Question

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)

Short answer

Isostasy is the state of gravitational equilibrium between Earth's lithosphere and asthenosphere, where the lithosphere "floats" on the denser asthenosphere. It affects the elevation of mountain ranges by influencing the thickness and density of the lithosphere beneath the mountains. Thicker and less dense lithosphere results in higher mountains, while thinner and denser lithosphere leads to lower elevations. Erosion, glacial deposition, and other processes that alter the mass of the lithosphere also contribute to isostatic compensation, impacting the elevation of mountain ranges over time.

Step-by-step solution

Define Isostasy

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.

Explain the Principles of Isostasy

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.

Relate Isostasy to Mountain Ranges

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.

Key concepts

Lithosphere

The lithosphere is a vital layer of the Earth, comprising both the crust and the uppermost portion of the mantle. It is essentially a rigid shell that covers our planet. Despite being solid, the lithosphere "floats" on the softer, more malleable asthenosphere beneath it.

• This floating occurs because the lithosphere is less dense compared to the asthenosphere, much like how wood floats on water.
• The thickness and density of the lithosphere play crucial roles in this floating mechanism. Where it's thicker and less dense, it will rise or stand at a higher elevation.
• Within the context of isostasy, the lithosphere seeks equilibrium, balancing its weight against the buoyant forces from the underlying asthenosphere.

This balancing act is constantly in motion, as changes such as erosion or deposition can alter the mass and thus the buoyancy of the lithosphere. Such dynamic adjustments help maintain the Earth's surface at various elevations.

Asthenosphere

The asthenosphere lies directly beneath the lithosphere and extends deeper into the Earth's mantle. It is a layer characterized by a semi-molten, ductile rock composition, giving it a consistency that allows for some movement.

• Its semi-fluid state is crucial for allowing the lithosphere to move and adjust, which is a key aspect of tectonic activity.
• The asthenosphere's density is higher relative to the lithosphere, adding to its ability to support the "floating lithosphere."
• This relationship allows for isostatic adjustments, whereby changes in the lithosphere's mass or density cause vertical shifts to maintain equilibrium.

The interplay between the lithosphere and asthenosphere is essential to isostasy and helps explain the continual balancing of Earth's surface materials, resulting in phenomena such as land subsidence and mountain stability.

Mountain Elevation

Mountain elevation is significantly influenced by the concepts of lithosphere and isostasy. These towering structures on Earth are formed primarily through tectonic forces that cause the lithosphere to thicken and uplift.

• Due to isostasy, the thicker and less dense lithosphere beneath mountains causes them to "rise," leading to higher elevations.
• Over time, erosion can wear down these mountains, reducing their mass and slightly altering their elevations as isostatic compensation allows them to "float" higher.
• Conversely, if additional weight is added, such as during periods of heavy glacier coverage, the lithosphere can sink, temporarily decreasing mountain height, only to rebalance once the weight is removed.

Understanding mountain elevation changes helps us grasp how Earth's surface reacts to tectonic activities and surface processes, maintaining a dynamic equilibrium through the principles of isostasy.