Which process occurs when water freezes and expands in rock cracks?

Correct answer: Frost weathering

The direct answer is A, frost weathering. When water enters a crack and freezes, it expands. The expanding ice presses against the sides of the crack. Repeated freezing and thawing gradually widens the crack until pieces of rock break away. This is a physical or mechanical weathering process because the rock is broken into smaller pieces without a necessary change in its chemical composition. Option A is correct because it specifically names the action of freezing water in rock cracks. Option B, oxidation, is a chemical reaction commonly involving oxygen and minerals such as iron; it may produce rust-like changes, not crack expansion by ice. Option C, carbonation, occurs when carbonic acid reacts with minerals, especially limestone; it is also chemical weathering. Option D, humus formation, is the production of dark organic matter from decayed plants and animals in soil, not the breaking of rock by ice. The exact chain is water enters crack, temperature falls, water freezes and expands, pressure increases, crack widens, and rock fragments detach. Memory cue: freeze, expand, crack, break.