spheroidal weathering

Spheroidal weathering is a form of exfoliation, a process where thin shells or spalls separate from massive rock. This type of weathering is typically observed in boulder-sized rocks and can occur at some depth within the Earth. It results in rounded boulders surrounded by disintegrated material.1

While large daily temperature variations were once thought to cause exfoliation through expansion and contraction, experiments have largely discredited this theory. A more common cause is the slow development of clay minerals within the rock. As moisture penetrates crevices and initiates decay, the resulting swelling causes flaking roughly parallel to the rock's outer surface.1