowing ice and stagnant ice, often near the headwall of a glacier; it is most visible in summer when covering snow is gone. An ice-carved rock basin left as a glacier retreats may fill with water to form a tarn; tarns in a string separated by moraines are paternoster lakes. Where a glacial trough joins the ocean and the glacier retreats, the sea extends inland to form a fjord.
All glacial deposits, whether ice-borne or meltwater- borne, constitute glacial drift. Direct deposits from ice consist of unstratified and unsorted till. Specific land- forms produced by the deposition of till at glacial margins are moraines. A lateral moraine forms along each side of a glacier; merging glaciers with lateral moraines form a medial moraine; and eroded debris dropped at the far- thest extent of a glacier’s terminus is a terminal moraine.
Recessional moraines mark temporary endpoints as the glacier advances and retreats over time.
A till plain forms behind a terminal moraine, fea- turing unstratified coarse till, low and rolling relief, and deranged drainage. Drumlins are elongated hills of deposited till, streamlined in the direction of continen- tal ice movement (blunt end upstream and tapered end downstream).
Glacial meltwater deposits are sorted and stratified, and called stratified drift, forming outwash plains fea- turing braided stream channels that carry a heavy sedi- ment load. An esker is a sinuously curving, narrow ridge of coarse sand and gravel that forms along the channel of a meltwater stream beneath a glacier. A kame is a small hill, knob, or mound of poorly sorted sand and gravel that is deposited directly on top of glacial ice, and then is deposited on the ground when the glacier melts. An iso- lated block of ice left by a retreating glacier becomes sur- rounded by debris; when the block finally melts, it leaves a steep-sided depression called a kettle that when filled with water forms a kettle lake.
roche moutonnée (p. 544) arête (p. 544)
col (p. 544)
horn (p. 544) bergschrund (p. 544) tarn (p. 544)
paternoster lake (p. 544) fjord (p. 546)
glacial drift (p. 547)
till (p. 547)
moraine (p. 547)
lateral moraine (p. 547) medial moraine (p. 547) terminal moraine (p. 547) till plain (p. 548) drumlin (p. 549) stratified drift (p. 549) outwash plain (p. 549) esker (p. 550)
kame (p. 550)
kettle (p. 550)
8. How does a glacier accomplish erosion?
9. Describe the transformation of a V-shaped stream
valley into a U-shaped glaciated valley. What
features are visible after the glacier retreats?
10. How is an arête formed? A col? A horn? Briefly
differentiate among them.
11. Differentiate between two forms of glacial drift—till
and glacial outwash.
12. What is a morainal deposit? What specific moraines
are created by alpine glaciers?
13. What is a common depositional feature encountered
in a till plain?
14. Contrast a roche moutonnée and a drumlin with
regard to appearance, orientation, and the way each forms.
■ Discuss the distribution of permafrost, and explain several periglacial processes.
The term periglacial describes cold-climate processes, landforms, and topographic features that exist along
Chapter 17 glacial and Periglacial Landscapes 565