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.

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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.