Chapter 18
Chapter 18: Amazing Ice: Glaciers and Ice Ages
Animation
Glacial advance and retreat is determined by the balance between the accumulation of snow and the removal of ice by sublimation, melting, and calving (ablation). When the rate of ablation below the snowline equals the rate of accumulation above it, the glacier is stationary, as in View 1. During glacial retreat, View 2, the rate of ablation exceeds the rate of accumulation, and the position of the toe retreats toward the origin of the glacier. Glacial advance, View 3, occurs when the rate of accumulation exceeds the rate of ablation. For all views, pay attention to the motion of the stones. Note that in all cases, ice flows downhill. For more information, see 18.2 The Nature of Glaciers starting on p. 470 and Figure 18.8 on p. 477 in your textbook.
Glaciers are rivers or sheets of ice that last all year and flow slowly. Continental glaciers, vast sheets of ice up to a few kilometers thick, covered extensive areas of land during times when Earth had a colder climate. For more information, see Geology at a Glance: Glaciers and Glacial Landforms on pp. 484-485 in your textbook.
Why do glaciers advance and retreat periodically during an ice age? In 1920, Mulutin Milankovitch showed that regular variations in the shape of Earth’s orbit and the orientation of its axis create variations of solar intensity at high latitudes: warm summers in which glaciers retreat, and cool summers when they advance. These climate cycles, called Milankovitch Cycles, are determined by three factors: orbital eccentricity, shown in View 1, changes in the tilt of Earth’s axis, View 2, and the precession of Earth’s axis, View 3. For more information, see 18.8 The Causes of the Ice Age starting on p. 494 and Figure 18.27 in your textbook.