Glaciers, immense bodies of ice, display deep cracks known as crevasses, which stretch across their surface. These fissures rarely exceed a depth of about 50 meters. Understanding this consistent depth limit reveals fundamental principles about the behavior of ice under pressure.
How Crevasses Form
Crevasses form when glacial ice experiences significant tensional stress. Glacial ice constantly flows and deforms under its own weight and gravity, but this movement is not uniform. When a glacier flows over uneven terrain, around obstacles, or down changes in slope, different parts of the ice mass move at varying speeds. This differential movement creates tensile forces, pulling the ice apart and leading to cracks in the ice.
The Ice’s Breaking Point
At shallow depths, glacier ice behaves as a brittle material. When subjected to tensional forces, this brittle ice fractures, initiating a crevasse. As depth increases, the immense pressure from overlying ice fundamentally alters the ice’s physical properties.
Below approximately 50 meters, ice transitions from this brittle state to a ductile, or plastic, state. This change is known as the brittle-ductile transition zone. In this deeper zone, ice crystals rearrange under high pressure. Instead of fracturing, they deform and slide past one another, allowing the ice to flow continuously. The pressure at these depths prevents the ice from sustaining an open fracture, effectively setting a limit on how deep a crevasse can propagate.
Plastic Ice Flow
Once ice enters the plastic flow regime below the brittle-ductile transition zone, its response to stress changes dramatically. At these greater depths, the ice flows and deforms continuously under overwhelming pressure. Any cracks attempting to extend deeper into this ductile ice are immediately “healed” or squeezed shut. The constant deformation of the ice at these pressures simply closes off any void. This continuous flow prevents crevasses from propagating beyond the brittle zone, ensuring they cannot penetrate further into the glacier.
Factors Influencing Depth
While 50 meters is a commonly cited average for crevasse depth, this figure is not an absolute constant. Several factors can cause slight variations. Colder ice tends to be more brittle, potentially allowing for slightly deeper cracks. The rate at which the ice is deforming, known as the strain rate, also plays a role, with faster deformation sometimes influencing crack propagation. The presence of impurities within the ice can affect its overall strength and how it responds to stress.
Furthermore, if a crevasse becomes filled with water, the hydrostatic pressure can exert additional force, allowing it to extend deeper. Despite these influencing factors, the brittle-ductile transition due to pressure remains the primary control, consistently limiting crevasse depth to this general range.