In physical weathering of rocks, the process in which outer layers of rock peel or flake off due to repeated expansion and contraction is known as what?

Introduction / Context:
Weathering is the breakdown of rocks at or near the Earth surface into smaller pieces or altered material. Mechanical or physical weathering works mainly through temperature changes, pressure release, and mechanical forces. This question deals with a particular type of physical weathering where rock surfaces peel off in sheets, often due to alternating heating and cooling, which causes expansion and contraction of the outer layers.

Given Data / Assumptions:

• The described process involves layers of rock peeling or flaking off.
• The peeling is related to expansion and contraction cycles.
• Options include shattering, block separation, granular disintegration, and exfoliation.
• We assume standard geomorphological definitions for these terms.

Concept / Approach:
Exfoliation is the process in which curved or planar sheets of rock break away from an exposed rock surface, often due to temperature changes or release of pressure. The repeated expansion when heated and contraction when cooled produces stress that separates outer layers from the interior. Shattering refers more to breaking into sharp fragments, block separation refers to larger blocky disintegration, and granular disintegration involves individual mineral grains breaking apart. The description of layers peeling off matches exfoliation most closely.

Step-by-Step Solution:
Step 1: Focus on the key description: layers of rock peel off and this effect is linked to repeated expansion and contraction. Step 2: Recall that exfoliation specifically refers to sheet like peeling, giving rock surfaces an onion skin appearance. Step 3: Shattering is usually associated with intense frost action or sudden stresses breaking rock into many fragments, not smooth peeling sheets. Step 4: Block separation describes rocks breaking into larger, more angular blocks along joints and fractures. Step 5: Granular disintegration occurs when individual mineral grains loosen and fall out, producing a sandy or granular breakdown, not large layers. Step 6: Because the question mentions layers peeling off due to expansion and contraction, exfoliation is the correct term.

Verification / Alternative check:
Geomorphology textbooks illustrate exfoliation domes and onion skin weathering, showing curved slabs of rock separating from underlying layers. These are often found in massive granitic outcrops subjected to intense daytime heating and cooler nights. The terminology consistently labels this peeling of sheets as exfoliation and differentiates it from other mechanical weathering forms, confirming the correct answer.

Why Other Options Are Wrong:

• Shattering: Emphasises fragmentation into many sharp pieces, not sheet like peeling.
• Block separation: Involves breaking into large, angular blocks along joints, rather than thin surface layers.
• Granular disintegration: Refers to breakdown into grains or sand like particles, not larger sheets.

Common Pitfalls:
Students sometimes confuse granular disintegration and exfoliation because both can be related to temperature changes. The key difference lies in the scale of breakdown: exfoliation involves slabs or sheets, while granular disintegration involves small grains. To avoid errors, remember the visual image of onion skin weathering when you see the word exfoliation and connect it to layers peeling from rock surfaces.

Final Answer:
The weathering process in which layers of rock peel off due to expansion and contraction is called exfoliation.