In rocks that are at or near the surface, water seeps along the joints, attacking unstable minerals. This causes rocks to decompose and disintegrate at their edges, opening the joints wider and allowing even more water to reach the surfaces. At corners where two or more joints meet, water attacks from more than one direction, causing more rapid decomposition by chemical weathering. This extra disintegration at joint intersections tends to change sharp corners into rounded surfaces. When the decomposed rock is removed from the widened joints by running water, wind or gravity, the unweathered portions of the rock form a complex of rounded boulders in their original positions.
Spheroidal weathering is most common among coarse-grained igneous rocks, especially granite and similar rock types. It is more likely to be found in warm climates, where mechanical weathering by freezing ice is less likely.