Plucking (glaciation)

Plucking, also referred to as quarrying, is a critical mechanism of glacial erosion responsible for the detachment and transport of bedrock. This process is most pronounced beneath the basal ice of valley glaciers. The fundamental driver is the repeated action of meltwater infiltrating pre-existing joints, fractures, or bedding planes within the bedrock.

Due to the immense pressure exerted by the overlying ice, the melting point of ice is lowered, leading to basal melting even at sub-zero ambient temperatures. This meltwater, acting as a fluid, penetrates deep into the bedrock's discontinuities. Upon a decrease in pressure or temperature, this water refreezes.

The volumetric expansion of water upon freezing (approximately 9%) generates significant tensile stress within the rock. This repeated hydraulic wedging, exacerbated by the glacier's movement and the cyclical nature of freeze-thaw, progressively widens and deepens these fractures, ultimately leading to the dislodgement of large rock masses known as joint blocks. The efficiency of plucking is directly proportional to the degree of jointing and fracturing in the bedrock, as well as the presence of exposed bed planes that facilitate water ingress.

Once joint blocks are detached through plucking, they become entrained within the basal ice of the glacier. This entrainment is facilitated by the process of regelation, where pressure variations cause localized melting and refreezing. As the glacier moves over obstacles or uneven bedrock, pressure increases on the up-ice side, causing melting, and decreases on the down-ice side, allowing refreezing.

This allows the ice to flow around obstacles and also helps to grip and incorporate the loosened rock fragments. The size of these entrained blocks can be substantial, with documented cases of joint blocks up to three meters in length being plucked and transported. These transported rock fragments are not merely passengers; they become active agents of further erosion.

As the glacier advances, these embedded rocks scour the bedrock beneath, a process known as glacial abrasion, which smooths and deepens valleys and produces features like striations and polishing. Thus, plucking and abrasion work in tandem to dramatically sculpt glacial landscapes.

The significance of plucking in geomorphology is profound. It is a primary process responsible for the characteristic erosional landforms created by glaciers, most notably the steep, often vertical, cirques and the broad, U-shaped valleys. The removal of large rock masses by plucking contributes directly to the overdeepening of valleys and the formation of features such as roche moutonnées, where the up-ice (stoss) side is often plucked and smoothed, while the down-ice (lee) side is steeper and more blocky due to plucking.

Plucking also leads to the formation of chatter marks, which are diagnostic crescentic fractures or wedge-shaped indentations left on the bedrock surface. These marks are indicative of the repeated impact and fracturing caused by the movement of plucked blocks. The effectiveness of plucking is maximized in areas with well-jointed or fractured bedrock, or where exposed bedding planes provide easy access for meltwater, highlighting the importance of pre-existing geological conditions in controlling glacial erosion rates.

While plucking is most famously associated with glacial environments, analogous processes can be observed in steep upland rivers, particularly during high-flow events. In these settings, the detachment of bedrock blocks from riverbeds and banks is driven by a combination of factors, including chemical and physical weathering along joints, hydraulic wedging by smaller transported clasts, and stresses induced by the impact of larger debris carried by the turbulent flow. The flexing of bedrock due to fluctuating water pressure during floods can also contribute to crack propagation.

However, the entrainment and transport efficiency of water, even in powerful floods, is generally considered less effective than that of ice in carrying large, heavy rock blocks over significant distances. Nevertheless, these fluvial processes demonstrate that the fundamental principles of wedging, fracturing, and detachment are applicable in various high-energy geomorphic systems, underscoring the universal nature of these erosional forces.