Glacial Landscapes in the UKCausation

Plucking (Quarrying): Step-by-Step Mechanism

Part of Glacial Processes · GCSE GCSE Geography revision

This causation covers Plucking (Quarrying): Step-by-Step Mechanism within Glacial Processes for GCSE Geography. Revise Glacial Processes in Glacial Landscapes in the UK for GCSE Geography with 15 exam-style questions and 20 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 5 of 17 in this topic. Use this causation to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 5 of 17

Practice

15 questions

Recall

20 flashcards

⛓️ Plucking (Quarrying): Step-by-Step Mechanism

Plucking (also called quarrying) is one of the two main glacial erosion processes. It involves the glacier physically tearing blocks of rock from the bedrock and incorporating them into the ice. It is most effective on the downhill (lee) side of rocky obstacles and on bedrock that is well-jointed (i.e. divided by natural fractures).

Step 1 — Glacier moves over bedrock: The base of the glacier slides over the underlying bedrock through basal sliding. As the ice flows over a rocky bump or obstacle, it exerts enormous pressure on the uphill face (stoss side) of the rock.

Step 2 — Pressure differences create meltwater: On the high-pressure (uphill) side of the obstacle, the increased pressure lowers the melting point of ice and creates a thin film of meltwater. On the low-pressure (downhill/lee) side of the obstacle, the pressure drops — and the meltwater refreezes.

Step 3 — Ice freezes onto rock: The refreezing meltwater on the lee side of the obstacle penetrates into the joints and cracks of the rock. As it freezes, it bonds the ice firmly to the rock, effectively welding the glacier to the bedrock at that point.

Step 4 — Glacier movement tears out blocks: As the glacier continues to move forward, it exerts a pulling force on the rock it has frozen onto. If this force exceeds the strength of the rock (which it readily does with well-jointed or weakened rock), the block is torn away — quarried — from the bedrock and incorporated into the base of the glacier.

Step 5 — Irregular, jagged surface left behind: Unlike abrasion, which smooths and polishes the bedrock, plucking produces an irregular, jagged, stepped surface. The plucked blocks become part of the subglacial debris load.

Step 6 — Positive feedback loop: The plucked blocks, now embedded in the base of the glacier, become tools for abrasion (see below). This means plucking and abrasion are linked processes: more plucking produces more abrasive tools, which in turn produces more rock flour, which lubricates the glacier base and allows it to slide faster, allowing more plucking. The two processes reinforce each other.

Plucking is most effective where rocks are well-jointed (fractured), where pre-existing freeze-thaw weathering has already opened and widened joints, and where the glacier moves quickly enough to exert significant pulling forces. Slow-moving cold-based glaciers, which lack basal sliding, pluck far less effectively than fast-moving temperate glaciers.

Plucking (Quarrying): Step-by-Step Mechanism

⛓️ Plucking (Quarrying): Step-by-Step Mechanism

Keep building this topic

Practice Questions for Glacial Processes

Quick Recall Flashcards

15 questions on Glacial Processes — practise free