OUTLINE Introduction Glaciers—Part of the Hydrologic Cycle How Glaciers Form and Move Types of Glaciers Accumulation and Wastage—The Glacial Budget Glacial Movement Erosion and Transport by Glaciers Glacial Deposits Causes of Ice Ages Geo-Recap

OBJECTIVES 1 Moving bodies of ice on land known as glaciers cover about 10% of Earth’s land surface, but they were much more widespread during the Pleistocene Epoch (Ice Age). 2 Glaciers constitute one reservoir in the hydrologic cycle. 3 In any area that has a yearly net accumulation of snow, the snow is converted first to firn and eventually to glacial ice. 4 Glaciers move by a combination of plastic flow and basal slip, with several factors determining the rate at which they move. 5 The concept of a glacial budget is important in considering the dynamics of any glacier. 6 Glaciers effectively erode, transport, and deposit sediment, thus accounting for the origin of distinctive landforms. 7 A theory explaining the onset of ice ages points to irregularities in Earth’s rotation and orbit as the cause of geologically recent ice ages.

A tributary glacier spills downward toward the Athabasca Glacier in the Columbia Icefield, Canada. Fig. 14-CO, p. 320

Figure 14.1: The Fox Glacier in New Zealand flows under its own weight to lower elevations. Fig. 14-1, p. 322

Table 14-1, p. 322

Figure 14.2a: The conversion of freshly fallen snow to firn and glacial ice. Fig. 14-2a, p. 323

Figure 14.2b: Firn sample in the Alps. Fig. 14-2b, p. 323

Figure 14.2c: The blue color common to glacial ice is visible here in New Zealand. Much of white light is absorbed by ice, but blue light is transmitted and scattered, giving the ice a blue hue. Fig. 14-2c, p. 323

Figure 14.3: Part of a glacier, showing how it moves by a combination of plastic flow and basal slip. Plastic flow takes place as the ice is internally deformed, whereas basal slip involves the glacier sliding over its underlying surface. If solidly frozen to its bed, a glacier moves only by plastic flow. Notice that the top of the glacier moves farther in a given time than the bottom does. Fig. 14-3, p. 324

Figure 14.4: Antarctica and Greenland are covered by continental glaciers. This image from Antarctica gives a hint of the scale of such glacial ice masses with only mountaintops visible above the thick ice. Fig. 14-4, p. 325

Figure 14.5: Response of a hypothetical glacier to changes in its budget. (a) If losses in the zone of wastage, shown by stippling, equal additions in the zone of accumulation, shown by crosshatching, the terminus of the glacier remains stationary. (b) Gains exceed losses, and the terminus advances. (c) Losses exceed gains, and the terminus retreats, although the glacier continues to flow. Fig. 14-5, p. 327

Figure 14.6a: Flow velocity, proportional to the length of the arrows, is greatest at the top center of this valley glacier because friction causes slower flow adjacent to the floor and walls of the valley. Notice the crevasses on the glacier’s surface. Fig. 14-6a, p. 328

Figure 14.6b: Deep crevasses are visible in summer on the top of Franz Josef Glacier in New Zealand. In winter, most glacial crevasses are covered in snow. Mountain climbers must be careful that they don’t “punch through” a snow bridge and fall into a crevasse. Fig. 14-6b, p. 328

Figure 14.7a: The rock face on this valley wall in New Zealand has been polished smooth by glacial abrasion. Fig. 14-7a, p. 329

Figure 14.7b: Both glacial polish and glacial striations created during the Permian Period are visible on this rock in Australia. A penknife provides a sense of scale. Fig. 14-7b, p. 329

Figure 14.7c: These pebbles’ striations indicate that they were once moved by a glacier. The largest pebble dimension is about 10 cm. Fig. 14-7c, p. 329

Figure 1: Sharp angular peaks and ridges and rounded valleys are typical of areas eroded by valley glaciers such as in Glacier National Park. Figure 1, p. 330

Figure 2: Grinnell Glacier in Glacier National Park Figure 2: Grinnell Glacier in Glacier National Park. In 1850, at the end of the Little Ice Age, the glacier extended much farther and covered about 2.33 km2. By 1981 its terminus had retreated to the position shown, and in 1993 it covered only about 0.88 km2. Figure 2, p. 331

Figure 14. 8: Erosional landforms produced by valley glaciers Figure 14.8: Erosional landforms produced by valley glaciers. (a) A mountain area before glaciation. (b) The same area during the maximum extent of the valley glaciers. (c) After glaciation. Fig. 14-8, p. 332

Figure 14. 8: Erosional landforms produced by valley glaciers Figure 14.8: Erosional landforms produced by valley glaciers. (a) A mountain area before glaciation. Fig. 14-8a, p. 332

Figure 14. 8: Erosional landforms produced by valley glaciers Figure 14.8: Erosional landforms produced by valley glaciers. (b) The same area during the maximum extent of the valley glaciers. Fig. 14-8b, p. 332

Figure 14. 8: Erosional landforms produced by valley glaciers Figure 14.8: Erosional landforms produced by valley glaciers. (c) After glaciation. Fig. 14-8c, p. 332

Figure 14.9a: The steep walls of this valley in New Zealand are part of its U-shape and indicate that it has been shaped by the valley glacier we still see within it. Fig. 14-9a, p. 332

Figure 14.9b: This much broader U-shaped valley is in Glacier National Park, Montana. Smaller tributary glaciers once fed into this valley so that it was carved by a larger body of ice than the valley shown in New Zealand. Fig. 14-9b, p. 332

Figure 14.10a: This bowl-shaped depression on Mount Wheeler in Great Basin National Park, Nevada, is a cirque. Notice that it has steep walls on three sides but opens out into a glacial trough in the foreground. Fig. 14-10a, p. 333

Figure 14.10b: Many cirques contain small lakes known as tarns, such as this one in Glacier National Park. The opaque, turquoise color of the water is characteristic of tarns. Such lakes have very fine rock particles suspended in the water, giving them their unusual color. The particles are called rock flour and are created by glacial abrasion. Fig. 14-10b, p. 333

Figure 14.11a: This sharp arête in New Zealand was created by valley glaciers eroding rock on either side of the ridge. Fig. 14-11a, p. 334

Figure 14.11b: The Matterhorn in Switzerland is a well-known horn. Fig. 14-11b, p. 334

Figure 14. 12a: Glacial till in northern Washington State Figure 14.12a: Glacial till in northern Washington State. Note the poor degree of sorting of the sediments. Fig. 14-12a, p. 335

Figure 14. 12b: The interior of a kame in upstate New York Figure 14.12b: The interior of a kame in upstate New York. These materials are better sorted because they have been moved by running water, not just glacial ice. Fig. 14-12b, p. 335

Figure 14. 13: (a) The origin of an end moraine Figure 14.13: (a) The origin of an end moraine. (b) The glacier retreats and its terminus stablilizes in a new position, and another moraine is deposited. Fig. 14-13ab, p. 336

Figure 14.13: (c) End moraines are described as terminal moraines or recessional moraines depending on their positions. Fig. 14-13c, p. 336

Figure 14.13: (d) Ground moraine in Montana. Fig. 14-13d, p. 336

Figure 14. 14: An end moraine deposited by a valley glacier Figure 14.14: An end moraine deposited by a valley glacier. This end moraine is also a terminal moraine because it is the one most distant from the glacier’s source. Fig. 14-14, p. 337

Figure 14. 15: (a) Lateral and medial moraines on a glacier in Alaska Figure 14.15: (a) Lateral and medial moraines on a glacier in Alaska. Notice that where the two large tributary glaciers converge, two lateral moraines merge and form a medial moraine. (b) The two parallel ridges extending from this mountain valley are lateral moraines. Fig. 14-15, p. 337

Figure 14. 15a: Lateral and medial moraines on a glacier in Alaska Figure 14.15a: Lateral and medial moraines on a glacier in Alaska. Notice that where the two large tributary glaciers converge, two lateral moraines merge and form a medial moraine. Fig. 14-15a, p. 337

Figure 14.15b: The two parallel ridges extending from this mountain valley are lateral moraines. Fig. 14-15b, p. 337

Figure 14.16: Two stages in the origin of kettles, kames, eskers, and an outwash plain: (a) during glaciation and (b) after glaciation. Fig. 14-16, p. 338

Figure 14.16: Two stages in the origin of kettles, kames, eskers, and an outwash plain: (a) during glaciation and (b) after glaciation. Fig. 14-16a, p. 338

Figure 14.16: Two stages in the origin of kettles, kames, eskers, and an outwash plain: (a) during glaciation and (b) after glaciation. Fig. 14-16b, p. 338

Figure 14. 17: Glacial deposits Figure 14.17: Glacial deposits. (a) These streamlined hills are drumlins. Fig. 14-17a, p. 339

Figure 14. 17: Glacial deposits Figure 14.17: Glacial deposits. (b) Braided streams discharging from a valley glacier such as this one in Alaska deposit a valley train of stratified drift. Outwash plains of continental glaciers are similar but more extensive. Fig. 14-17b, p. 339

Figure 14. 17: Glacial deposits Figure 14.17: Glacial deposits. (c) This small, conical hill in Wisconsin is a kame. Fig. 14-17c, p. 339

Figure 14. 17: Glacial deposits Figure 14.17: Glacial deposits. (d) The sinuous ridge in this image is an esker. Fig. 14-17d, p. 339

Figure 14. 18: Four stages in the evolution of the Great Lakes Figure 14.18: Four stages in the evolution of the Great Lakes. As the glacial ice retreated northward, the lake basins began filling with meltwater. The dotted lines indicate the present shorelines of the lakes. Fig. 14-18, p. 340

Figure 14. 18: Four stages in the evolution of the Great Lakes Figure 14.18: Four stages in the evolution of the Great Lakes. As the glacial ice retreated northward, the lake basins began filling with meltwater. The dotted lines indicate the present shorelines of the lakes. Fig. 14-18a, p. 340

Figure 14. 18: Four stages in the evolution of the Great Lakes Figure 14.18: Four stages in the evolution of the Great Lakes. As the glacial ice retreated northward, the lake basins began filling with meltwater. The dotted lines indicate the present shorelines of the lakes. Fig. 14-18b, p. 340

Figure 14. 18: Four stages in the evolution of the Great Lakes Figure 14.18: Four stages in the evolution of the Great Lakes. As the glacial ice retreated northward, the lake basins began filling with meltwater. The dotted lines indicate the present shorelines of the lakes. Fig. 14-18c, p. 340

Figure 14. 18: Four stages in the evolution of the Great Lakes Figure 14.18: Four stages in the evolution of the Great Lakes. As the glacial ice retreated northward, the lake basins began filling with meltwater. The dotted lines indicate the present shorelines of the lakes. Fig. 14-18d, p. 340

Figure 14. 19: Glacial varves are paired in light and dark layers Figure 14.19: Glacial varves are paired in light and dark layers. Each pair represents one year of deposition in a glacial lake environment. The dark portions were laid down during the winters and the wider and lighter layers in the warm months. Fig. 14-19, p. 340

Figure 14. 20: Variations in three parameters of Earth’s orbit Figure 14.20: Variations in three parameters of Earth’s orbit. (a) Earth’s orbit varies from nearly a circle (left) to an ellipse (right) and back again in about 100,000 years. (b) Earth moves around its orbit while spinning about its axis, which is tilted to the plane of its orbit around the Sun at 23.5 degrees and points toward the North Star. Earth’s axis of rotation slowly moves and traces out the path of a cone in space. (c) At present Earth is closest to the Sun in January, when the Northern Hemisphere experiences winter. (d) In about 11,000 years, as a result of precession, Earth will be closer to the Sun in July, when summer occurs in the Northern Hemisphere. Fig. 14-20, p. 341

Figure 14. 20: Variations in three parameters of Earth’s orbit Figure 14.20: Variations in three parameters of Earth’s orbit. (a) Earth’s orbit varies from nearly a circle (left) to an ellipse (right) and back again in about 100,000 years. Fig. 14-20a, p. 341

Figure 14. 20: Variations in three parameters of Earth’s orbit Figure 14.20: Variations in three parameters of Earth’s orbit. (b) Earth moves around its orbit while spinning about its axis, which is tilted to the plane of its orbit around the Sun at 23.5 degrees and points toward the North Star. Earth’s axis of rotation slowly moves and traces out the path of a cone in space. Fig. 14-20b, p. 341

Figure 14. 20: Variations in three parameters of Earth’s orbit Figure 14.20: Variations in three parameters of Earth’s orbit. (c) At present Earth is closest to the Sun in January, when the Northern Hemisphere experiences winter. Fig. 14-20c, p. 341

Figure 14. 20: Variations in three parameters of Earth’s orbit Figure 14.20: Variations in three parameters of Earth’s orbit. (d) In about 11,000 years, as a result of precession, Earth will be closer to the Sun in July, when summer occurs in the Northern Hemisphere. Fig. 14-20d, p. 341

CHAPTER SUMMARY Glaciers are moving bodies of ice that now cover about 10% of Earth’s surface and they are part of the hydrologic cycle. A glacier forms when winter snowfall in an area exceeds summer melt and therefore accumulates year after year. Snow is compacted and converted to glacial ice, and when the ice is about 40 m thick, pressure causes it to flow. Glaciers move by plastic flow and basal slip. Plastic flow involves deformation in response to pressure, whereas basal slip takes place when a glacier slides over its underlying surface. Valley glaciers are confined to mountain valleys and flow from higher to lower elevations, whereas continental glaciers cover vast areas and flow outward in all directions from a zone of accumulation. The behavior of a glacier depends on its budget, which is the relationship between accumulation and wastage. If a glacier possesses a balanced budget, its terminus remains stationary; a positive or negative budget results in advance or retreat of the terminus, respectively. Glaciers move at varying rates depending on slope, discharge, and season. Valley glaciers tend to flow more rapidly than continental glaciers.

CHAPTER SUMMARY Glaciers are powerful agents of erosion and transport because they are solids in motion. They are particularly effective at eroding soil and unconsolidated sediment, and they can transport any size sediment supplied to them. Continental glaciers transport most of their sediment in the lower part of the ice, whereas valley glaciers may carry sediment in all parts of the ice. Erosion of mountains by valley glaciers creates several sharp, angular landforms, including cirques, arêtes, and horns. U-shaped glacial troughs, fiords, and hanging valleys are also products of valley glaciation. Continental glaciers abrade and bevel high areas, producing smooth, rounded landscapes known as ice-scoured plains. Depositional landforms include moraines, which are ridgelike accumulations of till. Several types of moraines are recognized, including terminal, recessional, lateral, and medial moraines. Drumlins are composed of till that was apparently reshaped into streamlined hills by continental glaciers or floods of glacial meltwater.

CHAPTER SUMMARY Stratified drift in outwash plains and valley trains consists of sediments deposited in or by meltwater streams issuing from glaciers. Ridges called eskers and conical hills called kames also consist of strati- fied drift. Currently, the Milankovitch theory is widely accepted as the explanation for glacial–interglacial intervals. The reasons for short-term climatic changes, such as the Little Ice Age, are not understood. Two proposed causes are changes in the amount of solar energy received by Earth and volcanism.