A plastic flow glacier describes the primary mechanism by which glaciers move, involving the internal deformation of ice under stress, allowing it to behave like a very slow-moving fluid.
Understanding Plastic Flow in Glaciers
Plastic flow, also known as creep, is a fundamental process in glacial dynamics. Unlike brittle fracture, which results in the formation of crevasses, plastic flow involves the gradual movement of ice masses under the influence of gravity. This occurs deep within the ice mass, where immense pressure causes the ice crystals to deform and slide past one another without actually melting.
How Plastic Flow Occurs
Ice, under sufficient pressure and stress, behaves as a plastic material. This means it can deform permanently without breaking. Key aspects include:
- Internal Deformation: The primary driver of plastic flow is the internal rearrangement of ice crystals. As gravity pulls the glacier downhill, the immense weight of the overlying ice creates shear stress within the glacier. This stress causes individual ice crystals to reorient, slip along their internal planes, and even recrystallize, leading to a continuous, albeit slow, flow.
- Pressure and Temperature: Plastic flow is more pronounced in thicker glaciers and at temperatures closer to the melting point, where ice is more ductile. However, it can occur even in very cold ice, provided the stress is high enough.
- Comparison to Brittle Fracture: While plastic flow is a smooth, continuous process, brittle fracture occurs when ice is subjected to stress too quickly or at its surface, leading to cracks and the formation of crevasses. These two processes often occur simultaneously within different parts of a glacier.
| Feature | Plastic Flow | Brittle Fracture |
|---|---|---|
| Mechanism | Internal deformation of ice crystals | Cracking and breaking of ice |
| Result | Gradual, continuous movement; shaping of glacier | Formation of crevasses, seracs, icefalls |
| Location | Deep within the glacier, where pressure is high | Near the surface, where ice is more rigid |
| Contributing Factor | Sustained stress and pressure | Rapid stress changes, tension, or compression |
Importance in Glacial Movement
Plastic flow is the dominant mechanism for glacial movement, especially in temperate glaciers and in the lower parts of all glaciers where the ice is thickest and under the most pressure. It allows glaciers to:
- Move down slopes, even very gentle ones.
- Flow around obstacles in the terrain.
- Shape valleys and landscapes over long periods.
Without plastic flow, glaciers would behave like rigid, unmoving blocks of ice, unable to transport sediment or carve out the distinctive U-shaped valleys we see today. It is a critical process in the Earth's geomorphological cycle and contributes significantly to the dynamics of glacial systems.
