* Exposure: The surface is the only part of the rock directly exposed to the elements – rain, wind, ice, sunlight, and biological organisms.

* Physical forces: Physical weathering processes like frost wedging, abrasion, and thermal expansion/contraction primarily target the outer layers.

* Chemical reactions: Chemical weathering, such as oxidation and acid rain, also occurs most readily on the surface where the rock is in direct contact with the weathering agents.

This is why you'll often see rocks with a weathered exterior, while the interior remains relatively intact.