The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Describe the concept of isostasy and how it affects the elevation of a mountain range. (E, F)

Isostasy is the state of gravitational equilibrium between Earth's lithosphere and asthenosphere. This means that the lithosphere, which includes the crust and upper portion of the mantle, is buoyant and "floats" on the denser, partially molten asthenosphere. The concept of isostasy explains how Earth's lithosphere adjusts itself to maintain balance and equilibrium with the underlying asthenosphere.

Isostatic equilibrium occurs when the vertical forces acting on a column of lithosphere are balanced by the forces acting on an equivalent column of fluid (in this case, the asthenosphere). The height of a column of lithosphere above the asthenosphere (i.e., its elevation) is related to its thickness and density. Thicker and less dense lithosphere will have a higher elevation, while a thinner and denser lithosphere will have a lower elevation. Isostatic compensation occurs when the lithosphere adjusts its thickness and density to maintain equilibrium with the underlying asthenosphere.

Mountain ranges are formed through tectonic processes, such as continental collision or volcanic activity, which cause the lithosphere to thicken and rise. The concept of isostasy affects the elevation of a mountain range in several ways: 1. The thickness and density of the lithosphere beneath the mountains will determine their elevation. If the lithosphere is thicker and less dense, the mountain range will be higher due to the buoyancy of the lithosphere on the asthenosphere. 2. As erosion wears down the mountain range, removing mass from the upper portion of the lithosphere, the lithosphere will "float" higher due to isostatic compensation. This process keeps the elevation of the mountain range relatively constant over time, even as it is being eroded. 3. Similarly, when mass is added to the lithosphere, such as during glacial deposition, the added weight will cause the lithosphere to "sink" lower, causing isostatic adjustment and a decrease in elevation. Understanding isostasy and its effects on mountain ranges will help in comprehending tectonic processes and the dynamic nature of Earth's lithosphere.