Which one of the following is the main method by which potholes are created in bedrocks by the flowing stream?

Correct — A, Abrasion. A pothole is the circular hollow drilled into a stream's rocky bed, common in the upper course where the channel runs over hard rock. It forms when a small depression traps pebbles and cobbles and the current sets them spinning; those tools grind against the bedrock, deepening and widening the hollow until it is a smooth cylindrical pit, and neighbouring potholes eventually coalesce to lower the whole bed. That grinding by the load against the bed is abrasion, sometimes called corrasion. The distinction the option list is probing is the one between abrasion and corrosion, which sound alike and mean opposite things — the second is chemical.

• (b)Attrition — Attrition is the load wearing itself down — pebbles knocking against pebbles until they are smaller and rounder. It acts on the transported material, not on the channel floor, so it cannot cut a hollow in bedrock.
• (c)Corrosion — Corrosion, or solution, is the chemical dissolving of rock by water, and it matters in limestone country where it makes caves and sinkholes. It works on the whole wetted surface rather than drilling the neat cylindrical pits that abrasion produces, and it has little effect on the hard crystalline rock where potholes are most common.
• (d)Solifluction — Not a fluvial process at all. Solifluction is a slow mass movement of water-saturated soil over frozen subsoil in periglacial areas, and it has nothing to do with a stream bed.

A river erodes in four ways. Hydraulic action is the sheer force of moving water prising material loose; abrasion or corrasion is the load scraping and grinding against the bed and banks; attrition is the load wearing itself down as the fragments collide; and corrosion or solution is the chemical dissolving of soluble rock. Potholes, along with most channel deepening in hard rock, belong to abrasion, with hydraulic action helping to start the initial hollow.

The trap is purely lexical, and it is one worth fixing for good — abrasion is mechanical, corrosion is chemical, and attrition is what happens to the tools rather than to the bed. A test that always works: ask what is being worn away and by what. In a pothole, bedrock is being worn away by pebbles, so the answer must be the mechanical process. Potholes are worth recognising in the field too, as one of the standard upper-course landforms alongside V-shaped valleys, gorges, waterfalls and rapids.

• The four processes of river erosion are hydraulic action, abrasion or corrasion, attrition and corrosion or solution.
• Potholes are cylindrical hollows ground into a rocky stream bed by pebbles swirled in eddies.
• Abrasion wears the bed using the transported load as tools; attrition wears down the load itself.
• Corrosion is chemical solution and dominates in limestone, producing caves, sinkholes and karst.
• Merging potholes lower the whole channel floor, which is how a stream deepens its bed in hard rock.

• Choosing corrosion because it sounds like corrasion.
• Applying attrition to the bed instead of to the transported load.
• Picking a mass-movement term in a question about a stream.

A one-word process item built on a pair of near-identical technical terms with opposite meanings.

No directly related past PYQ was found.

• practice — not a real PYQ

  The process by which pebbles carried by a river become smaller and rounder as they collide with one another is

  • (a)abrasion
  • (b)attrition
  • (c)corrosion
  • (d)hydraulic action

  Answer(b) attrition — the load wears itself down, rather than wearing the channel.

• practice — not a real PYQ

  Solution as a process of erosion is most significant in which rock type?

  • (a)Granite
  • (b)Basalt
  • (c)Limestone
  • (d)Quartzite

  Answer(c) Limestone — it dissolves in slightly acidic water, producing caves and sinkholes.