Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does lithosphere respond to the weight of glacial ice? (B)

Short answer

The lithosphere responds to the weight of glacial ice through the processes of isostatic depression and isostatic rebound. As glacial ice accumulates on the Earth's surface, the weight causes the lithosphere to adjust its thickness and sink deeper into the asthenosphere, a process called isostatic depression. When the ice melts or retreats, the lithosphere begins to rise back to its previous elevation through isostatic rebound, maintaining isostatic equilibrium.

Step-by-step solution

1. Introduction to lithosphere and glacial ice

The lithosphere is the solid, outer part of the Earth, comprising the Earth's crust and upper mantle. Glacial ice is the accumulation of snow over time that forms large ice sheets and glaciers. When these ice sheets and glaciers form on the Earth's surface, they exert a significant downward pressure on the underlying lithosphere due to their immense weight.

2. Introduction to Isostasy

Isostasy is the state of gravitational equilibrium between the Earth's lithosphere and the underlying, more fluid asthenosphere. It's a term used to describe how the Earth's crust tends to adjust its thickness so that lighter rocks float on denser mantle rocks over long geological timescales. This concept helps explain how the Earth's crust responds to the distribution of mass, like the weight of glacial ice, on its surface.

3. Response of the lithosphere to glacial ice

When a significant amount of glacial ice accumulates on the Earth's surface, it adds extra weight to the lithosphere. In response, the lithosphere adjusts its thickness to maintain isostatic equilibrium, causing the crust to sink deeper into the more fluid asthenosphere. This process is called isostatic depression, where the crust bends downward under the weight of the ice.

4. Isostatic rebound

After the ice melts or retreats, the weight on the lithosphere is removed, and the crust begins to rise back up to its previous elevation, a process called isostatic rebound. The rate of rebound is influenced by the Earth's crust's elasticity and the rate at which glacial ice is removed. This process may take thousands of years to reach a new equilibrium. To sum up, the lithosphere responds to the weight of glacial ice by adjusting its thickness through the processes of isostatic depression and isostatic rebound. The Earth's crust gets pushed down under the weight of the ice and rises back up when the ice melts or retreats, in order to maintain isostatic equilibrium.

Key concepts

Isostasy

Isostasy is a concept crucial to understanding how the Earth's lithosphere and asthenosphere interact. It refers to the state of balance, or gravitational equilibrium, between these two layers. The lithosphere, which is the Earth's rigid outer shell, "floats" atop the more ductile and semi-fluid asthenosphere beneath it.
The idea here is similar to how an iceberg floats on water, with variations in buoyancy determining how high or low the lithosphere lies. When large weights like mountains or glaciers press down on the lithosphere, it sinks deeper into the asthenosphere. Conversely, when these weights are removed, the lithosphere rises. This balance of mass ensures that over geological time, the Earth remains stable.

• The lithosphere is the outer solid part of Earth.
• The asthenosphere is more fluid and lies beneath the lithosphere.
• Isostasy maintains Earth's balance through buoyancy adjustments.

Understanding isostasy helps us grasp the dynamic nature of Earth's surface and why it changes over time in response to shifting masses.

Isostatic Depression

Isostatic depression occurs when the lithosphere sinks due to the weight of an immense mass, such as a glacier. This process was especially prevalent during the last Ice Age, when massive ice sheets covered large parts of the planet.
The heavy weight of these glaciers pushes the lithosphere down further into the asthenosphere, much like a person sitting on a mattress makes it sink under their weight. This downward bending is what geologists refer to as isostatic depression.

• Occurs due to large weights pressing on the lithosphere.
• Results in the lithosphere sinking into the more fluid asthenosphere.
• Illustrates the Earth's capability to adapt to surface mass changes.

The process demonstrates Earth's remarkable flexibility, even in its solid form, and underscores how intertwined geological processes are.

Isostatic Rebound

Isostatic rebound is the counter-process to isostatic depression. It takes place when the weight compressing the lithosphere, such as a glacier, is removed. When glaciers melt and retreat, the lithosphere begins to bounce back or rise to its original position.
This process can take thousands of years and is affected by the elasticity of the crust and the speed of melting. In areas where glaciers once dominated, you might still observe evidence of this rebound even today.

• Occurs as the lithosphere rises after the removal of weight.
• Can take several millennia to complete.
• Influenced by both the crust's elasticity and the melting rate.

Isostatic rebound is vital in shaping current land elevations and continues to redefine our planet's landscapes today.

Gravitational Equilibrium

Gravitational equilibrium is a fundamental principle that ensures the balance of forces within the Earth. This concept underpins isostasy and guides how the Earth's crust behaves under varying loads.
At its core, gravitational equilibrium involves the balance between the lithosphere (the solid shell) and the asthenosphere (the more pliable layer beneath). The lithosphere adjusts its position to maintain this balance, rising or sinking in response to added or removed masses.

• Represents the balance of forces within the Earth.
• Ensures stability by adjusting the lithosphere's height.
• Directly influences isostatic processes.

Gravitational equilibrium is what keeps the Earth's crust stable over time, allowing it to accommodate changes in surface loads, such as ice caps and mountains.