The aggregates which are available near the river front are found to have a rounded shape due to
Correct Answer: C. attrition
🧱 Detailed Explanation: Why River Gravels Have Rounded Shapes — Attrition
When rock fragments are carried downstream by rivers, they undergo a remarkable transformation over thousands of years: sharp, angular edges are gradually worn away until the particles take on the smooth, rounded shape that characterises river gravel. This natural process is called attrition.
What is attrition? Attrition is the gradual wear and mass loss of particle surfaces caused by mutual rubbing, grinding, and friction between particles as they are transported by water. As the river current carries particles over the riverbed, they repeatedly collide with each other and the rocky bed, progressively rounding off corners and roughness.
Why C (Attrition) is correct and the others are not:
• Crushing (A): Mechanical crushing by jaw/cone crushers produces angular particles with fresh fracture faces — the opposite of rounded.
• Impaction (B): Sudden violent impact causes irregular breakage, producing angular and sub-angular fragments, not rounded shapes.
• Attrition (C) ✅: Prolonged gentle rubbing progressively removes surface material from all directions equally, producing the characteristic smooth, rounded form of river and seashore gravels.
• Segregation (D): A settling/sorting process that separates particles by size or density, but does NOT change particle shape.
• Crushing (A): Mechanical crushing by jaw/cone crushers produces angular particles with fresh fracture faces — the opposite of rounded.
• Impaction (B): Sudden violent impact causes irregular breakage, producing angular and sub-angular fragments, not rounded shapes.
• Attrition (C) ✅: Prolonged gentle rubbing progressively removes surface material from all directions equally, producing the characteristic smooth, rounded form of river and seashore gravels.
• Segregation (D): A settling/sorting process that separates particles by size or density, but does NOT change particle shape.
Natural vs. Mechanical Aggregate Shapes
| Source / Process | Mechanism | Resulting Shape |
|---|---|---|
| River bed / seashore | Prolonged attrition in water | Rounded, smooth |
| Mountain streams | Short transport — limited attrition | Sub-angular to sub-rounded |
| Rock quarry (crusher) | Mechanical crushing | Angular, rough surfaces |
| Desert / aeolian | Wind abrasion (attrition by wind-blown sand) | Rounded, frosted (matte) surface |
Key Concepts for Students
- Attrition = gradual surface wear by mutual rubbing during water transport. The longer the river journey, the more rounded the particles become.
- Do not confuse attrition with abrasion: both involve surface wear, but attrition is mutual rubbing between particles; abrasion involves a harder surface wearing a softer one.
- River gravels have the lowest void content (~32–33%) when packed due to their smooth, rounded form — meaning less cement paste is needed to fill voids.
- The smooth surface of river gravel provides poor bond with cement paste, making rounded aggregates unsuitable for high-strength structural concrete.