Glaciers are enormous, slow-moving rivers of ice driven by gravity. These ice masses move across the landscape, modifying the terrain beneath them through erosion, transport, and deposition. Glacial erosion is highly effective, transforming sharp mountain peaks and valleys into rounded, U-shaped features. Among the methods glaciers use to break down and remove the underlying rock, one of the most mechanically destructive is plucking.
Defining Glacial Plucking
Glacial plucking, also called quarrying, is an erosional process where a glacier removes large, intact blocks of bedrock from its bed. This action begins when the glacier’s basal ice is near the melting point, allowing meltwater to exist at the ice-rock interface. This water, generated by the immense pressure of the overlying ice or by friction, seeps into pre-existing cracks and fractures in the bedrock. The efficiency of plucking depends heavily on the presence of these fractures, as well as the rock type and joint density.
As the meltwater penetrates the rock, it refreezes and expands its volume by approximately nine percent. This volumetric expansion exerts mechanical stress, widening the fissures and loosening the blocks. Once the blocks are sufficiently stressed and separated, the glacier’s forward movement applies a shearing force. This force tears the joint blocks away from the bedrock, incorporating them into the basal ice layer for transport downslope.
The Critical Role of Basal Sliding
The actual removal of the loosened rock blocks requires a sustained, dragging force, which is provided by the movement of the ice over its bed, a process known as basal sliding. Basal sliding occurs when a film of meltwater lubricates the interface between the ice and the underlying terrain. This meltwater is often created by pressure melting, where the weight of the ice lowers the melting point of the water, causing ice to melt even at temperatures below zero degrees Celsius.
This sliding mechanism is distinct from the overall mass balance of the glacier, which determines whether the glacier’s terminus advances or retreats. Glacial flow is driven by gravity and occurs through both internal deformation and basal sliding. Glaciers that are warm-based, meaning they have meltwater at their bed, rely heavily on this sliding motion. This movement provides the necessary mechanical friction and pulling action to dislodge the fractured blocks during the plucking process.
Plucking During Periods of Retreat or Stagnation
Glacial plucking will, in fact, continue even if a glacier is not advancing, provided that the necessary conditions for basal sliding are met. The advance or retreat of a glacier refers only to the position of its terminus and is determined by the glacier’s mass balance—the difference between accumulation and ablation. A retreating glacier is one where the rate of ablation at the terminus exceeds the rate of ice flow delivering new mass to that point.
Crucially, the ice mass itself still flows downslope under the force of gravity, much like a conveyor belt, even as the terminus recedes. This internal flow and basal sliding, which is the true driver of plucking, persists as long as the glacier is thick enough to maintain the pressure required for basal meltwater production. Therefore, during periods of retreat or stagnation, the continuous movement of the ice over the bedrock still facilitates the freeze-thaw cycle and the shearing action that removes rock blocks. The erosive machinery of the glacier remains active, sculpting the landscape from below.