Glaciers are powerful agents of geological change, carving and reshaping the Earth’s surface. This immense erosive capability is executed through two primary processes: abrasion, the grinding action of embedded debris, and plucking, a mechanism that physically removes large chunks of bedrock. Also known as quarrying, glacial plucking is a mechanical process where the ice pulls rock fragments directly from the ground as it moves. This action is especially significant in mountainous terrain, creating the dramatic, rugged features associated with glaciated landscapes.
The Glacial Plucking Mechanism
The process begins when the enormous weight of the overlying ice creates intense pressure on the bedrock below. This pressure lowers the freezing point of water, causing the ice at the glacier’s base to melt and create a thin layer of subglacial meltwater, known as pressure melting. This liquid water is then forced into existing fractures, joints, and micro-cracks within the underlying rock structure.
As the glacier continues its forward movement, the water-filled cracks move into areas of lower pressure, typically downstream of a rock obstruction. In these low-pressure zones, the water quickly refreezes, bonding tightly to the surrounding rock surfaces. Water expands by approximately 9% of its volume when it transitions into solid ice.
This volumetric increase creates an immense expansive force, acting like a wedge to widen the cracks and loosen the block of rock from the bedrock mass. Once the rock fragment is sufficiently fractured and detached, the overriding ice completely freezes around it. The glacier then exerts a powerful tractive force, pulling the incorporated rock fragment out of its original position and carrying it downstream.
The cycle repeats continuously, as the glacier exploits new cracks and weaknesses in the newly exposed bedrock surface. Large pieces of rock can be removed and transported in this manner. These entrained fragments become part of the glacier’s debris load, contributing to the process of abrasion further down the flow path.
Prerequisites for Plucking Action
For plucking to occur effectively, a specific set of environmental and geological conditions must be met. The most fundamental requirement is the existence of pre-fractured or jointed bedrock beneath the glacier. Plucking is highly inefficient on smooth, unfractured rock because meltwater needs existing weaknesses to penetrate and exert its expansion force.
The process is most active beneath warm-based, or temperate, glaciers. In these ice masses, the basal ice is at or near the pressure melting point, ensuring a consistent supply of liquid water at the ice-bedrock interface. This water is necessary to infiltrate the rock joints and complete the freeze-thaw cycle that breaks the rock apart.
A fluctuating pressure regime is also necessary, often found where the glacier flows over an uneven bed. The cyclical change in pressure, from high over an obstruction to low in the lee, drives the sequence of melting and refreezing. This localized change allows the water to melt under high pressure and then refreeze under reduced pressure, maximizing the loosening effect.
Landforms Resulting from Plucking
Glacial plucking leaves behind distinct, observable topographical features. One of the most characteristic landforms is the roche moutonnée, an asymmetrical bedrock hill shaped by both abrasion and plucking. The upstream side, known as the stoss side, is gently sloping and smooth, resulting from the grinding action of abrasion.
In sharp contrast, the downstream or lee side of the roche moutonnée is steep, rough, and often jagged. This side is the direct result of plucking, where the ice has ripped away fractured rock fragments. The alternating smooth and rough surfaces illustrate the two modes of glacial erosion working in tandem on the same feature.
Plucking also plays a large role in shaping the U-shaped valleys typical of glaciated mountain ranges. The process contributes significantly to the vertical erosion of the valley sides, steepening and straightening the slopes by removing large blocks of rock. This action creates the characteristic broad floor and steep walls of a glacial trough.
A related feature is the crag and tail, which forms when a large, resistant rock mass obstructs the flow of the glacier. Plucking aggressively removes material from the exposed upstream face of the crag. The less resistant rock on the downstream side is protected from intense erosion, leaving a steep, plucked face and a streamlined, gradually tapering ridge of protected rock, or “tail.”