Question

Why are crevasses only about 40 meters deep, even though many glaciers are much thicker?

Short answer

Crevasses are limited to about 40 meters because only the upper ice layer is brittle.

Step-by-step solution

Understand Glacier Basics

Glaciers are massive bodies of ice that flow slowly due to gravity. They have a thick, dense outer layer at the top and often move more quickly near the surface than deeper.

Define Crevasses

Crevasses are deep cracks or fractures in a glacier. They form when the glacier's surface ice is pulled apart due to the stress of moving over uneven terrain or changes in velocity.

Explain Glacier's Brittle Zone

The surface of a glacier is brittle and fractures easily under stress. This brittle zone generally extends to about 40 meters in depth, where the ice behaves like a solid and cracks.

Introduce the Plastic Flow Zone

Below approximately 40 meters, the glacier ice transitions to a plastic flow state, where it deforms rather than cracks. Ice here behaves more like a viscous fluid, bending under stress instead of breaking.

Conclusion

Crevasses are limited to the glacier's brittle zone, which only reaches about 40 meters deep. Beyond this depth, the ice bends rather than breaks, preventing the formation of deeper crevasses.

Key concepts

Crevasses

Crevasses are the large fractures or openings that form on the surface of glaciers. They are visually striking and can be several meters wide and tens of meters deep. These cracks appear due to the pulling and stretching of the glacier as it moves.
When the surface ice layers are subjected to tension—such as when a glacier flows over a drop-off or uneven terrain—the ice fractures, creating crevasses.

The formation of crevasses is a fascinating indication of the glacier's movement and stress patterns. The depth limitation of about 40 meters is due to the properties of ice in this zone, which we will explore further in the following sections.

Brittle Zone

The brittle zone is the uppermost layer of the glacier where the ice behaves like a solid and cracks under stress. This zone is influenced by the low temperature and the great pressure of the overlying ice.
Within this zone, which extends approximately 40 meters deep, the ice is prone to breaking, resulting in the formation of crevasses. Beyond this depth, the conditions change, leading to different behavior of the ice mass.

Some features of the brittle zone include:

• Presence of crevasses primarily due to tensile stress.
• Visible cracks on the glacier surface.
• Ice behaves like a brittle solid.

This zone is crucial for understanding the dynamics of glacier surfaces and their susceptibility to environmental changes.

Plastic Flow Zone

Beneath the brittle zone, the properties of the glacier ice change significantly. From around 40 meters downwards, the ice enters what is known as the plastic flow zone. Here, the glacier ice transitions from a brittle state to one that flows plastically.
This means the ice deforms and bends under pressure rather than cracking. As a result, no crevasses form in this deeper part of the glacier.

Characteristics of the plastic flow zone include:

• The ice behaves more like a very slow-moving liquid.
• High pressures cause the ice to flow and adjust to terrain changes beneath it.
• Absence of surface cracks like those seen in the brittle zone.

This zone is integral to the glacier's ability to adapt its shape as it navigates mountainous terrains.