The correct option is Unloading.
Explanation
Exfoliation domes are large, rounded landforms that result from physical weathering. This phenomenon is most commonly observed in massive igneous rocks, such as granite, which were originally formed deep underground.
- Thermal expansion is incorrect: Thermal expansion involves the expansion and contraction of rock surfaces due to diurnal temperature changes. While this can cause surface flaking or "onion-skin" weathering on a smaller scale, it is not the primary mechanism responsible for the deep-seated sheeting joints required to form large exfoliation domes.
- Unloading is correct: The primary process is unloading (or pressure release). Massive rocks like granite solidify deep within the crust under high pressure from overlying materials. When erosion removes this overlying rock load, the vertical pressure is released. This causes the rock mass to expand upward, creating large fractures or "sheeting joints" roughly parallel to the ground surface. As these sheets peel away, a smooth, rounded dome structure remains.
- Oxidation is incorrect: Oxidation is a chemical weathering process where minerals react with oxygen (e.g., iron rusting). It leads to the decomposition of rock material rather than the mechanical sheeting associated with dome formation.
- Frost wedging is incorrect: Frost wedging is a physical weathering process driven by the freeze-thaw cycle of water in cracks. It typically results in the shattering of rocks into angular fragments, not the formation of smooth, curved surfaces characteristic of exfoliation domes.
Key Takeaway: Exfoliation domes are formed by unloading and expansion, where the removal of superincumbent load releases confining pressure, causing the rock to fracture in layers parallel to the surface.